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Implementations

Although ECMAScript Edition 4 isn’t an officially released standard, that hasn’t stopped both Microsoft

and Mozilla from implementing some of it.

Mozilla

Mozilla chose to implement some small features in the Web browser. For instance, it is possible to define

constant values in Mozilla using the

const

keyword:

const message = “Hello World!”;

This line causes an error in all other browsers because

const

is a reserved word in ECMAScript Edition 3.

In Mozilla, however, this line defines a constant string named

message

that cannot have its value changed.

Mozilla also chose to implement getters and setters for object properties, albeit with a different syntax.

You may remember getters and setters from earlier chapters of this book, but here is a brief example:

function MyClass() {

this.__color__ = “red”;

}

MyClass.prototype.color getter = function () {

return this.__color__;

};

MyClass.prototype.color setter = function (value) {

this.__color__ = value;

alert(“Color changed to “ + value);

};

var obj = new MyClass();

alert(obj.color); //outputs “red”

obj.color = “blue”; //outputs “Color change to blue”

This is an alternate syntax (so as not to break syntax in non-Mozilla browsers):

function MyClass() {

this.__color__ = “red”;

}

MyClass.prototype.__defineGetter__(“color”, function () {

return this.__color__;

});

MyClass.prototype.__defineSetter__(“color”, function (value) {

this.__color__ = value;

alert(“Color changed to “ + value);

});

var obj = new MyClass();

alert(obj.color); //outputs “red”

obj.color = “blue”; //outputs “Color change to blue”

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