Cannot read property 'getContext' of null, using canvas

You don't have to include JQuery.

In the index.html:
<canvas id="canvas" width="640" height="480"></canvas><script src="javascript/game.js"> This should work without JQuery...

Edit: You should put the script tag IN the body tag...

I guess the problem is your js runs before the html is loaded.

If you are using jquery, you can use the document ready function to wrap your code:

$(function() {
    var Grid = function(width, height) {
        // codes...
    }
});

Or simply put your js after the <canvas>.

You should put javascript tag in your html file. because browser load your webpage according to html flow, you should put your javascript file<script src="javascript/game.js"> after the <canvas>element tag. otherwise,if you put your javascript in the header of html.Browser load script first but it doesn't find the canvas. So your canvas doesn't work.

Put your JavaScript code after your tag <canvas></canvas>