jQuery使用HTML5 Canvas的方法

我使用的jQuery版本是2.1.1、網頁是HTML5,在取得canvas物件時,卻沒有反應。
原來在jQuery取得canvas物件需要有一點「眉角」

Javascript的語法如下:
    var canvas = document.getElementById('canvasTEST');
    var ctx = canvas.getContext('2d');
    ctx.beginPath();
    ctx.fillStyle = "#0000FF";
    ctx.fillRect(10,50,100,100);
canvas.addEventListener( "mousedown", mouseDown );
    function mouseDown() {
        ctx.beginPath();
        ctx.strokeStyle = "#00FF00";
        ctx.lineWidth = 10;
        ctx.moveTo(10,10);
   ctx.lineTo(50,200);
        ctx.stroke();
    }

JQuery的語法如下:
    var $canvas = $('#canvasTEST');
    var ctx = $canvas[0].getContext('2d');
    ctx.beginPath();
    ctx.fillStyle = "#0000FF";
    ctx.fillRect(10,50,100,100);
$canvas.mousedown(function (e) {
        ctx.beginPath();
        ctx.strokeStyle = "#00FF00";
        ctx.lineWidth = 10;
        ctx.moveTo(10,10);
   ctx.lineTo(50,200);
        ctx.stroke();
    });

兩者的差異在此:
var ctx = canvas.getContext('2d');
var ctx = $canvas[0].getContext('2d');

網路上有人解釋如下:
Because getContext('2d') works on DOM element,
where var 「$canvas = $('#canvas1');」return a jQuery object but node a DOM element.
And to retrieve a DOM element from jQuery object you need to use $canvas[0].

完整的HTML5語法如下:
<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8" />
        <title>TESTING</title>
    </head>

    <body>
   <canvas id="canvas1" width="300" height="300" style="border: 2px solid #999900" />
</body>
</html>

<script type="text/javascript" src="jquery-2.1.1.min.js"></script>
<script>
    var $canvas = $('#canvasTEST');
    var ctx = $canvas[0].getContext('2d');
    ctx.beginPath();
    ctx.fillStyle = "#0000FF";
    ctx.fillRect(10,50,100,100);
$canvas.mousedown(function (e) {
        ctx.beginPath();
        ctx.strokeStyle = "#00FF00";
        ctx.lineWidth = 10;
        ctx.moveTo(10,10);
   ctx.lineTo(50,200);
        ctx.stroke();
    });
</script>

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