randomPassword()

// ⭐️ 預設密碼長度：32
function randomPassword(length = 32, {
    digit      = true,      // 要有數字
    lowercase  = true,      // 要有小寫
    uppercase  = true,      // 要有大寫
    underscore = true,      // 可以有 "_"
    symbol     = true,      // 特殊符號
}={}){

    // 1. 字集
    let digits   = String.fromCharCodesBetween(48, 57);    // '0...9'
    let letters  = String.fromCharCodesBetween(97, 122);  // 'a...z'
    let capitals = String.fromCharCodesBetween(65, 90);   // 'A...Z'
    let symbols  = "!@#$%^&*";
    let underscores = "_";

    // 2. 準備組合密碼
    let pwd = '';                       // 密碼
    let candidates = '';                // 密碼候選字

    // 3. 從必用的字集中選字
    [
        [digit, digits],                // 數字
        [lowercase, letters],           // 小寫
        [uppercase, capitals],          // 大寫
        [underscore, underscores],      // 底線 "_"
        [symbol, symbols],              // 特殊符號

    ].forEach(([include, chars]) => {
        if(include){                    // 如果要加入 
            pwd += chars.random();      // 先加入一個字元
            candidates += chars;        // 整組字集加入候選字
        }
    });

    // 4. 如果密碼長度不足，就繼續選字
    let k = pwd.length;                 // 目前密碼長度
    let n = candidates.length;          // 選用字總長度

    while(k < length){
        pwd += candidates.random();
        k += 1;
    }

    return pwd.shuffle();
}

const { log } = console;
const { floor, random: rand } = Math;

/* --------------- helpers --------------- */

// ⭐️ random int: 0 ... n-1
//    randomInt(5): choose from 0, 1, 2, 3, 4
function randomInt(n){
    return floor( n * rand() );
}

// ⭐️ array.random()
Array.prototype.random = function(){
    return this[randomInt(this.length)];
};

// ⭐️ string.random()
// ⭐️ Note: this may not work well with Unicode❗️ 
String.prototype.random = function(){
    return this[randomInt(this.length)];
}

// ⭐️ string.shuffle()
String.prototype.shuffle = function(){

    var a = this.split("");
    var n = a.length;

    for(var i = n - 1; i > 0; i--) {
        var j = randomInt(i+1);
        [a[i], a[j]] = [a[j], a[i]];
    }

    return a.join("");
}

// ⭐️ String.fromCharCodesBetween(65, 70) -> 'ABCDEF'
String.fromCharCodesBetween = function(from, to){
    let array = [];
    for(let i = from; i <= to; i++) array.push(i);
    return array
        .map(code => String.fromCharCode(code))
        .join('');
};