Question

I am learning the Javascript tips from this http://code.google.com/p/jslibs/wiki/JavascriptTips site.

I came to see about the Factory method pattern. They shown the following example

Complex = new function() {

        function Complex(a, b) {
                // ...
        }

        this.fromCartesian = function(real, mag) {

                return new Complex(real, imag);
        }

        this.fromPolar = function(rho, theta) {

                return new Complex(rho * Math.cos(theta), rho * Math.sin(theta));
        }
}


var c = Complex.fromPolar(1, Math.pi); // Same as fromCartesian(-1, 0);

But I can’t guess why actually this is called as Factory method pattern. I want to know about this Factory method pattern and advantage of using this and especially to know where to exactly use this.

Answer 1

The Factory pattern may be described as a function that creates a class instance (like a real factory produces cars or toys, etc.) In your example, Complex is a factory that can create instances in two ways: cartesian or polar.

The Factory pattern is useful in a number of scenarios, including:

• the Factory maintains information needed (such as a sequence number, or usually something more complex) for creating instances
• the Factory maintains the instances over their lifetimes (such a pool of resources).