Returning an object but not using the new keyword, Javascript?

Question

I am trying to create an object that gets returned without the new keyword in javascript?

My code structure so far;

myLib.func = (function() {

    "use strict";

    function func() {

        this._init();
    };

    func.prototype._init = function() {

        this.someVar = 5;
    };

    Return func;

})();

This obviously only works when using the new keyword;

new myLib.func();

How can I make it so that I can just do;

var func = myLib.func();

But it would still return an object that is exactly the same as the first example?

What I have tried

myLib.func = (function() {

    "use strict";

    function func() {

        if (window === this) {
            return new myLib.func();
        } else {
            this._init();
        }
    };

    func.prototype._init = function() {

        this.someVar = 5;
    };

    Return func;

})();

This does not work I learned from an example on slide 25 of John Resig's tips on building a library, http://ejohn.org/blog/building-a-javascript-library/

I know there are already existing frameworks, but rolling my own will make me learn alot, and as you can see that isn't alot at the moment!

user2251919 · Accepted Answer · 2013-05-13 08:50:52Z

2

In strict mode, the this will be undefined by default. You can account for this feature by adding it to your condition:

    if (window === this || undefined === this) {
        return new myLib.func();
    } else {
        this._init();
    }

or by checking whether the current object is an instance of the constructor (using instanceof):

    if (this instanceof func) {
        this._init();
    } else {
        return new func();
    }

(PS. You've got a typo in your code; JavaScript is case-sensitive, so you should use return instead of Return at the end)

edited May 13, 2013 at 8:50

user2251919

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answered May 13, 2013 at 8:31

Rob W

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2 Comments

user2251919 Over a year ago

This is awesome! Thank you alot :) I think I will use the second example it is more concise and easier to read! Thanks for the typo tip

Rob W Over a year ago

@user2251919 One last thing, I would just write new func(); instead of new myLib.func();. By doing this, the code becomes more portable. For instance, if you decide to rename myLib, or re-assign myLib.func, then the code will still behave as expected.

chtenb · Accepted Answer · 2013-05-13 08:31:17Z

1

If myLib.func is your class name, you cannot call it like var func = myLib.func();. However, you could wrap this latter into another function, such that you get

var factory = function(){
    var func = myLib.func();
    return func;
}

answered May 13, 2013 at 8:31

chtenb

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Comments

Draykos · Accepted Answer · 2013-05-13 08:35:59Z

0

If you don't really need public / private methods, you can use object literal

var myLib = {}; 
myLib.func = {
     _init: function() {
         this.someVar = 5;
     },
     doSomething: function(a, b) {
        return a + b;
     }
};

This will be a singleton object, usually it's enough for javascript applciation. If you need many instances of an object, you'll be forced to put a "new" keyword somewhere.

edited May 13, 2013 at 8:35

answered May 13, 2013 at 8:29

Draykos

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