I'm trying to create multiple instances of a JavaScript "class" or object but it seems to be acting as a singleton... Could someone assist me with the following code?

        (function() {
            /*
             * Canvas object
             */
            var Canvas = function( el ) {
                _self = this;
                _self.el = el;
            }
            Canvas.prototype.init = function() {
                _self.el.style.backgroundColor = "#000";
                _self.el.addEventListener( "mousedown", _self.onMouseDown );
            }
            Canvas.prototype.onMouseDown = function() {
                console.log( 'onMouseDown: ', _self.el );
            }

            var cOne = new Canvas( document.getElementById('one') );
            var cTwo = new Canvas( document.getElementById('two') );
            cOne.init();
            cTwo.init();
        })();

I'm trying to create multiple instances of a JavaScript "class" or object but it seems to be acting as a singleton... Could someone assist me with the following code?

        (function() {
            /*
             * Canvas object
             */
            var Canvas = function( el ) {
                _self = this;
                _self.el = el;
            }
            Canvas.prototype.init = function() {
                _self.el.style.backgroundColor = "#000";
                _self.el.addEventListener( "mousedown", _self.onMouseDown );
            }
            Canvas.prototype.onMouseDown = function() {
                console.log( 'onMouseDown: ', _self.el );
            }

            var cOne = new Canvas( document.getElementById('one') );
            var cTwo = new Canvas( document.getElementById('two') );
            cOne.init();
            cTwo.init();
        })();

• javascript
• object
• multiple-instances

Share Improve this question Follow asked Feb 7, 2013 at 1:41 user1960364user1960364 2,01966 gold badges2929 silver badges4848 bronze badges 2

• How is it acting as a singleton? May we see your HTML. Create a fiddle perhaps. – Aadit M Shah Commented Feb 7, 2013 at 1:45
• nothing obviously wrong. Please explain what behavior you're seeing thats incorrect. – Ben McCormick Commented Feb 7, 2013 at 1:45

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2 Answers 2

Sorted by: Reset to default 4

When not using var on a variable declaration, it bees a global variable. So when you create a new instance, it updates the global variable.

An alternative to this approach is to simple make a el an object property:

var Canvas = function(el) {
    this.el = el;
};

jsFiddle Demo

---

Moreover, you should consider binding your .onMouseDown method to the current object. Use this instead:

this.el.addEventListener(..., this.onMouseDown.bind(this));

or this:

Canvas.prototype.init = function() {
    var self = this;
    ...
    this.el.addEventListener(..., function() {
        self.onMouseDown.call(self);
    });
};

var Canvas = function( el ) {
   var _self = this;
    _self.el = el;
}
Canvas.prototype.init = function() {
    this.el.style.backgroundColor = "#000";
    this.el.addEventListener( "mousedown", this.onMouseDown.bind(this) );
}
Canvas.prototype.onMouseDown = function() {
    console.log( 'onMouseDown: ', this.el );
}

no var you make _self a globle val