Added automatic insertion of missing "return" statement into simple function…

Added automatic insertion of missing "return" statement into simple function strings. Added an option to turn this feature off.

common/lib/xmodule/xmodule/js/src/graphical_slider_tool/graph.js

@@ -512,12 +512,13 @@ define('Graph', ['logme'], function (logme) {
                     obj['@label'],
                     obj['@point_size'],
                     obj['@fill_area'],
-                    obj['@bar']
+                    obj['@bar'],
+                    obj['@disable_auto_return']
                 );
             }
 
             function addFunction(funcString, color, line, dot, label,
-                                 pointSize, fillArea, bar) {
+                                 pointSize, fillArea, bar, disableAutoReturn) {
                 var newFunctionObject, func, paramNames;
 
                 // The main requirement is function string. Without it we can't
@@ -532,6 +533,30 @@ define('Graph', ['logme'], function (logme) {
                 // will break.
                 funcString = $('<div>').html(funcString).text();
 
+                // If the user did not specifically turn off this feature,
+                // check if the function string contains a 'return', and
+                // prepend a 'return ' to the string if one, or more, is not
+                // found.
+                if (
+                    (disableAutoReturn === undefined) ||
+                    (
+                        (typeof disableAutoReturn === 'string') &&
+                        (disableAutoReturn.toLowerCase() !== 'true')
+                    )
+                ) {
+                    if (funcString.search(/return/i) === -1) {
+                        funcString = 'return ' + funcString;
+                    }
+                } else {
+                    if (funcString.search(/return/i) === -1) {
+                        logme(
+                            'ERROR: You have specified a JavaScript ' +
+                            'function without a "return" statemnt. Your ' +
+                            'function will return "undefined" by default.'
+                        );
+                    }
+                }
+
                 // Some defaults. If no options are set for the graph, we will
                 // make sure that at least a line is drawn for a function.
                 newFunctionObject = {

docs/source/graphical_slider_tool.rst

@@ -266,6 +266,12 @@ Optional parameters::
             with id set in 'el_id' attribute.
     el_id:  Id of html element, defined in 'render' section. Value of
             function will be rendered to content of this element.
+    disable_auto_return: By default, if JavaScript function string is written
+                         without a "return" statement, one will be prepended
+                         to it. Set to "true" to disable this functionality.
+                         This is done so that simple functions can be defined
+                         in an easy fashion (for example, "a", which will be
+                         translated into "return a").
 
 With ``output`` and ``el_id`` set together you can update html elements with
 function value, also function will not be plotted.