random_password_string.js

const random = (length, chars) => {
  let mask = "";
  if (chars.indexOf("a") > -1) mask += "abcdefghijklmnopqrstuvwxyz";
  if (chars.indexOf("A") > -1) mask += "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  if (chars.indexOf("#") > -1) mask += "0123456789";
  if (chars.indexOf("!") > -1) mask += "~`!@#$%^&*()_+-={}[]:\";'<>?,./|\\";

  let result = "";
  for (let i = length; i > 0; --i) {
    result += mask[Math.round(Math.random() * (mask.length - 1))];
  }
  return result;
};

random(20, "aA#!");
// TdIXwXCO\Q#Rse68H:"O

console.log(random(20, "aA#"));
// 20ZomWHbTkPvMjXfSxE9

random(20, "A#");
// 271GLIZB4DD4NU3KVZM3

random(20, "aA");
// BoIkVWcMVcCDCohUBcLc