我收到错误消息Uncaught TypeError: Cannot read property 'getContext' of null，文件中的重要部分在…我想知道，因为game.js在下面的目录中，所以找不到画布？我该怎么办？

./index.html：

<canvas id="canvas" width="640" height="480"></canvas>

./javascript/game.js：

var Grid = function(width, height) {
        ...
        this.draw = function() {
        var canvas = document.getElementById("canvas");
        if(canvas.getContext) {
            var context = canvas.getContext("2d");
            for(var i = 0; i < width; i++) {
                for(var j = 0; j < height; j++) {
                    if(isLive(i, j)) {
                        context.fillStyle = "lightblue";
                    }
                    else {
                        context.fillStyle = "yellowgreen";
                    }
                    context.fillRect(i*15, j*15, 14, 14);
                }
            }
        }
    }
}