Question

I am learning Object Oriented Java Script. I have below code for Factory Method.

function Foo() {
    var value = 1;
    return {
        method: function() {
            return value;
        },
        value:value
    }
}
Foo.prototype = {
    bar: function() {}
};

new Foo();
Foo();

method Foo can be called by two ways. new Foo(); or Foo(); Both do the same thing and output is same. what is actual difference in java script processing ?

Answer 1

In normal cases, new should be used when creating objects from a constructor function and eschewed when performing any other function call. When using new on a function 1) a new object is created; 2) the function is called with the value of this bound to that new object, and 3) the value returned from the function (by default) is the object created in step one.

However, in your case you’re returning a completely new object from the “constructor” (different from the object in 1) above), which means there is no practical difference. The value of this will still be different inside the function, but both of these will return false:

new Foo() instanceof Foo;
Foo() instanceof Foo;

To illustrate the this difference, add the following to Foo:

alert(this instanceof Foo)

When called with new this alerts true; when called without it, false.

Furthermore, there’s no point in assigning an object to Foo.prototype because you’ll never create any instances of Foo that would make use of it (because, again, you’re returning something completely different from Foo).

If you’re going to return a custom object from Foo then you should prefer the version without new; there’s no point in creating a new instance of Foo if you’re going to ignore it and return something completely different, anyway.