getContext is not a function

Question

I'm working on making a basic web application using canvas that dynamically changes the canvas size when the window resizes. I've gotten it to work statically without any flaws but when I create an object to make it dynamically it throws an error

in chrome the error is:

Uncaught TypeError: Object [object Object] has no method 'getContext'

and in firefox it is:

this.element.getContext is not a function

I've searched the web and it seems like a common problem but none of the fixes mentioned make any difference.

The code is in question is as follows:

layer['background'] = new canvasLayer("body","background"); function canvasLayer(location,id){     $(location).append("<canvas id='"+id+"'>unsupported browser</canvas>");     this.element=$(id);     this.context = this.element.getContext("2d"); //this is the line that throws the error     this.width=$(window).width(); //change the canvas size     this.height=$(window).height();     $(id).width($(window).width()); //change the canvas tag size to maintain proper scale     $(id).height($(window).height()); } 

Thanks in advance.

Answer 1

Your value:

this.element = $(id); 

is a jQuery object, not a pure Canvas element.

To turn it back so you can call getContext(), call this.element.get(0), or better yet store the real element and not the jQuery object:

function canvasLayer(location, id) {      this.width = $(window).width();     this.height = $(window).height();     this.element = document.createElement('canvas');      $(this.element)        .attr('id', id)        .text('unsupported browser')        .attr('width', this.width)       // for pixels        .attr('height', this.height)        .width(this.width)               // for CSS scaling        .height(this.height)        .appendTo(location);      this.context = this.element.getContext("2d"); } 

See running code at http://jsfiddle.net/alnitak/zbaMh/, ideally using the Chrome Javascript Console so you can see the resulting object in the debug output.