Javascript variable scope and inheritance

Question

I'm stuck in some part of my code. I created something like model class BoardEx and I want to create Iterator for it, here's some example:

Game.BoardEx = function(params){
    ...
    this.board = Array(cols, rows);
    ...
}

where Game is the "parent" object of all children "classes". Then I declared Iterator:

Game.BoardEx.prototype.Iterator = function(){
    var pos = {x: 0, y: -1};
    this.next = function(){
        ...     
        return this.board[pos.x][pos.y];
    };

Later usage is:

var board = new Game.BoardEx();
var iter = new board.Iterator();

It seems that I can't call this.board in this scope. Should I use bind or other method or just can't I call this at all?

This looks like it should work, assuming you're doing new Iterator and copying the board over (or a reference to it). Remember, this.board belongs to the created Iterator now and not the BoardEx object. Also, you want a method like .iterate on the BoardEx instance that does return new Iterator(this) with the board passed so you can have access to it. — Benjamin Gruenbaum
– Benjamin Gruenbaum, Commented Jun 5, 2014 at 14:01
I use it later as: var board = new Game.BoardEx(); var iter = new board.Iterator(); — Selenir
– Selenir, Commented Jun 5, 2014 at 14:02
@user3664767 Ah, I understand your problem much better now, please edit that into the question — Benjamin Gruenbaum
– Benjamin Gruenbaum, Commented Jun 5, 2014 at 14:03

Benjamin Gruenbaum · Accepted Answer · 2014-06-05 14:05:42Z

2

You can't access this of the calling object when you're inside a new call.

Performing a new "construct" call changes the context to the newly created object.

Instead, you can use a builder function:

Game.BoardEx.prototype.getIterator = function(){
    function Iterator(board){
        this.board = board;
        this.pos = {x:0, y:-1};
    }
    Iterator.prototype.next = function(){
       return this.board[this.pos.x][this.pos.y];
       ...
    }
    return new Iterator(this.board); // create new instance with the board.
}

You can define Iterator completely externally to the Board, as long as the getIterator function passes a valid board to it. This would also let you call it from the outside and might be more effecient. It will also make instanceof work but IMO it's really not a big deal.

answered Jun 5, 2014 at 14:05

Benjamin Gruenbaum

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3 Comments

spender Over a year ago

I was just writing the same answer. +1 for GetIterator on the BoardEx instance. Definitely worth hiding the Interator constructor in closure, but maybe in the scope just above this function.

Selenir Over a year ago

Okay, it worked like magic, but can I modify this.board in this scope or just iterate through it? E.g var it = new board.getIterator(); it.setval(123);

Benjamin Gruenbaum Over a year ago

@user3664767 since board is an object, it is passed by reference value, you can change the contents of the board from the iterator, (but not swap the board altogether).

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