JavaScript: The Definitive Guide, 6th Edition

Augmenting Classes

JavaScript’s prototype-based inheritance mechanism is dynamic: an object inherits properties from its prototype, even if the properties of the prototype change after the object is created. This means that we can augment JavaScript classes simply by adding new methods to their prototype objects. Here is code that adds a method for computing the complex conjugate to the Complex class of Example 9-3:

// Return a complex number that is the complex conjugate of this one.
Complex.prototype.conj = function() { return new Complex(this.r, -this.i); };

The prototype object of built-in JavaScript classes is also “open” like this, which means that we can add methods to numbers, strings, arrays, functions, and so on. We did this in Example 8-5 when we added a bind() method to the function class in ECMAScript 3 implementations where it did not already exist:

if (!Function.prototype.bind) {
    Function.prototype.bind = function(o /*, args */) {
       // Code for the bind method goes here...
    };
}

Here are some other examples:

// Invoke the function f this many times, passing the iteration number
// For example, to print "hello" 3 times:
//     var n = 3;
//     n.times(function(n) { console.log("hello"); });
Number.prototype.times = function(f, context) {
    var n = this.valueOf();
    for(var i = 0; i < n; i++) f.call(context, i);
};

// Define the ES5 String.trim() method if one does not already exist.
// This method returns a string with whitespace removed from the start and end.
String.prototype.trim ...