javascript canvas

// javascript
/*global
alert, confirm, console, Debug, opera, prompt, WSH, strict
*/
var canvas = document.getElementById("canvas"),
    context = canvas.getContext("2d"),
    width = window.innerWidth,
    height = window.innerHeight,
    ratio = window.devicePixelRatio;
canvas.width  = width  * ratio;
canvas.height = height * ratio;
canvas.style.width  = width  + "px";
canvas.style.height = height + "px";
context.scale(ratio, ratio);


document.addEventListener('keydown', function (e) {

    "use strict";
    if (e.keyCode === 'D'.charCodeAt(0)) {
        alert("right");

    } else if (e.keyCode === 'A'.charCodeAt(0)) {
        alert("left");

    } else if (e.keyCode === 'W'.charCodeAt(0)) {
        alert("up");

    } else if (e.keyCode === 'S'.charCodeAt(0)) {
        alert("down");


    }
});

// html
<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8"/>
        <title>CHANGE HISTORY</title>
    </head>
    <body style="margin: 0px; overflow: hidden">

        <canvas id="canvas"></canvas>

        <script type="text/javascript" src="main%20(2).lnk"></script>

    </body>
</html>