Merge pull request #12176 from edx/robrap/linter-javascript

TNL-4325: Linter Enhancements for JavaScript files

scripts/safe_template_linter.py

@@ -40,7 +40,7 @@ def _is_skip_dir(skip_dirs, directory):
     return False
 
 
-def _load_file(self, file_full_path):
+def _load_file(file_full_path):
     """
     Loads a file into a string.
 
@@ -56,6 +56,77 @@ def _load_file(self, file_full_path):
         return file_contents.decode(encoding='utf-8')
 
 
+def _find_closing_char_index(start_delim, open_char, close_char, template, start_index, num_open_chars=0, strings=None):
+    """
+    Finds the index of the closing char that matches the opening char.
+
+    For example, this could be used to find the end of a Mako expression,
+    where the open and close characters would be '{' and '}'.
+
+    Arguments:
+        start_delim: If provided (e.g. '${' for Mako expressions), the
+            closing character must be found before the next start_delim.
+        open_char: The opening character to be matched (e.g '{')
+        close_char: The closing character to be matched (e.g '}')
+        template: The template to be searched.
+        start_index: The start index of the last open char.
+        num_open_chars: The current number of open chars.
+        strings: A list of ParseStrings already parsed
+
+    Returns:
+        A dict containing the following, or None if unparseable:
+            close_char_index: The index of the closing character
+            strings: a list of ParseStrings
+
+    """
+    strings = [] if strings is None else strings
+    close_char_index = template.find(close_char, start_index)
+    if close_char_index < 0:
+        # if we can't find an end_char, let's just quit
+        return None
+    open_char_index = template.find(open_char, start_index, close_char_index)
+    parse_string = ParseString(template, start_index, close_char_index)
+
+    valid_index_list = [close_char_index]
+    if 0 <= open_char_index:
+        valid_index_list.append(open_char_index)
+    if parse_string.start_index is not None:
+        valid_index_list.append(parse_string.start_index)
+    min_valid_index = min(valid_index_list)
+
+    if parse_string.start_index == min_valid_index:
+        strings.append(parse_string)
+        if parse_string.end_index is None:
+            return None
+        else:
+            return _find_closing_char_index(
+                start_delim, open_char, close_char, template, start_index=parse_string.end_index,
+                num_open_chars=num_open_chars, strings=strings
+            )
+
+    if open_char_index == min_valid_index:
+        if start_delim is not None:
+            # if we find another starting delim, consider this unparseable
+            start_delim_index = template.find(start_delim, start_index, close_char_index)
+            if 0 <= start_delim_index < open_char_index:
+                return None
+        return _find_closing_char_index(
+            start_delim, open_char, close_char, template, start_index=open_char_index + 1,
+            num_open_chars=num_open_chars + 1, strings=strings
+        )
+
+    if num_open_chars == 0:
+        return {
+            'close_char_index': close_char_index,
+            'strings': strings,
+        }
+    else:
+        return _find_closing_char_index(
+            start_delim, open_char, close_char, template, start_index=close_char_index + 1,
+            num_open_chars=num_open_chars - 1, strings=strings
+        )
+
+
 class StringLines(object):
     """
     StringLines provides utility methods to work with a string in terms of
@@ -72,7 +143,10 @@ class StringLines(object):
 
         """
         self._string = string
-        self._line_breaks = self._process_line_breaks(string)
+        self._line_start_indexes = self._process_line_breaks(string)
+        # this is an exclusive index used in the case that the template doesn't
+        # end with a new line
+        self.eof_index = len(string)
 
     def _process_line_breaks(self, string):
         """
@@ -83,19 +157,18 @@ class StringLines(object):
             string: The string in which to find line breaks.
 
         Returns:
-             A list of indices into the string at which each line break can be
-             found.
+             A list of indices into the string at which each line begins.
 
         """
-        line_breaks = [0]
+        line_start_indexes = [0]
         index = 0
         while True:
             index = string.find('\n', index)
             if index < 0:
                 break
             index += 1
-            line_breaks.append(index)
-        return line_breaks
+            line_start_indexes.append(index)
+        return line_start_indexes
 
     def get_string(self):
         """
@@ -116,7 +189,7 @@ class StringLines(object):
 
         """
         current_line_number = 0
-        for line_break_index in self._line_breaks:
+        for line_break_index in self._line_start_indexes:
             if line_break_index <= index:
                 current_line_number += 1
             else:
@@ -154,6 +227,20 @@ class StringLines(object):
         line_number = self.index_to_line_number(index)
         return self.line_number_to_start_index(line_number)
 
+    def index_to_line_end_index(self, index):
+        """
+        Gets the index of the end of the line of the given index.
+
+        Arguments:
+            index: The index into the original string.
+
+        Returns:
+            The index of the end of the line of the given index.
+
+        """
+        line_number = self.index_to_line_number(index)
+        return self.line_number_to_end_index(line_number)
+
     def line_number_to_start_index(self, line_number):
         """
         Gets the starting index for the provided line number.
@@ -166,7 +253,26 @@ class StringLines(object):
             The starting index for the provided line number.
 
         """
-        return self._line_breaks[line_number - 1]
+        return self._line_start_indexes[line_number - 1]
+
+    def line_number_to_end_index(self, line_number):
+        """
+        Gets the ending index for the provided line number.
+
+        Arguments:
+            line_number: The line number of the line for which we want to find
+                the end index.
+
+        Returns:
+            The ending index for the provided line number.
+
+        """
+        if line_number < len(self._line_start_indexes):
+            return self._line_start_indexes[line_number]
+        else:
+            # an exclusive index in the case that the file didn't end with a
+            # newline.
+            return self.eof_index
 
     def line_number_to_line(self, line_number):
         """
@@ -179,11 +285,11 @@ class StringLines(object):
             The line of text designated by the provided line number.
 
         """
-        start_index = self._line_breaks[line_number - 1]
-        if len(self._line_breaks) == line_number:
+        start_index = self._line_start_indexes[line_number - 1]
+        if len(self._line_start_indexes) == line_number:
             line = self._string[start_index:]
         else:
-            end_index = self._line_breaks[line_number]
+            end_index = self._line_start_indexes[line_number]
             line = self._string[start_index:end_index - 1]
         return line
 
@@ -191,7 +297,7 @@ class StringLines(object):
         """
         Gets the number of lines in the string.
         """
-        return len(self._line_breaks)
+        return len(self._line_start_indexes)
 
 
 class Rules(Enum):
@@ -222,6 +328,14 @@ class Rules(Enum):
         'mako-invalid-js-filter',
         'The expression is using an invalid filter in a JavaScript context.'
     )
+    mako_js_missing_quotes = (
+        'mako-js-missing-quotes',
+        'An expression using js_escaped_string must be wrapped in quotes.'
+    )
+    mako_js_html_string = (
+        'mako-js-html-string',
+        'A JavaScript string containing HTML should not have an embedded Mako expression.'
+    )
     mako_deprecated_display_name = (
         'mako-deprecated-display-name',
         'Replace deprecated display_name_with_default_escaped with display_name_with_default.'
@@ -254,12 +368,95 @@ class Rules(Enum):
         'underscore-not-escaped',
         'Expressions should be escaped using <%- expression %>.'
     )
+    javascript_jquery_append = (
+        'javascript-jquery-append',
+        'Use HtmlUtils.append() or .append(HtmlUtils.xxx().toString()).'
+    )
+    javascript_jquery_prepend = (
+        'javascript-jquery-prepend',
+        'Use HtmlUtils.prepend() or .prepend(HtmlUtils.xxx().toString()).'
+    )
+    javascript_jquery_insertion = (
+        'javascript-jquery-insertion',
+        'JQuery DOM insertion calls that take content must use HtmlUtils (e.g. $el.after(HtmlUtils.xxx().toString()).'
+    )
+    javascript_jquery_insert_into_target = (
+        'javascript-jquery-insert-into-target',
+        'JQuery DOM insertion calls that take a target can only be called from elements (e.g. .$el.appendTo()).'
+    )
+    javascript_jquery_html = (
+        'javascript-jquery-html',
+        "Use HtmlUtils.setHtml(), .html(HtmlUtils.xxx().toString()), or JQuery's text() function."
+    )
+    javascript_concat_html = (
+        'javascript-concat-html',
+        'Use HtmlUtils functions rather than concatenating strings with HTML.'
+    )
+    javascript_escape = (
+        'javascript-escape',
+        "Avoid calls to escape(), especially in Backbone. Use templates, HtmlUtils, or JQuery's text() function."
+    )
+    javascript_interpolate = (
+        'javascript-interpolate',
+        'Use StringUtils.interpolate() or HtmlUtils.interpolateHtml() as appropriate.'
+    )
 
     def __init__(self, rule_id, rule_summary):
         self.rule_id = rule_id
         self.rule_summary = rule_summary
 
 
+class Expression(object):
+    """
+    Represents an arbitrary expression.
+
+    An expression can be any type of code snippet. It will sometimes have a
+    starting and ending delimiter, but not always.
+
+    Here are some example expressions::
+
+        ${x | n, decode.utf8}
+        <%= x %>
+        function(x)
+        "<p>" + message + "</p>"
+
+    Other details of note:
+    - Only a start_index is required for a valid expression.
+    - If end_index is None, it means we couldn't parse the rest of the
+    expression.
+    - All other details of the expression are optional, and are only added if
+    and when supplied and needed for additional checks.  They are not necessary
+    for the final results output.
+
+    """
+
+    def __init__(self, start_index, end_index=None, template=None, start_delim="", end_delim="", strings=None):
+        """
+        Init method.
+
+        Arguments:
+            start_index: the starting index of the expression
+            end_index: the index immediately following the expression, or None
+                if the expression was unparseable
+            template: optional template code in which the expression was found
+            start_delim: optional starting delimiter of the expression
+            end_delim: optional ending delimeter of the expression
+            strings: optional list of ParseStrings
+
+        """
+        self.start_index = start_index
+        self.end_index = end_index
+        self.start_delim = start_delim
+        self.end_delim = end_delim
+        self.strings = strings
+        if template is not None and self.end_index is not None:
+            self.expression = template[start_index:end_index]
+            self.expression_inner = self.expression[len(start_delim):-len(end_delim)].strip()
+        else:
+            self.expression = None
+            self.expression_inner = None
+
+
 class RuleViolation(object):
     """
     Base class representing a rule violation which can be used for reporting.
@@ -312,6 +509,18 @@ class RuleViolation(object):
                 self.is_disabled = True
                 return
 
+    def sort_key(self):
+        """
+        Returns a key that can be sorted on
+        """
+        return 0
+
+    def first_line(self):
+        """
+        Since a file level rule has no first line, returns empty string.
+        """
+        return ''
+
     def prepare_results(self, full_path, string_lines):
         """
         Preps this instance for results reporting.
@@ -349,7 +558,7 @@ class ExpressionRuleViolation(RuleViolation):
 
         Arguments:
             rule: The Rule which was violated.
-            expression: The expression that was in violation.
+            expression: The Expression that was in violation.
 
         """
         super(ExpressionRuleViolation, self).__init__(rule)
@@ -400,6 +609,18 @@ class ExpressionRuleViolation(RuleViolation):
         line_to_check = string_lines.line_number_to_line(self.start_line)
         self._mark_disabled(line_to_check, scope_start_string=False)
 
+    def sort_key(self):
+        """
+        Returns a key that can be sorted on
+        """
+        return (self.start_line, self.start_column)
+
+    def first_line(self):
+        """
+        Returns the initial line of code of the violation.
+        """
+        return self.lines[0]
+
     def prepare_results(self, full_path, string_lines):
         """
         Preps this instance for results reporting.
@@ -411,11 +632,11 @@ class ExpressionRuleViolation(RuleViolation):
 
         """
         self.full_path = full_path
-        start_index = self.expression['start_index']
+        start_index = self.expression.start_index
         self.start_line = string_lines.index_to_line_number(start_index)
         self.start_column = string_lines.index_to_column_number(start_index)
-        end_index = self.expression['end_index']
-        if end_index > 0:
+        end_index = self.expression.end_index
+        if end_index is not None:
             self.end_line = string_lines.index_to_line_number(end_index)
             self.end_column = string_lines.index_to_column_number(end_index)
         else:
@@ -467,17 +688,21 @@ class FileResults(object):
         self.is_file = os.path.isfile(full_path)
         self.violations = []
 
-    def prepare_results(self, file_string):
+    def prepare_results(self, file_string, line_comment_delim=None):
         """
         Prepares the results for output for this file.
 
         Arguments:
             file_string: The string of content for this file.
+            line_comment_delim: A string representing the start of a line
+                comment. For example "##" for Mako and "//" for JavaScript.
 
         """
         string_lines = StringLines(file_string)
         for violation in self.violations:
             violation.prepare_results(self.full_path, string_lines)
+        if line_comment_delim is not None:
+            self._filter_commented_code(line_comment_delim)
 
     def print_results(self, options, out):
         """
@@ -489,14 +714,49 @@ class FileResults(object):
                     all violations.
             out: output file
 
+        Returns:
+            The number of violations. When using --quiet, returns number of
+            files with violations.
 
         """
+        num_violations = 0
         if options['is_quiet']:
-            print(self.full_path, file=out)
+            if self.violations is not None and 0 < len(self.violations):
+                num_violations += 1
+                print(self.full_path, file=out)
         else:
+            self.violations.sort(key=lambda violation: violation.sort_key())
             for violation in self.violations:
                 if not violation.is_disabled:
+                    num_violations += 1
                     violation.print_results(out)
+        return num_violations
+
+    def _filter_commented_code(self, line_comment_delim):
+        """
+        Remove any violations that were found in commented out code.
+
+        Arguments:
+            line_comment_delim: A string representing the start of a line
+                comment. For example "##" for Mako and "//" for JavaScript.
+
+        """
+        self.violations = [v for v in self.violations if not self._is_commented(v, line_comment_delim)]
+
+    def _is_commented(self, violation, line_comment_delim):
+        """
+        Checks if violation line is commented out.
+
+        Arguments:
+            violation: The violation to check
+            line_comment_delim: A string representing the start of a line
+                comment. For example "##" for Mako and "//" for JavaScript.
+
+        Returns:
+            True if the first line of the violation is actually commented out,
+            False otherwise.
+        """
+        return violation.first_line().lstrip().startswith(line_comment_delim)
 
 
 class ParseString(object):
@@ -504,8 +764,8 @@ class ParseString(object):
     ParseString is the result of parsing a string out of a template.
 
     A ParseString has the following attributes:
-        start_index: The index of the first quote, or -1 if none found
-        end_index: The index following the closing quote, or -1 if
+        start_index: The index of the first quote, or None if none found
+        end_index: The index following the closing quote, or None if
             unparseable
         quote_length: The length of the quote.  Could be 3 for a Python
             triple quote.  Or None if none found.
@@ -525,12 +785,12 @@ class ParseString(object):
             end_index: The end index to search before.
 
         """
-        self.end_index = -1
+        self.end_index = None
         self.quote_length = None
         self.string = None
         self.string_inner = None
         self.start_index = self._find_string_start(template, start_index, end_index)
-        if 0 <= self.start_index:
+        if self.start_index is not None:
             result = self._parse_string(template, self.start_index)
             if result is not None:
                 self.end_index = result['end_index']
@@ -549,13 +809,13 @@ class ParseString(object):
             end_index: The end index to search before.
 
         Returns:
-            The start index of the first single or double quote, or -1 if
-            no quote was found.
+            The start index of the first single or double quote, or None if no
+            quote was found.
         """
         quote_regex = re.compile(r"""['"]""")
         start_match = quote_regex.search(template, start_index, end_index)
         if start_match is None:
-            return -1
+            return None
         else:
             return start_match.start()
 
@@ -603,207 +863,807 @@ class ParseString(object):
                 }
 
 
-class MakoTemplateLinter(object):
+class UnderscoreTemplateLinter(object):
     """
-    The linter for Mako template files.
+    The linter for Underscore.js template files.
     """
 
-    _skip_mako_dirs = _skip_dirs
+    _skip_underscore_dirs = _skip_dirs + ('test',)
 
     def process_file(self, directory, file_name):
         """
-        Process file to determine if it is a Mako template file and
+        Process file to determine if it is an Underscore template file and
         if it is safe.
 
         Arguments:
             directory (string): The directory of the file to be checked
-            file_name (string): A filename for a potential Mako file
+            file_name (string): A filename for a potential underscore file
 
         Returns:
             The file results containing any violations.
 
         """
-        mako_file_full_path = os.path.normpath(directory + '/' + file_name)
-        results = FileResults(mako_file_full_path)
-
-        if not results.is_file:
-            return results
+        full_path = os.path.normpath(directory + '/' + file_name)
+        results = FileResults(full_path)
 
         if not self._is_valid_directory(directory):
             return results
 
-        # TODO: When safe-by-default is turned on at the platform level, will we:
-        # 1. Turn it on for .html only, or
-        # 2. Turn it on for all files, and have different rulesets that have
-        #    different rules of .xml, .html, .js, .txt Mako templates (e.g. use
-        #    the n filter to turn off h for some of these)?
-        # For now, we only check .html and .xml files
-        if not (file_name.lower().endswith('.html') or file_name.lower().endswith('.xml')):
+        if not file_name.lower().endswith('.underscore'):
             return results
 
-        return self._load_and_check_mako_file_is_safe(mako_file_full_path, results)
+        return self._load_and_check_underscore_file_is_safe(full_path, results)
 
     def _is_valid_directory(self, directory):
         """
         Determines if the provided directory is a directory that could contain
-        Mako template files that need to be linted.
+        Underscore.js template files that need to be linted.
 
         Arguments:
             directory: The directory to be linted.
 
         Returns:
-            True if this directory should be linted for Mako template violations
-            and False otherwise.
+            True if this directory should be linted for Underscore.js template
+            violations and False otherwise.
         """
-        if _is_skip_dir(self._skip_mako_dirs, directory):
+        if _is_skip_dir(self._skip_underscore_dirs, directory):
             return False
 
-        # TODO: This is an imperfect guess concerning the Mako template
-        # directories. This needs to be reviewed before turning on safe by
-        # default at the platform level.
-        if ('/templates/' in directory) or directory.endswith('/templates'):
-            return True
-
-        return False
+        return True
 
-    def _load_and_check_mako_file_is_safe(self, mako_file_full_path, results):
+    def _load_and_check_underscore_file_is_safe(self, file_full_path, results):
         """
-        Loads the Mako template file and checks if it is in violation.
+        Loads the Underscore.js template file and checks if it is in violation.
 
         Arguments:
-            mako_file_full_path: The file to be loaded and linted.
+            file_full_path: The file to be loaded and linted
 
         Returns:
             The file results containing any violations.
 
         """
-        mako_template = _load_file(self, mako_file_full_path)
-        self._check_mako_file_is_safe(mako_template, results)
+        underscore_template = _load_file(file_full_path)
+        self.check_underscore_file_is_safe(underscore_template, results)
         return results
 
-    def _check_mako_file_is_safe(self, mako_template, results):
+    def check_underscore_file_is_safe(self, underscore_template, results):
         """
-        Checks for violations in a Mako template.
+        Checks for violations in an Underscore.js template.
 
         Arguments:
-            mako_template: The contents of the Mako template.
+            underscore_template: The contents of the Underscore.js template.
             results: A file results objects to which violations will be added.
 
         """
-        if self._is_django_template(mako_template):
-            return
-        has_page_default = False
-        if self._has_multiple_page_tags(mako_template):
-            results.violations.append(RuleViolation(Rules.mako_multiple_page_tags))
-        else:
-            has_page_default = self._has_page_default(mako_template)
-            if not has_page_default:
-                results.violations.append(RuleViolation(Rules.mako_missing_default))
-        self._check_mako_expressions(mako_template, has_page_default, results)
-        results.prepare_results(mako_template)
+        self._check_underscore_expressions(underscore_template, results)
+        results.prepare_results(underscore_template)
 
-    def _is_django_template(self, mako_template):
+    def _check_underscore_expressions(self, underscore_template, results):
         """
-            Determines if the template is actually a Django template.
+        Searches for Underscore.js expressions that contain violations.
 
         Arguments:
-            mako_template: The template code.
+            underscore_template: The contents of the Underscore.js template.
+            results: A list of results into which violations will be added.
+
+        """
+        expressions = self._find_unescaped_expressions(underscore_template)
+        for expression in expressions:
+            if not self._is_safe_unescaped_expression(expression):
+                results.violations.append(ExpressionRuleViolation(
+                    Rules.underscore_not_escaped, expression
+                ))
+
+    def _is_safe_unescaped_expression(self, expression):
+        """
+        Determines whether an expression is safely escaped, even though it is
+        using the expression syntax that doesn't itself escape (i.e. <%= ).
+
+        In some cases it is ok to not use the Underscore.js template escape
+        (i.e. <%- ) because the escaping is happening inside the expression.
+
+        Safe examples::
+
+            <%= HtmlUtils.ensureHtml(message) %>
+            <%= _.escape(message) %>
+
+        Arguments:
+            expression: The Expression being checked.
 
         Returns:
-            True if this is really a Django template, and False otherwise.
+            True if the Expression has been safely escaped, and False otherwise.
 
         """
-        if re.search('({%.*%})|({{.*}})', mako_template) is not None:
+        if expression.expression_inner.startswith('HtmlUtils.'):
+            return True
+        if expression.expression_inner.startswith('_.escape('):
             return True
         return False
 
-    def _has_multiple_page_tags(self, mako_template):
+    def _find_unescaped_expressions(self, underscore_template):
         """
-        Checks if the Mako template contains more than one page expression.
+        Returns a list of unsafe expressions.
+
+        At this time all expressions that are unescaped are considered unsafe.
 
         Arguments:
-            mako_template: The contents of the Mako template.
+            underscore_template: The contents of the Underscore.js template.
 
+        Returns:
+            A list of Expressions.
         """
-        count = len(re.findall('<%page ', mako_template, re.IGNORECASE))
-        return count > 1
+        unescaped_expression_regex = re.compile("<%=.*?%>", re.DOTALL)
 
-    def _has_page_default(self, mako_template):
-        """
-        Checks if the Mako template contains the page expression marking it as
-        safe by default.
+        expressions = []
+        for match in unescaped_expression_regex.finditer(underscore_template):
+            expression = Expression(
+                match.start(), match.end(), template=underscore_template, start_delim="<%=", end_delim="%>"
+            )
+            expressions.append(expression)
+        return expressions
 
-        Arguments:
-            mako_template: The contents of the Mako template.
 
-        """
-        page_h_filter_regex = re.compile('<%page[^>]*expression_filter=(?:"h"|\'h\')[^>]*/>')
-        page_match = page_h_filter_regex.search(mako_template)
-        return page_match
+class JavaScriptLinter(object):
+    """
+    The linter for JavaScript and CoffeeScript files.
+    """
 
-    def _check_mako_expressions(self, mako_template, has_page_default, results):
+    _skip_javascript_dirs = _skip_dirs + ('i18n', 'static/coffee')
+    _skip_coffeescript_dirs = _skip_dirs
+    underScoreLinter = UnderscoreTemplateLinter()
+
+    def process_file(self, directory, file_name):
         """
-        Searches for Mako expressions and then checks if they contain
-        violations.
+        Process file to determine if it is a JavaScript file and
+        if it is safe.
 
         Arguments:
-            mako_template: The contents of the Mako template.
-            has_page_default: True if the page is marked as default, False
-                otherwise.
-            results: A list of results into which violations will be added.
+            directory (string): The directory of the file to be checked
+            file_name (string): A filename for a potential JavaScript file
+
+        Returns:
+            The file results containing any violations.
 
         """
-        expressions = self._find_mako_expressions(mako_template)
-        contexts = self._get_contexts(mako_template)
-        for expression in expressions:
-            if expression['expression'] is None:
-                results.violations.append(ExpressionRuleViolation(
-                    Rules.mako_unparseable_expression, expression
-                ))
-                continue
+        file_full_path = os.path.normpath(directory + '/' + file_name)
+        results = FileResults(file_full_path)
 
-            context = self._get_context(contexts, expression['start_index'])
-            self._check_filters(mako_template, expression, context, has_page_default, results)
-            self._check_deprecated_display_name(expression, results)
-            self._check_html_and_text(expression, has_page_default, results)
+        if not results.is_file:
+            return results
 
-    def _check_deprecated_display_name(self, expression, results):
+        if file_name.lower().endswith('.js') and not file_name.lower().endswith('.min.js'):
+            skip_dirs = self._skip_javascript_dirs
+        elif file_name.lower().endswith('.coffee'):
+            skip_dirs = self._skip_coffeescript_dirs
+        else:
+            return results
+
+        if not self._is_valid_directory(skip_dirs, directory):
+            return results
+
+        return self._load_and_check_javascript_file_is_safe(file_full_path, results)
+
+    def _is_valid_directory(self, skip_dirs, directory):
         """
-        Checks that the deprecated display_name_with_default_escaped is not
-        used. Adds violation to results if there is a problem.
+        Determines if the provided directory is a directory that could contain
+        a JavaScript file that needs to be linted.
 
         Arguments:
-            expression: A dict containing the start_index, end_index, and
-                expression (text) of the expression.
-            results: A list of results into which violations will be added.
+            skip_dirs: The directories to be skipped.
+            directory: The directory to be linted.
 
+        Returns:
+            True if this directory should be linted for JavaScript violations
+            and False otherwise.
         """
-        if '.display_name_with_default_escaped' in expression['expression']:
-            results.violations.append(ExpressionRuleViolation(
-                Rules.mako_deprecated_display_name, expression
-            ))
+        if _is_skip_dir(skip_dirs, directory):
+            return False
 
-    def _check_html_and_text(self, expression, has_page_default, results):
+        return True
+
+    def _load_and_check_javascript_file_is_safe(self, file_full_path, results):
         """
-        Checks rules related to proper use of HTML() and Text().
+        Loads the JavaScript file and checks if it is in violation.
 
         Arguments:
-            expression: A dict containing the start_index, end_index, and
-                expression (text) of the expression.
-            has_page_default: True if the page is marked as default, False
-                otherwise.
+            file_full_path: The file to be loaded and linted.
+
+        Returns:
+            The file results containing any violations.
+
+        """
+        file_contents = _load_file(file_full_path)
+        self.check_javascript_file_is_safe(file_contents, results)
+        return results
+
+    def check_javascript_file_is_safe(self, file_contents, results):
+        """
+        Checks for violations in a JavaScript file.
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            results: A file results objects to which violations will be added.
+
+        """
+        no_caller_check = None
+        no_argument_check = None
+        self._check_jquery_function(
+            file_contents, "append", Rules.javascript_jquery_append, no_caller_check,
+            self._is_jquery_argument_safe, results
+        )
+        self._check_jquery_function(
+            file_contents, "prepend", Rules.javascript_jquery_prepend, no_caller_check,
+            self._is_jquery_argument_safe, results
+        )
+        self._check_jquery_function(
+            file_contents, "unwrap|wrap|wrapAll|wrapInner|after|before|replaceAll|replaceWith",
+            Rules.javascript_jquery_insertion, no_caller_check, self._is_jquery_argument_safe, results
+        )
+        self._check_jquery_function(
+            file_contents, "appendTo|prependTo|insertAfter|insertBefore",
+            Rules.javascript_jquery_insert_into_target, self._is_jquery_insert_caller_safe, no_argument_check, results
+        )
+        self._check_jquery_function(
+            file_contents, "html", Rules.javascript_jquery_html, no_caller_check,
+            self._is_jquery_html_argument_safe, results
+        )
+        self._check_javascript_interpolate(file_contents, results)
+        self._check_javascript_escape(file_contents, results)
+        self._check_concat_with_html(file_contents, results)
+        self.underScoreLinter.check_underscore_file_is_safe(file_contents, results)
+        results.prepare_results(file_contents, line_comment_delim='//')
+
+    def _get_expression_for_function(self, file_contents, function_start_match):
+        """
+        Returns an expression that matches the function call opened with
+        function_start_match.
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            function_start_match: A regex match representing the start of the function
+                call (e.g. ".escape(").
+
+        Returns:
+            An Expression that best matches the function.
+
+        """
+        start_index = function_start_match.start()
+        inner_start_index = function_start_match.end()
+        result = _find_closing_char_index(
+            None, "(", ")", file_contents, start_index=inner_start_index
+        )
+        if result is not None:
+            end_index = result['close_char_index'] + 1
+            expression = Expression(
+                start_index, end_index, template=file_contents, start_delim=function_start_match.group(), end_delim=")"
+            )
+        else:
+            expression = Expression(start_index)
+        return expression
+
+    def _check_javascript_interpolate(self, file_contents, results):
+        """
+        Checks that interpolate() calls are safe.
+
+        Only use of StringUtils.interpolate() or HtmlUtils.interpolateText()
+        are safe.
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            results: A file results objects to which violations will be added.
+
+        """
+        # Ignores calls starting with "StringUtils.", because those are safe
+        regex = re.compile(r"(?<!StringUtils).interpolate\(")
+        for function_match in regex.finditer(file_contents):
+            expression = self._get_expression_for_function(file_contents, function_match)
+            results.violations.append(ExpressionRuleViolation(Rules.javascript_interpolate, expression))
+
+    def _check_javascript_escape(self, file_contents, results):
+        """
+        Checks that only necessary escape() are used.
+
+        Allows for _.escape(), although this shouldn't be the recommendation.
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            results: A file results objects to which violations will be added.
+
+        """
+        # Ignores calls starting with "_.", because those are safe
+        regex = regex = re.compile(r"(?<!_).escape\(")
+        for function_match in regex.finditer(file_contents):
+            expression = self._get_expression_for_function(file_contents, function_match)
+            results.violations.append(ExpressionRuleViolation(Rules.javascript_escape, expression))
+
+    def _check_jquery_function(self, file_contents, function_names, rule, is_caller_safe, is_argument_safe, results):
+        """
+        Checks that the JQuery function_names (e.g. append(), prepend()) calls
+        are safe.
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            function_names: A pipe delimited list of names of the functions
+                (e.g. "wrap|after|before").
+            rule: The name of the rule to use for validation errors (e.g.
+                Rules.javascript_jquery_append).
+            is_caller_safe: A function to test if caller of the JQuery function
+                is safe.
+            is_argument_safe: A function to test if the argument passed to the
+                JQuery function is safe.
+            results: A file results objects to which violations will be added.
+
+        """
+        # Ignores calls starting with "HtmlUtils.", because those are safe
+        regex = re.compile(r"(?<!HtmlUtils).(?:{})\(".format(function_names))
+        for function_match in regex.finditer(file_contents):
+            is_violation = True
+            expression = self._get_expression_for_function(file_contents, function_match)
+            if expression.end_index is not None:
+                start_index = expression.start_index
+                inner_start_index = function_match.end()
+                close_paren_index = expression.end_index - 1
+                function_argument = file_contents[inner_start_index:close_paren_index].strip()
+                if is_argument_safe is not None and is_caller_safe is None:
+                    is_violation = is_argument_safe(function_argument) is False
+                elif is_caller_safe is not None and is_argument_safe is None:
+                    line_start_index = StringLines(file_contents).index_to_line_start_index(start_index)
+                    caller_line_start = file_contents[line_start_index:start_index]
+                    is_violation = is_caller_safe(caller_line_start) is False
+                else:
+                    raise ValueError("Must supply either is_argument_safe, or is_caller_safe, but not both.")
+            if is_violation:
+                results.violations.append(ExpressionRuleViolation(rule, expression))
+
+    def _is_jquery_argument_safe_html_utils_call(self, argument):
+        """
+        Checks that the argument sent to a jQuery DOM insertion function is a
+        safe call to HtmlUtils.
+
+        A safe argument is of the form:
+        - HtmlUtils.xxx(anything).toString()
+        - edx.HtmlUtils.xxx(anything).toString()
+
+        Arguments:
+            argument: The argument sent to the jQuery function (e.g.
+            append(argument)).
+
+        Returns:
+            True if the argument is safe, and False otherwise.
+
+        """
+        # match on HtmlUtils.xxx().toString() or edx.HtmlUtils
+        match = re.search(r"(?:edx\.)?HtmlUtils\.[a-zA-Z0-9]+\(.*\)\.toString\(\)", argument)
+        return match is not None and match.group() == argument
+
+    def _is_jquery_argument_safe(self, argument):
+        """
+        Check the argument sent to a jQuery DOM insertion function (e.g.
+        append()) to check if it is safe.
+
+        Safe arguments include:
+        - the argument can end with ".el", ".$el" (with no concatenation)
+        - the argument can be a single variable ending in "El" or starting with
+            "$". For example, "testEl" or "$test".
+        - the argument can be a single string literal with no HTML tags
+        - the argument can be a call to $() with the first argument a string
+            literal with a single HTML tag.  For example, ".append($('<br/>'))"
+            or ".append($('<br/>'))".
+        - the argument can be a call to HtmlUtils.xxx(html).toString()
+
+        Arguments:
+            argument: The argument sent to the jQuery function (e.g.
+            append(argument)).
+
+        Returns:
+            True if the argument is safe, and False otherwise.
+
+        """
+        match_variable_name = re.search("[_$a-zA-Z]+[_$a-zA-Z0-9]*", argument)
+        if match_variable_name is not None and match_variable_name.group() == argument:
+            if argument.endswith('El') or argument.startswith('$'):
+                return True
+        elif argument.startswith('"') or argument.startswith("'"):
+            # a single literal string with no HTML is ok
+            # 1. it gets rid of false negatives for non-jquery calls (e.g. graph.append("g"))
+            # 2. JQuery will treat this as a plain text string and will escape any & if needed.
+            string = ParseString(argument, 0, len(argument))
+            if string.string == argument and "<" not in argument:
+                return True
+        elif argument.startswith('$('):
+            # match on JQuery calls with single string and single HTML tag
+            # Examples:
+            #    $("<span>")
+            #    $("<div/>")
+            #    $("<div/>", {...})
+            match = re.search(r"""\$\(\s*['"]<[a-zA-Z0-9]+\s*[/]?>['"]\s*[,)]""", argument)
+            if match is not None:
+                return True
+        elif self._is_jquery_argument_safe_html_utils_call(argument):
+            return True
+        # check rules that shouldn't use concatenation
+        elif "+" not in argument:
+            if argument.endswith('.el') or argument.endswith('.$el'):
+                return True
+        return False
+
+    def _is_jquery_html_argument_safe(self, argument):
+        """
+        Check the argument sent to the jQuery html() function to check if it is
+        safe.
+
+        Safe arguments to html():
+        - no argument (i.e. getter rather than setter)
+        - empty string is safe
+        - the argument can be a call to HtmlUtils.xxx(html).toString()
+
+        Arguments:
+            argument: The argument sent to html() in code (i.e. html(argument)).
+
+        Returns:
+            True if the argument is safe, and False otherwise.
+
+        """
+        if argument == "" or argument == "''" or argument == '""':
+            return True
+        elif self._is_jquery_argument_safe_html_utils_call(argument):
+            return True
+        return False
+
+    def _is_jquery_insert_caller_safe(self, caller_line_start):
+        """
+        Check that the caller of a jQuery DOM insertion function that takes a
+        target is safe (e.g. thisEl.appendTo(target)).
+
+        If original line was::
+
+            draggableObj.iconEl.appendTo(draggableObj.containerEl);
+
+        Parameter caller_line_start would be:
+
+            draggableObj.iconEl
+
+        Safe callers include:
+        - the caller can be ".el", ".$el"
+        - the caller can be a single variable ending in "El" or starting with
+            "$". For example, "testEl" or "$test".
+
+        Arguments:
+            caller_line_start: The line leading up to the jQuery function call.
+
+        Returns:
+            True if the caller is safe, and False otherwise.
+
+        """
+        # matches end of line for caller, which can't itself be a function
+        caller_match = re.search(r"(?:\s*|[.])([_$a-zA-Z]+[_$a-zA-Z0-9])*$", caller_line_start)
+        if caller_match is None:
+            return False
+        caller = caller_match.group(1)
+        if caller is None:
+            return False
+        elif caller.endswith('El') or caller.startswith('$'):
+            return True
+        elif caller == 'el' or caller == 'parentNode':
+            return True
+        return False
+
+    def _check_concat_with_html(self, file_contents, results):
+        """
+        Checks that strings with HTML are not concatenated
+
+        Arguments:
+            file_contents: The contents of the JavaScript file.
+            results: A file results objects to which violations will be added.
+
+        """
+        lines = StringLines(file_contents)
+        last_expression = None
+        # attempt to match a string that starts with '<' or ends with '>'
+        regex_string_with_html = r"""["'](?:\s*<.*|.*>\s*)["']"""
+        regex_concat_with_html = r"(\+\s*{}|{}\s*\+)".format(regex_string_with_html, regex_string_with_html)
+        for match in re.finditer(regex_concat_with_html, file_contents):
+            found_new_violation = False
+            if last_expression is not None:
+                last_line = lines.index_to_line_number(last_expression.start_index)
+                # check if violation should be expanded to more of the same line
+                if last_line == lines.index_to_line_number(match.start()):
+                    last_expression = Expression(
+                        last_expression.start_index, match.end(), template=file_contents
+                    )
+                else:
+                    results.violations.append(ExpressionRuleViolation(
+                        Rules.javascript_concat_html, last_expression
+                    ))
+                    found_new_violation = True
+            else:
+                found_new_violation = True
+            if found_new_violation:
+                last_expression = Expression(
+                    match.start(), match.end(), template=file_contents
+                )
+
+        # add final expression
+        if last_expression is not None:
+            results.violations.append(ExpressionRuleViolation(
+                Rules.javascript_concat_html, last_expression
+            ))
+
+
+class MakoTemplateLinter(object):
+    """
+    The linter for Mako template files.
+    """
+
+    _skip_mako_dirs = _skip_dirs
+    javaScriptLinter = JavaScriptLinter()
+
+    def process_file(self, directory, file_name):
+        """
+        Process file to determine if it is a Mako template file and
+        if it is safe.
+
+        Arguments:
+            directory (string): The directory of the file to be checked
+            file_name (string): A filename for a potential Mako file
+
+        Returns:
+            The file results containing any violations.
+
+        """
+        mako_file_full_path = os.path.normpath(directory + '/' + file_name)
+        results = FileResults(mako_file_full_path)
+
+        if not results.is_file:
+            return results
+
+        if not self._is_valid_directory(directory):
+            return results
+
+        # TODO: When safe-by-default is turned on at the platform level, will we:
+        # 1. Turn it on for .html only, or
+        # 2. Turn it on for all files, and have different rulesets that have
+        #    different rules of .xml, .html, .js, .txt Mako templates (e.g. use
+        #    the n filter to turn off h for some of these)?
+        # For now, we only check .html and .xml files
+        if not (file_name.lower().endswith('.html') or file_name.lower().endswith('.xml')):
+            return results
+
+        return self._load_and_check_mako_file_is_safe(mako_file_full_path, results)
+
+    def _is_valid_directory(self, directory):
+        """
+        Determines if the provided directory is a directory that could contain
+        Mako template files that need to be linted.
+
+        Arguments:
+            directory: The directory to be linted.
+
+        Returns:
+            True if this directory should be linted for Mako template violations
+            and False otherwise.
+        """
+        if _is_skip_dir(self._skip_mako_dirs, directory):
+            return False
+
+        # TODO: This is an imperfect guess concerning the Mako template
+        # directories. This needs to be reviewed before turning on safe by
+        # default at the platform level.
+        if ('/templates/' in directory) or directory.endswith('/templates'):
+            return True
+
+        return False
+
+    def _load_and_check_mako_file_is_safe(self, mako_file_full_path, results):
+        """
+        Loads the Mako template file and checks if it is in violation.
+
+        Arguments:
+            mako_file_full_path: The file to be loaded and linted.
+
+        Returns:
+            The file results containing any violations.
+
+        """
+        mako_template = _load_file(mako_file_full_path)
+        self._check_mako_file_is_safe(mako_template, results)
+        return results
+
+    def _check_mako_file_is_safe(self, mako_template, results):
+        """
+        Checks for violations in a Mako template.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            results: A file results objects to which violations will be added.
+
+        """
+        if self._is_django_template(mako_template):
+            return
+        has_page_default = self._has_page_default(mako_template, results)
+        self._check_mako_expressions(mako_template, has_page_default, results)
+        results.prepare_results(mako_template, line_comment_delim='##')
+
+    def _is_django_template(self, mako_template):
+        """
+            Determines if the template is actually a Django template.
+
+        Arguments:
+            mako_template: The template code.
+
+        Returns:
+            True if this is really a Django template, and False otherwise.
+
+        """
+        if re.search('({%.*%})|({{.*}})', mako_template) is not None:
+            return True
+        return False
+
+    def _get_page_tag_count(self, mako_template):
+        """
+        Determines the number of page expressions in the Mako template. Ignores
+        page expressions that are commented out.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+
+        Returns:
+            The number of page expressions
+        """
+        count = len(re.findall('<%page ', mako_template, re.IGNORECASE))
+        count_commented = len(re.findall(r'##\s+<%page ', mako_template, re.IGNORECASE))
+        return max(0, count - count_commented)
+
+    def _has_page_default(self, mako_template, results):
+        """
+        Checks if the Mako template contains the page expression marking it as
+        safe by default.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
             results: A list of results into which violations will be added.
 
+        Side effect:
+            Adds violations regarding page default if necessary
+
+        Returns:
+            True if the template has the page default, and False otherwise.
+
+        """
+        page_tag_count = self._get_page_tag_count(mako_template)
+        # check if there are too many page expressions
+        if 2 <= page_tag_count:
+            results.violations.append(RuleViolation(Rules.mako_multiple_page_tags))
+            return False
+        # make sure there is exactly 1 page expression, excluding commented out
+        # page expressions, before proceeding
+        elif page_tag_count != 1:
+            results.violations.append(RuleViolation(Rules.mako_missing_default))
+            return False
+        # check that safe by default (h filter) is turned on
+        page_h_filter_regex = re.compile('<%page[^>]*expression_filter=(?:"h"|\'h\')[^>]*/>')
+        page_match = page_h_filter_regex.search(mako_template)
+        if not page_match:
+            results.violations.append(RuleViolation(Rules.mako_missing_default))
+        return page_match
+
+    def _check_mako_expressions(self, mako_template, has_page_default, results):
         """
-        # strip '${' and '}' and whitespace from ends
-        expression_inner = expression['expression'][2:-1].strip()
-        # find the template relative inner expression start index
-        # - find used to take into account above strip()
-        template_inner_start_index = expression['start_index'] + expression['expression'].find(expression_inner)
+        Searches for Mako expressions and then checks if they contain
+        violations, including checking JavaScript contexts for JavaScript
+        violations.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            has_page_default: True if the page is marked as default, False
+                otherwise.
+            results: A list of results into which violations will be added.
+
+        """
+        expressions = self._find_mako_expressions(mako_template)
+        contexts = self._get_contexts(mako_template)
+        self._check_javascript_contexts(mako_template, contexts, results)
+        for expression in expressions:
+            if expression.end_index is None:
+                results.violations.append(ExpressionRuleViolation(
+                    Rules.mako_unparseable_expression, expression
+                ))
+                continue
+
+            context = self._get_context(contexts, expression.start_index)
+            self._check_filters(mako_template, expression, context, has_page_default, results)
+            self._check_deprecated_display_name(expression, results)
+            self._check_html_and_text(expression, has_page_default, results)
+
+    def _check_javascript_contexts(self, mako_template, contexts, results):
+        """
+        Lint the JavaScript contexts for JavaScript violations inside a Mako
+        template.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            contexts: A list of context dicts with 'type' and 'index'.
+            results: A list of results into which violations will be added.
+
+        Side effect:
+            Adds JavaScript violations to results.
+        """
+        javascript_start_index = None
+        for context in contexts:
+            if context['type'] == 'javascript':
+                if javascript_start_index < 0:
+                    javascript_start_index = context['index']
+            else:
+                if javascript_start_index is not None:
+                    javascript_end_index = context['index']
+                    javascript_code = mako_template[javascript_start_index:javascript_end_index]
+                    self._check_javascript_context(javascript_code, javascript_start_index, results)
+                    javascript_start_index = None
+        if javascript_start_index is not None:
+            javascript_code = mako_template[javascript_start_index:]
+            self._check_javascript_context(javascript_code, javascript_start_index, results)
+
+    def _check_javascript_context(self, javascript_code, start_offset, results):
+        """
+        Lint a single JavaScript context for JavaScript violations inside a Mako
+        template.
+
+        Arguments:
+            javascript_code: The template contents of the JavaScript context.
+            start_offset: The offset of the JavaScript context inside the
+                original Mako template.
+            results: A list of results into which violations will be added.
+
+        Side effect:
+            Adds JavaScript violations to results.
+
+        """
+        javascript_results = FileResults("")
+        self.javaScriptLinter.check_javascript_file_is_safe(javascript_code, javascript_results)
+        # translate the violations into the location within the original
+        # Mako template
+        for violation in javascript_results.violations:
+            expression = violation.expression
+            expression.start_index += start_offset
+            if expression.end_index is not None:
+                expression.end_index += start_offset
+            results.violations.append(ExpressionRuleViolation(violation.rule, expression))
+
+    def _check_deprecated_display_name(self, expression, results):
+        """
+        Checks that the deprecated display_name_with_default_escaped is not
+        used. Adds violation to results if there is a problem.
+
+        Arguments:
+            expression: An Expression
+            results: A list of results into which violations will be added.
+
+        """
+        if '.display_name_with_default_escaped' in expression.expression:
+            results.violations.append(ExpressionRuleViolation(
+                Rules.mako_deprecated_display_name, expression
+            ))
+
+    def _check_html_and_text(self, expression, has_page_default, results):
+        """
+        Checks rules related to proper use of HTML() and Text().
+
+        Arguments:
+            expression: A Mako Expression.
+            has_page_default: True if the page is marked as default, False
+                otherwise.
+            results: A list of results into which violations will be added.
+
+        """
+        expression_inner = expression.expression_inner
+        # use find to get the template relative inner expression start index
+        # due to possible skipped white space
+        template_inner_start_index = expression.start_index
+        template_inner_start_index += expression.expression.find(expression_inner)
         if 'HTML(' in expression_inner:
             if expression_inner.startswith('HTML('):
-                close_paren_index = self._find_closing_char_index(
-                    None, "(", ")", expression_inner, start_index=len('HTML('), num_open_chars=0, strings=[]
+                close_paren_index = _find_closing_char_index(
+                    None, "(", ")", expression_inner, start_index=len('HTML(')
                 )['close_char_index']
                 # check that the close paren is at the end of the stripped expression.
                 if close_paren_index != len(expression_inner) - 1:
@@ -821,13 +1681,11 @@ class MakoTemplateLinter(object):
                 ))
 
         # strings to be checked for HTML
-        unwrapped_html_strings = expression['strings']
+        unwrapped_html_strings = expression.strings
         for match in re.finditer(r"(HTML\(|Text\()", expression_inner):
-            result = self._find_closing_char_index(
-                None, "(", ")", expression_inner, start_index=match.end(), num_open_chars=0, strings=[]
-            )
-            close_paren_index = result['close_char_index']
-            if 0 <= close_paren_index:
+            result = _find_closing_char_index(None, "(", ")", expression_inner, start_index=match.end())
+            if result is not None:
+                close_paren_index = result['close_char_index']
                 # the argument sent to HTML() or Text()
                 argument = expression_inner[match.end():close_paren_index]
                 if ".format(" in argument:
@@ -865,8 +1723,7 @@ class MakoTemplateLinter(object):
 
         Arguments:
             mako_template: The contents of the Mako template.
-            expression: A dict containing the start_index, end_index, and
-                expression (text) of the expression.
+            expression: A Mako Expression.
             context: The context of the page in which the expression was found
                 (e.g. javascript, html).
             has_page_default: True if the page is marked as default, False
@@ -874,9 +1731,9 @@ class MakoTemplateLinter(object):
             results: A list of results into which violations will be added.
 
         """
-        # finds "| n, h}" when given "${x | n, h}"
-        filters_regex = re.compile('\|[a-zA-Z_,\s]*\}')
-        filters_match = filters_regex.search(expression['expression'])
+        # Example: finds "| n, h}" when given "${x | n, h}"
+        filters_regex = re.compile(r'\|([.,\w\s]*)\}')
+        filters_match = filters_regex.search(expression.expression)
         if filters_match is None:
             if context == 'javascript':
                 results.violations.append(ExpressionRuleViolation(
@@ -884,12 +1741,12 @@ class MakoTemplateLinter(object):
                 ))
             return
 
-        filters = filters_match.group()[1:-1].replace(" ", "").split(",")
-        if (len(filters) == 2) and (filters[0] == 'n') and (filters[1] == 'unicode'):
-            # {x | n, unicode} is valid in any context
+        filters = filters_match.group(1).replace(" ", "").split(",")
+        if filters == ['n', 'decode.utf8']:
+            # {x | n, decode.utf8} is valid in any context
             pass
         elif context == 'html':
-            if (len(filters) == 1) and (filters[0] == 'h'):
+            if filters == ['h']:
                 if has_page_default:
                     # suppress this violation if the page default hasn't been set,
                     # otherwise the template might get less safe
@@ -900,27 +1757,103 @@ class MakoTemplateLinter(object):
                 results.violations.append(ExpressionRuleViolation(
                     Rules.mako_invalid_html_filter, expression
                 ))
-
-        else:
-            if (len(filters) == 2) and (filters[0] == 'n') and (filters[1] == 'dump_js_escaped_json'):
+        elif context == 'javascript':
+            self._check_js_expression_not_with_html(mako_template, expression, results)
+            if filters == ['n', 'dump_js_escaped_json']:
                 # {x | n, dump_js_escaped_json} is valid
                 pass
-            elif (len(filters) == 2) and (filters[0] == 'n') and (filters[1] == 'js_escaped_string'):
+            elif filters == ['n', 'js_escaped_string']:
                 # {x | n, js_escaped_string} is valid, if surrounded by quotes
-                pass
+                self._check_js_string_expression_in_quotes(mako_template, expression, results)
             else:
                 results.violations.append(ExpressionRuleViolation(
                     Rules.mako_invalid_js_filter, expression
                 ))
 
+    def _check_js_string_expression_in_quotes(self, mako_template, expression, results):
+        """
+        Checks that a Mako expression using js_escaped_string is surrounded by
+        quotes.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            expression: A Mako Expression.
+            results: A list of results into which violations will be added.
+        """
+        parse_string = self._find_string_wrapping_expression(mako_template, expression)
+        if parse_string is None:
+            results.violations.append(ExpressionRuleViolation(
+                Rules.mako_js_missing_quotes, expression
+            ))
+
+    def _check_js_expression_not_with_html(self, mako_template, expression, results):
+        """
+        Checks that a Mako expression in a JavaScript context does not appear in
+        a string that also contains HTML.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            expression: A Mako Expression.
+            results: A list of results into which violations will be added.
+        """
+        parse_string = self._find_string_wrapping_expression(mako_template, expression)
+        if parse_string is not None and re.search('[<>]', parse_string.string) is not None:
+            results.violations.append(ExpressionRuleViolation(
+                Rules.mako_js_html_string, expression
+            ))
+
+    def _find_string_wrapping_expression(self, mako_template, expression):
+        """
+        Finds the string wrapping the Mako expression if there is one.
+
+        Arguments:
+            mako_template: The contents of the Mako template.
+            expression: A Mako Expression.
+
+        Returns:
+            ParseString representing a scrubbed version of the wrapped string,
+            where the Mako expression was replaced with "${...}", if a wrapped
+            string was found.  Otherwise, returns None if none found.
+        """
+        lines = StringLines(mako_template)
+        start_index = lines.index_to_line_start_index(expression.start_index)
+        if expression.end_index is not None:
+            end_index = lines.index_to_line_end_index(expression.end_index)
+        else:
+            return None
+        # scrub out the actual expression so any code inside the expression
+        # doesn't interfere with rules applied to the surrounding code (i.e.
+        # checking JavaScript).
+        scrubbed_lines = "".join((
+            mako_template[start_index:expression.start_index],
+            "${...}",
+            mako_template[expression.end_index:end_index]
+        ))
+        adjusted_start_index = expression.start_index - start_index
+        start_index = 0
+        while True:
+            parse_string = ParseString(scrubbed_lines, start_index, len(scrubbed_lines))
+            # check for validly parsed string
+            if 0 <= parse_string.start_index < parse_string.end_index:
+                # check if expression is contained in the given string
+                if parse_string.start_index < adjusted_start_index < parse_string.end_index:
+                    return parse_string
+                else:
+                    # move to check next string
+                    start_index = parse_string.end_index
+            else:
+                break
+        return None
+
     def _get_contexts(self, mako_template):
         """
         Returns a data structure that represents the indices at which the
         template changes from HTML context to JavaScript and back.
 
         Return:
-            A list of dicts where each dict contains the 'index' of the context
-            and the context 'type' (e.g. 'html' or 'javascript').
+            A list of dicts where each dict contains:
+                - index: the index of the context.
+                - type: the context type (e.g. 'html' or 'javascript').
         """
         contexts_re = re.compile(r"""
             <script.*?>|  # script tag start
@@ -930,6 +1863,7 @@ class MakoTemplateLinter(object):
         media_type_re = re.compile(r"""type=['"].*?['"]""", re.IGNORECASE)
 
         contexts = [{'index': 0, 'type': 'html'}]
+        javascript_types = ['text/javascript', 'text/ecmascript', 'application/ecmascript', 'application/javascript']
         for context in contexts_re.finditer(mako_template):
             match_string = context.group().lower()
             if match_string.startswith("<script"):
@@ -939,15 +1873,10 @@ class MakoTemplateLinter(object):
                     # get media type (e.g. get text/javascript from
                     # type="text/javascript")
                     match_type = match_type.group()[6:-1].lower()
-                    if match_type not in [
-                        'text/javascript',
-                        'text/ecmascript',
-                        'application/ecmascript',
-                        'application/javascript',
-                    ]:
-                        #TODO: What are other types found, and are these really
+                    if match_type not in javascript_types:
+                        # TODO: What are other types found, and are these really
                         # html?  Or do we need to properly handle unknown
-                        # contexts?
+                        # contexts? Only "text/template" is a known alternative.
                         context_type = 'html'
                 contexts.append({'index': context.end(), 'type': context_type})
             elif match_string.startswith("</script"):
@@ -989,14 +1918,7 @@ class MakoTemplateLinter(object):
             mako_template: The content of the Mako template.
 
         Returns:
-            A list of dicts for each expression, where the dict contains the
-            following:
-
-                start_index: The index of the start of the expression.
-                end_index: The index immediately following the expression, or -1
-                    if unparseable.
-                expression: The text of the expression.
-                strings: a list of ParseStrings
+            A list of Expressions.
 
         """
         start_delim = '${'
@@ -1008,249 +1930,28 @@ class MakoTemplateLinter(object):
             if start_index < 0:
                 break
 
-            result = self._find_closing_char_index(
-                start_delim, '{', '}', mako_template, start_index=start_index + len(start_delim),
-                num_open_chars=0, strings=[]
+            result = _find_closing_char_index(
+                start_delim, '{', '}', mako_template, start_index=start_index + len(start_delim)
             )
-            close_char_index = result['close_char_index']
-            if close_char_index < 0:
-                expression = None
+            if result is None:
+                expression = Expression(start_index)
+                # for parsing error, restart search right after the start of the
+                # current expression
+                start_index = start_index + len(start_delim)
             else:
+                close_char_index = result['close_char_index']
                 expression = mako_template[start_index:close_char_index + 1]
-
-            expression = {
-                'start_index': start_index,
-                'end_index': close_char_index + 1,
-                'expression': expression,
-                'strings': result['strings'],
-            }
-            expressions.append(expression)
-
-            # end_index of -1 represents a parsing error and we may find others
-            start_index = max(start_index + len(start_delim), close_char_index)
-
-        return expressions
-
-    def _find_closing_char_index(
-            self, start_delim, open_char, close_char, template, start_index, num_open_chars, strings
-    ):
-        """
-        Finds the index of the closing char that matches the opening char.
-
-        For example, this could be used to find the end of a Mako expression,
-        where the open and close characters would be '{' and '}'.
-
-        Arguments:
-            start_delim: If provided (e.g. '${' for Mako expressions), the
-                closing character must be found before the next start_delim.
-            open_char: The opening character to be matched (e.g '{')
-            close_char: The closing character to be matched (e.g '}')
-            template: The template to be searched.
-            start_index: The start index of the last open char.
-            num_open_chars: The current number of open chars.
-            strings: A list of ParseStrings already parsed
-
-        Returns:
-            A dict containing the following:
-                close_char_index: The index of the closing character, or -1 if
-                unparseable.
-                strings: a list of ParseStrings
-
-        """
-        unparseable_result = {'close_char_index': -1, 'strings': []}
-        close_char_index = template.find(close_char, start_index)
-        if close_char_index < 0:
-            # if we can't find an end_char, let's just quit
-            return unparseable_result
-        open_char_index = template.find(open_char, start_index, close_char_index)
-        parse_string = ParseString(template, start_index, close_char_index)
-
-        valid_index_list = [close_char_index]
-        if 0 <= open_char_index:
-            valid_index_list.append(open_char_index)
-        if 0 <= parse_string.start_index:
-            valid_index_list.append(parse_string.start_index)
-        min_valid_index = min(valid_index_list)
-
-        if parse_string.start_index == min_valid_index:
-            strings.append(parse_string)
-            if parse_string.end_index < 0:
-                return unparseable_result
-            else:
-                return self._find_closing_char_index(
-                    start_delim, open_char, close_char, template, start_index=parse_string.end_index,
-                    num_open_chars=num_open_chars, strings=strings
+                expression = Expression(
+                    start_index,
+                    end_index=close_char_index + 1,
+                    template=mako_template,
+                    start_delim=start_delim,
+                    end_delim='}',
+                    strings=result['strings'],
                 )
-
-        if open_char_index == min_valid_index:
-            if start_delim is not None:
-                # if we find another starting delim, consider this unparseable
-                start_delim_index = template.find(start_delim, start_index, close_char_index)
-                if 0 <= start_delim_index < open_char_index:
-                    return unparseable_result
-            return self._find_closing_char_index(
-                start_delim, open_char, close_char, template, start_index=open_char_index + 1,
-                num_open_chars=num_open_chars + 1, strings=strings
-            )
-
-        if num_open_chars == 0:
-            return {
-                'close_char_index': close_char_index,
-                'strings': strings,
-            }
-        else:
-            return self._find_closing_char_index(
-                start_delim, open_char, close_char, template, start_index=close_char_index + 1,
-                num_open_chars=num_open_chars - 1, strings=strings
-            )
-
-
-class UnderscoreTemplateLinter(object):
-    """
-    The linter for Underscore.js template files.
-    """
-
-    _skip_underscore_dirs = _skip_dirs + ('test',)
-
-    def process_file(self, directory, file_name):
-        """
-        Process file to determine if it is an Underscore template file and
-        if it is safe.
-
-        Arguments:
-            directory (string): The directory of the file to be checked
-            file_name (string): A filename for a potential underscore file
-
-        Returns:
-            The file results containing any violations.
-
-        """
-        full_path = os.path.normpath(directory + '/' + file_name)
-        results = FileResults(full_path)
-
-        if not self._is_valid_directory(directory):
-            return results
-
-        if not file_name.lower().endswith('.underscore'):
-            return results
-
-        return self._load_and_check_underscore_file_is_safe(full_path, results)
-
-    def _is_valid_directory(self, directory):
-        """
-        Determines if the provided directory is a directory that could contain
-        Underscore.js template files that need to be linted.
-
-        Arguments:
-            directory: The directory to be linted.
-
-        Returns:
-            True if this directory should be linted for Underscore.js template
-            violations and False otherwise.
-        """
-        if _is_skip_dir(self._skip_underscore_dirs, directory):
-            return False
-
-        return True
-
-    def _load_and_check_underscore_file_is_safe(self, file_full_path, results):
-        """
-        Loads the Underscore.js template file and checks if it is in violation.
-
-        Arguments:
-            file_full_path: The file to be loaded and linted
-
-        Returns:
-            The file results containing any violations.
-
-        """
-        underscore_template = _load_file(self, file_full_path)
-        self._check_underscore_file_is_safe(underscore_template, results)
-        return results
-
-    def _check_underscore_file_is_safe(self, underscore_template, results):
-        """
-        Checks for violations in an Underscore.js template.
-
-        Arguments:
-            underscore_template: The contents of the Underscore.js template.
-            results: A file results objects to which violations will be added.
-
-        """
-        self._check_underscore_expressions(underscore_template, results)
-        results.prepare_results(underscore_template)
-
-    def _check_underscore_expressions(self, underscore_template, results):
-        """
-        Searches for Underscore.js expressions that contain violations.
-
-        Arguments:
-            underscore_template: The contents of the Underscore.js template.
-            results: A list of results into which violations will be added.
-
-        """
-        expressions = self._find_unescaped_expressions(underscore_template)
-        for expression in expressions:
-            if not self._is_safe_unescaped_expression(expression):
-                results.violations.append(ExpressionRuleViolation(
-                    Rules.underscore_not_escaped, expression
-                ))
-
-    def _is_safe_unescaped_expression(self, expression):
-        """
-        Determines whether an expression is safely escaped, even though it is
-        using the expression syntax that doesn't itself escape (i.e. <%= ).
-
-        In some cases it is ok to not use the Underscore.js template escape
-        (i.e. <%- ) because the escaping is happening inside the expression.
-
-        Safe examples::
-
-            <%= HtmlUtils.ensureHtml(message) %>
-            <%= _.escape(message) %>
-
-        Arguments:
-            expression: The expression being checked.
-
-        Returns:
-            True if the expression has been safely escaped, and False otherwise.
-
-        """
-        if expression['expression_inner'].startswith('HtmlUtils.'):
-            return True
-        if expression['expression_inner'].startswith('_.escape('):
-            return True
-        return False
-
-    def _find_unescaped_expressions(self, underscore_template):
-        """
-        Returns a list of unsafe expressions.
-
-        At this time all expressions that are unescaped are considered unsafe.
-
-        Arguments:
-            underscore_template: The contents of the Underscore.js template.
-
-        Returns:
-            A list of dicts for each expression, where the dict contains the
-            following:
-
-                start_index: The index of the start of the expression.
-                end_index: The index of the end of the expression.
-                expression: The text of the expression.
-        """
-        unescaped_expression_regex = re.compile("<%=(.*?)%>", re.DOTALL)
-
-        expressions = []
-        for match in unescaped_expression_regex.finditer(underscore_template):
-            expression = {
-                'start_index': match.start(),
-                'end_index': match.end(),
-                'expression': match.group(),
-                'expression_inner': match.group(1).strip()
-            }
+                # restart search after the current expression
+                start_index = expression.end_index
             expressions.append(expression)
-
         return expressions
 
 
@@ -1265,12 +1966,17 @@ def _process_file(full_path, template_linters, options, out):
         options: A list of the options.
         out: output file
 
+    Returns:
+        The number of violations.
+
     """
+    num_violations = 0
     directory = os.path.dirname(full_path)
-    file = os.path.basename(full_path)
+    file_name = os.path.basename(full_path)
     for template_linter in template_linters:
-        results = template_linter.process_file(directory, file)
-        results.print_results(options, out)
+        results = template_linter.process_file(directory, file_name)
+        num_violations += results.print_results(options, out)
+    return num_violations
 
 
 def _process_current_walk(current_walk, template_linters, options, out):
@@ -1284,12 +1990,17 @@ def _process_current_walk(current_walk, template_linters, options, out):
         options: A list of the options.
         out: output file
 
+    Returns:
+        The number of violations.
+
     """
+    num_violations = 0
     walk_directory = os.path.normpath(current_walk[0])
     walk_files = current_walk[2]
     for walk_file in walk_files:
         full_path = os.path.join(walk_directory, walk_file)
-        _process_file(full_path, template_linters, options, out)
+        num_violations += _process_file(full_path, template_linters, options, out)
+    return num_violations
 
 
 def _process_os_walk(starting_dir, template_linters, options, out):
@@ -1302,29 +2013,14 @@ def _process_os_walk(starting_dir, template_linters, options, out):
         options: A list of the options.
         out: output file
 
-    """
-    for current_walk in os.walk(starting_dir):
-        _process_current_walk(current_walk, template_linters, options, out)
-
-
-def _parse_arg(arg, option):
-    """
-    Parses an argument searching for --[option]=[OPTION_VALUE]
-
-    Arguments:
-        arg: The system argument
-        option: The specific option to be searched for (e.g. "file")
-
     Returns:
-        The option value for a match, or None if arg is not for this option
+        The number of violations.
+
     """
-    if arg.startswith('--{}='.format(option)):
-        option_value = arg.split('=')[1]
-        if option_value.startswith("'") or option_value.startswith('"'):
-            option_value = option_value[1:-1]
-        return option_value
-    else:
-        return None
+    num_violations = 0
+    for current_walk in os.walk(starting_dir):
+        num_violations += _process_current_walk(current_walk, template_linters, options, out)
+    return num_violations
 
 
 def main():
@@ -1352,15 +2048,20 @@ def main():
         'is_quiet': args.quiet,
     }
 
-    template_linters = [MakoTemplateLinter(), UnderscoreTemplateLinter()]
+    template_linters = [MakoTemplateLinter(), UnderscoreTemplateLinter(), JavaScriptLinter()]
     if args.file is not None:
         if os.path.isfile(args.file[0]) is False:
             raise ValueError("File [{}] is not a valid file.".format(args.file[0]))
-        _process_file(args.file[0], template_linters, options, out=sys.stdout)
+        num_violations = _process_file(args.file[0], template_linters, options, out=sys.stdout)
     else:
         if os.path.exists(args.directory[0]) is False or os.path.isfile(args.directory[0]) is True:
             raise ValueError("Directory [{}] is not a valid directory.".format(args.directory[0]))
-        _process_os_walk(args.directory[0], template_linters, options, out=sys.stdout)
+        num_violations = _process_os_walk(args.directory[0], template_linters, options, out=sys.stdout)
+
+    if options['is_quiet'] is False:
+        # matches output of jshint for simplicity
+        print("")
+        print("{} violations found".format(num_violations))
 
 
 if __name__ == "__main__":

scripts/tests/templates/test.coffee

+var x = "<string>" + message + "</strong>"
+var template = "<%= invalid %>"

scripts/tests/templates/test.js

+var message = "Rock & Roll";
+var x = "<string>" + message + "</strong>";
+var template = "<%= invalid %>";
+// quiet the linter
+alert(x);
+alert(template);

scripts/tests/templates/test.underscore

+<%= invalid %>

scripts/tests/test_safe_template_linter.py

+# -*- coding: utf-8 -*-
 """
 Tests for safe_template_linter.py
 """
@@ -9,16 +10,68 @@ import textwrap
 from unittest import TestCase
 
 from ..safe_template_linter import (
-    _process_os_walk, FileResults, MakoTemplateLinter, ParseString, UnderscoreTemplateLinter, Rules
+    _process_os_walk, FileResults, JavaScriptLinter, MakoTemplateLinter, ParseString, StringLines,
+    UnderscoreTemplateLinter, Rules
 )
 
 
+@ddt
+class TestStringLines(TestCase):
+    """
+    Test StringLines class.
+    """
+    @data(
+        {'string': 'test', 'index': 0, 'line_start_index': 0, 'line_end_index': 4},
+        {'string': 'test', 'index': 2, 'line_start_index': 0, 'line_end_index': 4},
+        {'string': 'test', 'index': 3, 'line_start_index': 0, 'line_end_index': 4},
+        {'string': '\ntest', 'index': 0, 'line_start_index': 0, 'line_end_index': 1},
+        {'string': '\ntest', 'index': 2, 'line_start_index': 1, 'line_end_index': 5},
+        {'string': '\ntest\n', 'index': 0, 'line_start_index': 0, 'line_end_index': 1},
+        {'string': '\ntest\n', 'index': 2, 'line_start_index': 1, 'line_end_index': 6},
+        {'string': '\ntest\n', 'index': 6, 'line_start_index': 6, 'line_end_index': 6},
+    )
+    def test_string_lines_start_end_index(self, data):
+        """
+        Test StringLines index_to_line_start_index and index_to_line_end_index.
+        """
+        lines = StringLines(data['string'])
+        self.assertEqual(lines.index_to_line_start_index(data['index']), data['line_start_index'])
+        self.assertEqual(lines.index_to_line_end_index(data['index']), data['line_end_index'])
+
+    @data(
+        {'string': 'test', 'line_number': 1, 'line': 'test'},
+        {'string': '\ntest', 'line_number': 1, 'line': ''},
+        {'string': '\ntest', 'line_number': 2, 'line': 'test'},
+        {'string': '\ntest\n', 'line_number': 1, 'line': ''},
+        {'string': '\ntest\n', 'line_number': 2, 'line': 'test'},
+        {'string': '\ntest\n', 'line_number': 3, 'line': ''},
+    )
+    def test_string_lines_start_end_index(self, data):
+        """
+        Test line_number_to_line.
+        """
+        lines = StringLines(data['string'])
+        self.assertEqual(lines.line_number_to_line(data['line_number']), data['line'])
+
+
+class TestLinter(TestCase):
+    """
+    Test Linter base class
+    """
+    def _validate_data_rule(self, data, results):
+        if data['rule'] is None:
+            self.assertEqual(len(results.violations), 0)
+        else:
+            self.assertEqual(len(results.violations), 1)
+            self.assertEqual(results.violations[0].rule, data['rule'])
+
+
 class TestSafeTemplateLinter(TestCase):
     """
     Test some top-level linter functions
     """
 
-    def test_process_os_walk_with_includes(self):
+    def test_process_os_walk(self):
         """
         Tests the top-level processing of template files, including Mako
         includes.
@@ -29,17 +82,25 @@ class TestSafeTemplateLinter(TestCase):
             'is_quiet': False,
         }
 
-        template_linters = [MakoTemplateLinter(), UnderscoreTemplateLinter()]
+        template_linters = [MakoTemplateLinter(), JavaScriptLinter(), UnderscoreTemplateLinter()]
 
-        with mock.patch.object(MakoTemplateLinter, '_is_valid_directory', return_value=True) as mock_is_valid_directory:
-            _process_os_walk('scripts/tests/templates', template_linters, options, out)
+        with mock.patch.object(MakoTemplateLinter, '_is_valid_directory', return_value=True):
+            with mock.patch.object(JavaScriptLinter, '_is_valid_directory', return_value=True):
+                with mock.patch.object(UnderscoreTemplateLinter, '_is_valid_directory', return_value=True):
+                    num_violations = _process_os_walk('scripts/tests/templates', template_linters, options, out)
 
         output = out.getvalue()
-        self.assertIsNotNone(re.search('test\.html.*mako-missing-default', out.getvalue()))
+        self.assertEqual(num_violations, 6)
+        self.assertIsNotNone(re.search('test\.html.*mako-missing-default', output))
+        self.assertIsNotNone(re.search('test\.coffee.*javascript-concat-html', output))
+        self.assertIsNotNone(re.search('test\.coffee.*underscore-not-escaped', output))
+        self.assertIsNotNone(re.search('test\.js.*javascript-concat-html', output))
+        self.assertIsNotNone(re.search('test\.js.*underscore-not-escaped', output))
+        self.assertIsNotNone(re.search('test\.underscore.*underscore-not-escaped', output))
 
 
 @ddt
-class TestMakoTemplateLinter(TestCase):
+class TestMakoTemplateLinter(TestLinter):
     """
     Test MakoTemplateLinter
     """
@@ -63,36 +124,40 @@ class TestMakoTemplateLinter(TestCase):
     @data(
         {
             'template': '\n <%page expression_filter="h"/>',
-            'violations': 0,
             'rule': None
         },
         {
             'template':
                 '\n <%page args="section_data" expression_filter="h" /> ',
-            'violations': 0,
             'rule': None
         },
         {
+            'template': '\n ## <%page expression_filter="h"/>',
+            'rule': Rules.mako_missing_default
+        },
+        {
             'template':
                 '\n <%page expression_filter="h" /> '
                 '\n <%page args="section_data"/>',
-            'violations': 1,
             'rule': Rules.mako_multiple_page_tags
         },
         {
+            'template':
+                '\n <%page expression_filter="h" /> '
+                '\n ## <%page args="section_data"/>',
+            'rule': None
+        },
+        {
             'template': '\n <%page args="section_data" /> ',
-            'violations': 1,
             'rule': Rules.mako_missing_default
         },
         {
             'template':
                 '\n <%page args="section_data"/> <some-other-tag expression_filter="h" /> ',
-            'violations': 1,
             'rule': Rules.mako_missing_default
         },
         {
             'template': '\n',
-            'violations': 1,
             'rule': Rules.mako_missing_default
         },
     )
@@ -105,16 +170,18 @@ class TestMakoTemplateLinter(TestCase):
 
         linter._check_mako_file_is_safe(data['template'], results)
 
-        self.assertEqual(len(results.violations), data['violations'])
-        if data['violations'] > 0:
+        num_violations = 0 if data['rule'] is None else 1
+        self.assertEqual(len(results.violations), num_violations)
+        if num_violations > 0:
             self.assertEqual(results.violations[0].rule, data['rule'])
 
     @data(
         {'expression': '${x}', 'rule': None},
         {'expression': '${{unbalanced}', 'rule': Rules.mako_unparseable_expression},
         {'expression': '${x | n}', 'rule': Rules.mako_invalid_html_filter},
-        {'expression': '${x | n, unicode}', 'rule': None},
+        {'expression': '${x | n, decode.utf8}', 'rule': None},
         {'expression': '${x | h}', 'rule': Rules.mako_unwanted_html_filter},
+        {'expression': '  ## ${commented_out | h}', 'rule': None},
         {'expression': '${x | n, dump_js_escaped_json}', 'rule': Rules.mako_invalid_html_filter},
     )
     def test_check_mako_expressions_in_html(self, data):
@@ -358,7 +425,7 @@ class TestMakoTemplateLinter(TestCase):
         {'expression': '${x | n}', 'rule': Rules.mako_invalid_js_filter},
         {'expression': '${x | h}', 'rule': Rules.mako_invalid_js_filter},
         {'expression': '${x | n, dump_js_escaped_json}', 'rule': None},
-        {'expression': '${x | n, unicode}', 'rule': None},
+        {'expression': '${x | n, decode.utf8}', 'rule': None},
     )
     def test_check_mako_expressions_in_javascript(self, data):
         """
@@ -381,7 +448,7 @@ class TestMakoTemplateLinter(TestCase):
 
     @data(
         {'expression': '${x}', 'rule': Rules.mako_invalid_js_filter},
-        {'expression': '${x | n, js_escaped_string}', 'rule': None},
+        {'expression': '"${x | n, js_escaped_string}"', 'rule': None},
     )
     def test_check_mako_expressions_in_require_js(self, data):
         """
@@ -458,6 +525,63 @@ class TestMakoTemplateLinter(TestCase):
         self.assertEqual(results.violations[3].rule, Rules.mako_invalid_js_filter)
         self.assertEqual(results.violations[4].rule, Rules.mako_unwanted_html_filter)
 
+    def test_check_mako_expressions_javascript_strings(self):
+        """
+        Test _check_mako_file_is_safe javascript string specific rules.
+        - mako_js_missing_quotes
+        - mako_js_html_string
+        """
+        linter = MakoTemplateLinter()
+        results = FileResults('')
+
+        mako_template = textwrap.dedent("""
+            <%page expression_filter="h"/>
+            <script type="text/javascript">
+                var valid1 = '${x | n, js_escaped_string} ${y | n, js_escaped_string}'
+                var valid2 = '${x | n, js_escaped_string} ${y | n, js_escaped_string}'
+                var valid3 = 'string' + ' ${x | n, js_escaped_string} '
+                var valid4 = "${Text(_('Some mixed text{begin_span}with html{end_span}')).format(
+                    begin_span=HTML('<span>'),
+                    end_span=HTML('</span>'),
+                ) | n, js_escaped_string}"
+                var valid5 = " " + "${Text(_('Please {link_start}send us e-mail{link_end}.')).format(
+                    link_start=HTML('<a href="#" id="feedback_email">'),
+                    link_end=HTML('</a>'),
+                ) | n, js_escaped_string}";
+                var invalid1 = ${x | n, js_escaped_string};
+                var invalid2 = '<strong>${x | n, js_escaped_string}</strong>'
+                var invalid3 = '<strong>${x | n, dump_js_escaped_json}</strong>'
+            </script>
+        """)
+
+        linter._check_mako_file_is_safe(mako_template, results)
+
+        self.assertEqual(len(results.violations), 3)
+        self.assertEqual(results.violations[0].rule, Rules.mako_js_missing_quotes)
+        self.assertEqual(results.violations[1].rule, Rules.mako_js_html_string)
+        self.assertEqual(results.violations[2].rule, Rules.mako_js_html_string)
+
+    def test_check_javascript_in_mako_javascript_context(self):
+        """
+        Test _check_mako_file_is_safe with JavaScript error in JavaScript
+        context.
+        """
+        linter = MakoTemplateLinter()
+        results = FileResults('')
+
+        mako_template = textwrap.dedent("""
+            <%page expression_filter="h"/>
+            <script type="text/javascript">
+                var message = '<p>' + msg + '</p>';
+            </script>
+        """)
+
+        linter._check_mako_file_is_safe(mako_template, results)
+
+        self.assertEqual(len(results.violations), 1)
+        self.assertEqual(results.violations[0].rule, Rules.javascript_concat_html)
+        self.assertEqual(results.violations[0].start_line, 4)
+
     @data(
         {'template': "\n${x | n}", 'parseable': True},
         {
@@ -516,10 +640,10 @@ class TestMakoTemplateLinter(TestCase):
         expressions = linter._find_mako_expressions(data['template'])
 
         self.assertEqual(len(expressions), 1)
-        start_index = expressions[0]['start_index']
-        end_index = expressions[0]['end_index']
+        start_index = expressions[0].start_index
+        end_index = expressions[0].end_index
         self.assertEqual(data['template'][start_index:end_index], data['template'].strip())
-        self.assertEqual(expressions[0]['expression'], data['template'].strip())
+        self.assertEqual(expressions[0].expression, data['template'].strip())
 
     @data(
         {'template': " ${{unparseable} ${}", 'start_index': 1},
@@ -533,8 +657,8 @@ class TestMakoTemplateLinter(TestCase):
 
         expressions = linter._find_mako_expressions(data['template'])
         self.assertTrue(2 <= len(expressions))
-        self.assertEqual(expressions[0]['start_index'], data['start_index'])
-        self.assertIsNone(expressions[0]['expression'])
+        self.assertEqual(expressions[0].start_index, data['start_index'])
+        self.assertIsNone(expressions[0].expression)
 
     @data(
         {
@@ -557,6 +681,10 @@ class TestMakoTemplateLinter(TestCase):
             'template': r""" ${" \" \\"} """,
             'result': {'start_index': 3, 'end_index': 11, 'quote_length': 1}
         },
+        {
+            'template': "${'broken string}",
+            'result': {'start_index': 2, 'end_index': None, 'quote_length': None}
+        },
     )
     def test_parse_string(self, data):
         """
@@ -572,28 +700,22 @@ class TestMakoTemplateLinter(TestCase):
         }
 
         self.assertDictEqual(string_dict, data['result'])
-        self.assertEqual(data['template'][parse_string.start_index:parse_string.end_index], parse_string.string)
-        start_index = parse_string.start_index + parse_string.quote_length
-        end_index = parse_string.end_index - parse_string.quote_length
-        self.assertEqual(data['template'][start_index:end_index], parse_string.string_inner)
-
-    def _validate_data_rule(self, data, results):
-        if data['rule'] is None:
-            self.assertEqual(len(results.violations), 0)
-        else:
-            self.assertEqual(len(results.violations), 1)
-            self.assertEqual(results.violations[0].rule, data['rule'])
+        if parse_string.end_index is not None:
+            self.assertEqual(data['template'][parse_string.start_index:parse_string.end_index], parse_string.string)
+            start_inner_index = parse_string.start_index + parse_string.quote_length
+            end_inner_index = parse_string.end_index - parse_string.quote_length
+            self.assertEqual(data['template'][start_inner_index:end_inner_index], parse_string.string_inner)
 
 
 @ddt
-class TestUnderscoreTemplateLinter(TestCase):
+class TestUnderscoreTemplateLinter(TestLinter):
     """
     Test UnderscoreTemplateLinter
     """
 
     def test_check_underscore_file_is_safe(self):
         """
-        Test _check_underscore_file_is_safe with safe template
+        Test check_underscore_file_is_safe with safe template
         """
         linter = UnderscoreTemplateLinter()
         results = FileResults('')
@@ -606,13 +728,13 @@ class TestUnderscoreTemplateLinter(TestCase):
             %>
         """)
 
-        linter._check_underscore_file_is_safe(template, results)
+        linter.check_underscore_file_is_safe(template, results)
 
         self.assertEqual(len(results.violations), 0)
 
     def test_check_underscore_file_is_not_safe(self):
         """
-        Test _check_underscore_file_is_safe with unsafe template
+        Test check_underscore_file_is_safe with unsafe template
         """
         linter = UnderscoreTemplateLinter()
         results = FileResults('')
@@ -625,7 +747,7 @@ class TestUnderscoreTemplateLinter(TestCase):
             %>
         """)
 
-        linter._check_underscore_file_is_safe(template, results)
+        linter.check_underscore_file_is_safe(template, results)
 
         self.assertEqual(len(results.violations), 2)
         self.assertEqual(results.violations[0].rule, Rules.underscore_not_escaped)
@@ -683,12 +805,12 @@ class TestUnderscoreTemplateLinter(TestCase):
     )
     def test_check_underscore_file_disable_rule(self, data):
         """
-        Test _check_underscore_file_is_safe with various disabled pragmas
+        Test check_underscore_file_is_safe with various disabled pragmas
         """
         linter = UnderscoreTemplateLinter()
         results = FileResults('')
 
-        linter._check_underscore_file_is_safe(data['template'], results)
+        linter.check_underscore_file_is_safe(data['template'], results)
 
         violation_count = len(data['is_disabled'])
         self.assertEqual(len(results.violations), violation_count)
@@ -697,7 +819,7 @@ class TestUnderscoreTemplateLinter(TestCase):
 
     def test_check_underscore_file_disables_one_violation(self):
         """
-        Test _check_underscore_file_is_safe with disabled before a line only
+        Test check_underscore_file_is_safe with disabled before a line only
         disables for the violation following
         """
         linter = UnderscoreTemplateLinter()
@@ -709,7 +831,7 @@ class TestUnderscoreTemplateLinter(TestCase):
             <%= message %>
         """)
 
-        linter._check_underscore_file_is_safe(template, results)
+        linter.check_underscore_file_is_safe(template, results)
 
         self.assertEqual(len(results.violations), 2)
         self.assertEqual(results.violations[0].is_disabled, True)
@@ -721,12 +843,212 @@ class TestUnderscoreTemplateLinter(TestCase):
     )
     def test_check_underscore_no_escape_allowed(self, data):
         """
-        Test _check_underscore_file_is_safe with expressions that are allowed
+        Test check_underscore_file_is_safe with expressions that are allowed
         without escaping because the internal calls properly escape.
         """
         linter = UnderscoreTemplateLinter()
         results = FileResults('')
 
-        linter._check_underscore_file_is_safe(data['template'], results)
+        linter.check_underscore_file_is_safe(data['template'], results)
 
         self.assertEqual(len(results.violations), 0)
+
+
+@ddt
+class TestJavaScriptLinter(TestLinter):
+    """
+    Test JavaScriptLinter
+    """
+    @data(
+        {'template': 'var m = "Plain text " + message + "plain text"', 'rule': None},
+        {'template': 'var m = "檌檒濦 " + message + "plain text"', 'rule': None},
+        {'template': 'var m = "<p>" + message + "</p>"', 'rule': Rules.javascript_concat_html},
+        {'template': '  // var m = "<p>" + commentedOutMessage + "</p>"', 'rule': None},
+        {'template': 'var m = " <p> " + message + " </p> "', 'rule': Rules.javascript_concat_html},
+        {'template': 'var m = " <p> " + message + " broken string', 'rule': Rules.javascript_concat_html},
+    )
+    def test_concat_with_html(self, data):
+        """
+        Test check_javascript_file_is_safe with concatenating strings and HTML
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': 'test.append( test.render().el )', 'rule': None},
+        {'template': 'test.append(test.render().el)', 'rule': None},
+        {'template': 'test.append(test.render().$el)', 'rule': None},
+        {'template': 'test.append(testEl)', 'rule': None},
+        {'template': 'test.append($test)', 'rule': None},
+        # plain text is ok because any & will be escaped, and it stops false
+        # negatives on some other objects with an append() method
+        {'template': 'test.append("plain text")', 'rule': None},
+        {'template': 'test.append("<div/>")', 'rule': Rules.javascript_jquery_append},
+        {'template': 'graph.svg.append("g")', 'rule': None},
+        {'template': 'test.append( $( "<div>" ) )', 'rule': None},
+        {'template': 'test.append($("<div>"))', 'rule': None},
+        {'template': 'test.append($("<div/>"))', 'rule': None},
+        {'template': 'test.append(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'HtmlUtils.append($el, someHtml)', 'rule': None},
+        {'template': 'test.append("fail on concat" + test.render().el)', 'rule': Rules.javascript_jquery_append},
+        {'template': 'test.append("fail on concat" + testEl)', 'rule': Rules.javascript_jquery_append},
+        {'template': 'test.append(message)', 'rule': Rules.javascript_jquery_append},
+    )
+    def test_jquery_append(self, data):
+        """
+        Test check_javascript_file_is_safe with JQuery append()
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': 'test.prepend( test.render().el )', 'rule': None},
+        {'template': 'test.prepend(test.render().el)', 'rule': None},
+        {'template': 'test.prepend(test.render().$el)', 'rule': None},
+        {'template': 'test.prepend(testEl)', 'rule': None},
+        {'template': 'test.prepend($test)', 'rule': None},
+        {'template': 'test.prepend("text")', 'rule': None},
+        {'template': 'test.prepend( $( "<div>" ) )', 'rule': None},
+        {'template': 'test.prepend($("<div>"))', 'rule': None},
+        {'template': 'test.prepend($("<div/>"))', 'rule': None},
+        {'template': 'test.prepend(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'HtmlUtils.prepend($el, someHtml)', 'rule': None},
+        {'template': 'test.prepend("broken string)', 'rule': Rules.javascript_jquery_prepend},
+        {'template': 'test.prepend("fail on concat" + test.render().el)', 'rule': Rules.javascript_jquery_prepend},
+        {'template': 'test.prepend("fail on concat" + testEl)', 'rule': Rules.javascript_jquery_prepend},
+        {'template': 'test.prepend(message)', 'rule': Rules.javascript_jquery_prepend},
+    )
+    def test_jquery_prepend(self, data):
+        """
+        Test check_javascript_file_is_safe with JQuery prepend()
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': 'test.unwrap(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.wrap(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.wrapAll(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.wrapInner(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.after(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.before(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.replaceAll(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.replaceWith(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'test.replaceWith(edx.HtmlUtils.HTML(htmlString).toString())', 'rule': None},
+        {'template': 'test.unwrap(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.wrap(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.wrapAll(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.wrapInner(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.after(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.before(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.replaceAll(anything)', 'rule': Rules.javascript_jquery_insertion},
+        {'template': 'test.replaceWith(anything)', 'rule': Rules.javascript_jquery_insertion},
+    )
+    def test_jquery_insertion(self, data):
+        """
+        Test check_javascript_file_is_safe with JQuery insertion functions
+        other than append(), prepend() and html() that take content as an
+        argument (e.g. before(), after()).
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': '  element.parentNode.appendTo(target);', 'rule': None},
+        {'template': '  test.render().el.appendTo(target);', 'rule': None},
+        {'template': '  test.render().$el.appendTo(target);', 'rule': None},
+        {'template': '  test.$element.appendTo(target);', 'rule': None},
+        {'template': '  test.testEl.appendTo(target);', 'rule': None},
+        {'template': '$element.appendTo(target);', 'rule': None},
+        {'template': 'el.appendTo(target);', 'rule': None},
+        {'template': 'testEl.appendTo(target);', 'rule': None},
+        {'template': 'testEl.prependTo(target);', 'rule': None},
+        {'template': 'testEl.insertAfter(target);', 'rule': None},
+        {'template': 'testEl.insertBefore(target);', 'rule': None},
+        {'template': 'anycall().appendTo(target)', 'rule': Rules.javascript_jquery_insert_into_target},
+        {'template': 'anything.appendTo(target)', 'rule': Rules.javascript_jquery_insert_into_target},
+        {'template': 'anything.prependTo(target)', 'rule': Rules.javascript_jquery_insert_into_target},
+        {'template': 'anything.insertAfter(target)', 'rule': Rules.javascript_jquery_insert_into_target},
+        {'template': 'anything.insertBefore(target)', 'rule': Rules.javascript_jquery_insert_into_target},
+    )
+    def test_jquery_insert_to_target(self, data):
+        """
+        Test check_javascript_file_is_safe with JQuery insert to target
+        functions that take a target as an argument, like appendTo() and
+        prependTo().
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': 'test.html()', 'rule': None},
+        {'template': 'test.html( )', 'rule': None},
+        {'template': "test.html( '' )", 'rule': None},
+        {'template': "test.html('')", 'rule': None},
+        {'template': 'test.html("")', 'rule': None},
+        {'template': 'test.html(HtmlUtils.ensureHtml(htmlSnippet).toString())', 'rule': None},
+        {'template': 'HtmlUtils.setHtml($el, someHtml)', 'rule': None},
+        {'template': 'test.html("any string")', 'rule': Rules.javascript_jquery_html},
+        {'template': 'test.html("broken string)', 'rule': Rules.javascript_jquery_html},
+        {'template': 'test.html("檌檒濦")', 'rule': Rules.javascript_jquery_html},
+        {'template': 'test.html(anything)', 'rule': Rules.javascript_jquery_html},
+    )
+    def test_jquery_html(self, data):
+        """
+        Test check_javascript_file_is_safe with JQuery html()
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': 'StringUtils.interpolate()', 'rule': None},
+        {'template': 'HtmlUtils.interpolateHtml()', 'rule': None},
+        {'template': 'interpolate(anything)', 'rule': Rules.javascript_interpolate},
+    )
+    def test_javascript_interpolate(self, data):
+        """
+        Test check_javascript_file_is_safe with interpolate()
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)
+
+    @data(
+        {'template': '_.escape(message)', 'rule': None},
+        {'template': 'anything.escape(message)', 'rule': Rules.javascript_escape},
+    )
+    def test_javascript_interpolate(self, data):
+        """
+        Test check_javascript_file_is_safe with interpolate()
+        """
+        linter = JavaScriptLinter()
+        results = FileResults('')
+
+        linter.check_javascript_file_is_safe(data['template'], results)
+
+        self._validate_data_rule(data, results)