大神们这段代码到底是怎么运算的？？？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>style样式</title>
<script type="text/javascript">
var numresult;
var str;
function onclicknum(nums) {
str = document.getElementById("nummessege");
str.value = str.value + nums;
}
</script>
</head>
<body>
<input type="text" id="nummessege" />
<input type="button" value="4" id="4" onclick="onclicknum(4)">
</body>
</html>
str = document.getElementById("nummessege");
str.value = str.value + nums; 这段代码有点不理解
