//文件rand.js
function MyRand(min, max){
this.min = min;
this.max = max;
}
MyRand.prototype.getRand = function(){
//parseInt(this.max * random()) 取整,此值肯定小于给的最大值
var temp = parseInt(this.max * Math.random());
while(temp < this.min){
temp = parseInt(this.max * Math.random());
}
return temp;
};
//文件index.html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>index</title>
<meta name="author" content="Administrator" />
<script type="text/javascript" src="rand.js"></script>
<!-- Date: 2015-12-29 -->
<script type="text/javascript">
var rand = new MyRand(20, 30);
var res = rand.getRand();
alert(res);
rand = null;//及时释放类存
</script>
</head>
<body>
</body>
</html>
原文：http://www.cnblogs.com/yuwensong/p/5086588.html