随机产生字符串（积累）

var chars='0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghijklmnopqrstuvwxyz'.split('');
var length = Math.floor(Math.random() * chars.length);
var str = '';

for (var i = 0; i < length; i++) {
    str += chars[Math.floor(Math.random() * chars.length)];
}