随机字符串

function randomString(length) {
    let str = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    let result = '';
    for (let i = length; i > 0; --i) 
        result += str[Math.floor(Math.random() * str.length)];
    return result;
}