想试着做一个composite的演示，但是无法将<a>的文本赋到变量中，这是为什么呢？
这是html：
<!doctype html>
<html>
<head>
<meta charset="utf-8"></meta>
<title>合成属性演示</title>
<script type="text/javascript" src="合成.js"></script>
<style type="text/css">
#buttons{
width:600px;
font:14px 微软雅黑;
color: #0277bf;
}
#buttons a{
text-decoration: none;
}
</style>
</head>
<body>
<canvas id='canvas'>看到这段文字请更换浏览器</canvas>
<div id='buttons'>
<a href="#">source-over</a>
<a href="#">source-in</a>
<a href="#">source-out</a>
<a href="#">source-atop</a>
<a href="#">destination-over</a>
<a href="#">destination-in</a>
<a href="#">destination-out</a>
<a href="#">destination-atop</a>
<a href="#">lighter</a>
<a href="#">copy</a>
<a href="#">xor</a>
</div>
</body>
</html>这是js：
window.onload = function(){
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var buttons = document.getElementById('buttons');
var aS = buttons.getElementsByTagName('a');
var compositeType = 'source-out';
for(var i = 0;i<aS.length;i++){
aS[i].onclick = function(){
compositeType = this.text;
}
}
canvas.width = 600;
canvas.height = 400;
context.globalAlpha = 1;
context.globalCompositeOperation = compositeType;
context.beginPath();
context.rect(100,100,200,100);
context.fillStyle = 'red';
context.fill();
context.closePath();
context.beginPath();
context.arc(300,200,75,0,2*Math.PI);
context.fillStyle = 'blue';
context.fill();
context.closePath();
}
