js生成随机id方法

function randomRangeId(num){
var returnStr = "",
charStr = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
for(var i=0; i<num; i++){
var index = Math.round(Math.random() * (charStr.length-1));
returnStr += charStr.substring(index,index+1);
}
return returnStr;
}