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# The MathJax Object-Oriented Programming Model

MathJax uses an object-oriented programming model for its main components, such as the Input jax, Output jax, and Element jax. The model is intended to be light-weight and is based on JavaScript’s prototype inheritance mechanism. Object classes are created by making subclasses of MathJax.Object or one of its subclasses, and are instantiated by calling the object class as you would a function.

For example:

    MathJax.Object.Foo = MathJax.Object.Subclass({
      Init: function (x) {this.setX(x)},
      getX: function () {return this.x},
      setX: function (x) {this.x = x}
    });
    var foo = MathJax.Object.Foo("bar");
    foo.getX();                // returns "bar"
    foo.setX("foobar");
    foo.getX();                // returns "foobar"

Object classes can have static properties and methods, which are accessed via the object class variable. E.g., `MathJax.Object.Foo.SUPER` or `MathJax.Object.Foo.Augment()` for the object in the example above. Static values are not inherited by subclasses.

## Static Properties

`SUPER`

  Pointer to the super class for this subclass. (It is a reference to MathJax.Object in the example above.)

## Static Methods

`Subclass`(def[, static])

  Creates a subclass of the given class using the contents of the def object to define new methods and properties of the object class, and the contents of the optional static object to define new static methods and properties.

  Parameters : | def — object that defines the properties and methods static — object that defines static properties and methods
  Returns : | the new object class

`Augment`(def[, static])

  Adds new properties and methods to the class prototype. All instances of the object already in existence will receive the new properties and methods automatically.

  Parameters : | def — object that defines the properties and methods static — object that defines static properties and methods
  Returns : | the object class itself

## Properties

`constructor`

  Pointer to the constructor function for this class. E.g., `foo.constructor` would be a reference to `MathJax.Object.Foo` in the example above.

## Methods

`Init`([data])

  An optional function that is called when an instance of the class is created. When called, the this variable is set to the newly instantiated object, and the data is whatever was passed to the object constructor. For instance, in the example above, the variable `foo` is created by calling `MathJax.Object.Foo("bar")`, which calls the `MathJax.Object.Foo` object’s `Init()` method with data equal to `"bar"`. If desired, the `Init()` method can create a different object, and return that, in which case this becomes the return value for the object constructor.

  Parameters : | data — the data from the constructor call
  Returns : | `null` or the object to be returned by the constructor

`isa`(class)

  Returns `true` if the object is an instance of the given class, or of a subclass of the given class, and `false` otherwise. So using the `foo` value defined above,

      foo.isa(MathJax.Object);      // returns true
      foo.isa(MathJax.Object.Foo);  // returns true
      foo.isa(MathJax.InputJax);    // returns false

`can`(method)

  Checks if the object has the given method and returns `true` if so, otherwise returns `false`. This allows you to test if an object has a particular function available before trying to call it (i.e., if an object implements a particular feature). For example:

      foo.can("getX");  // returns true
      foo.can("bar");   // returns false

`has`(property)

  Checks if the object has the given property and returns `true` if so, otherwise returns `false`. This allows you to test if an object has a particular property available before trying to use it. For example:

      foo.has("getX");  // returns true
      foo.has("x");     // returns true
      foo.has("bar");   // returns false

## Accessing the Super Class

If a subclass overrides a method of its parent class, it may want to call the original function as part of its replacement method. The semantics for this are a bit awkward, but work efficiently. Within a method, the value `arguments.callee.SUPER` refers to the super class, so you can access any method of the superclass using that. In order to have this refer to the current object when you call the super class, however, you need to use `call()` or `apply()` to access the given method.

For example, `arguments.callee.SUPER.method.call(this,data)` would call the superclass’ method and pass it data as its argument, properly passing the current object as this. Alternatively, you can use `this.SUPER(arguments)` in place of `arguments.callee.SUPER`. It is also possible to refer to the super class explicitly rather than through `arguments.callee.SUPER`, as in the following example:

    MathJax.Class1 = MathJax.Object.Subclass({
      Init: function(x) {this.x = x},
      XandY: function(y) {return "Class1: x and y = " + this.x + " and " + y}
    });

    MathJax.Class2 = MathJax.Class1.Subclass({
      XandY: function (y) {return "Class2: "+arguments.callee.SUPER.XandY.call(this,y)}
    });

    MathJax.Class3 = MathJax.Class2.Subclass({
      XandY: function (y) {return "Class3: "+MathJax.Class2.prototype.XandY.call(this,y)}
    });

    MathJax.Class4 = MathJax.Class1.Subclass({
      XandY: function (y) {return "Class4: "+this.SUPER(arguments).XandY.call(this,y)}
    });

    var foo = MathJax.Class2("foo");
    foo.XandY("bar");   // returns "Class2: Class1: x and y = foo and bar"
    var bar = MathJax.Class3("bar");
    bar.XandY("foo");   // returns "Class3: Class2: Class1: x and y = bar and foo"
    var moo = MathJax.Class4("moo");
    moo.XandY("cow");   // returns "Class4: Class1: x and y = moo and cow"

Since both of these mechanisms are rather awkward, MathJax provides an alternative syntax that is easier on the programmer, but at the cost of some inefficiency in creating the subclass and in calling methods that access the super class.

Since most calls to the super class are to the overridden method, not to some other method, the method name and the `call()` are essentially redundant. You can get a more convenient syntax by wrapping the def for the `Subclass()` call in a call to `MathJax.Object.SimpleSUPER()`, as in the following example:

    MathJax.Class1 = MathJax.Object.Subclass({
      Init: function (x) {this.x = x},
      XandY: function (y) {return "Class1: x and y = " + this.x + " and " + y}
    });

    MathJax.Class2 = MathJax.Class1.Subclass(
      MathJax.Object.SimpleSUPER({
        XandY: function (y) {return "Class2: "+this.SUPER(y)},
        AnotherMethod: function () {return this.x}              // it's OK if a method doesn't use SUPER
      })
    );

    var foo = MathJax.Class2("foo");
    foo.XandY("bar");     // returns "Class2: Class1: x and y = foo and bar"

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