How to return an object from an objects function - javascript?

Question

I have an object like below

 function obj() {

     this.cellSize = 50;

     this.createBlock = function() { // creates a block object
        this.x = 0;
        this.y = 0 - (this.cellSize * 1.5);
        this.baseVelocity = 1;
        this.velocity = this.baseVelocity;
        return this;
    };

    this.activeBlock = this.createBlock(); // active block object

    this.nextBlock = this.createBlock(); // next block object
 }

When I check obj.activeBlock I am not getting the object which should be returned from obj.createBlock?

Thanks,

Are you using new keyword? var a = new obj(); console.log(a.activeBlock);. — dfsq
– dfsq, Commented Feb 25, 2013 at 6:06
Yes I am but obj.activeBlock is the same as obj, rather than the object having what is outlined in createBlock? — ShaShads
– ShaShads, Commented Feb 25, 2013 at 6:10
"When I check obj.activeBlock..." Seems you're guessing at how this in JavaScript works. — the system
– the system, Commented Feb 25, 2013 at 6:11
@thesystem I am not guessing just trying to understand, I have only been learning for 2 days — ShaShads
– ShaShads, Commented Feb 25, 2013 at 6:14
@ShaShads: Alright, well there's no assignment of .activeBlock to obj. The obj is a function, and this isn't a reference to it. — the system
– the system, Commented Feb 25, 2013 at 6:16

Ted Hopp · Accepted Answer · 2013-02-25 06:09:34Z

2

You probably want something like:

function obj() {
     var that = this;
     this.cellSize = 50;

     this.createBlock = function() { // creates a block object
        this.x = 0;
        this.y = 0 - (that.cellSize * 1.5);
        this.baseVelocity = 1;
        this.velocity = this.baseVelocity;
        return this;
    };

    this.activeBlock = new this.createBlock(); // active block object

    this.nextBlock = new this.createBlock(); // next block object
}

The this in the createBlock function should be different from the this of obj(). You also need to create a new object using new for each block. If cellSize is supposed to be a constant, you can rewrite the code as a closure:

function obj() {
     var cellSize = 50;

     this.createBlock = function() { // creates a block object
        this.x = 0;
        this.y = 0 - (cellSize * 1.5);
        this.baseVelocity = 1;
        this.velocity = this.baseVelocity;
        return this;
    };

    this.activeBlock = new this.createBlock(); // active block object

    this.nextBlock = new this.createBlock(); // next block object
}

answered Feb 25, 2013 at 6:09

Ted Hopp

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3 Comments

ShaShads Over a year ago

Thank you this was exactly where I was going wrong, thanks for the closure tip :)

the system Over a year ago

@TedHopp: If OP did that, the function would no longer have access to the variable scope being used in your answer. And since .createBlock is invoked as a constructor, it also wouldn't have access to any properties of the object on which it was invoked.

Ted Hopp Over a year ago

@thesystem - Yeah, I just realized that. Best would be a factory function rather than a constructor. Deleting the comment.

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