<html>
<head>
</head>
<body>
<button id="a1">点击</button>
<script>
function test(){
document.getElementById("a1").onclick=function(){alert('this is a1');};
//下面这种写法是不对的
//document.getElementById("a1").onclick="alert('this is a1');";
}
test();
</script>
</body>
</html>
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