const LETTERS = 'qwertyuiopasdfghjklzxcvbnm';
const CAPITAL_LETTERS = LETTERS.toUpperCase();
const DIGITS = '1234567890';
const SYMBOLS = '!@#$%^&*()-_=+[]\\{}|;\':",./<>?`';

const generatePassword = ({ length, pool }) => Array.from({ length }).reduce(
  (password) => password + drawElement(drawPoolEntry(pool).characters),
  ''
);
const drawElement = (array) => array[drawIndex(array)];
const drawIndex = (array) => Math.floor(Math.random() * array.length);
const drawPoolEntry = (pool) => {
  const ranges = buildPoolRanges(pool);
  const random = Math.random();
  const rangeIndex = ranges.findIndex((range) => isValueInRange(random, range));
  return pool[rangeIndex];
};
const buildPoolRanges = (pool) => {
  const weightSum = pool.reduce((sum, { weight }) => sum + weight, 0);
  const ranges = [];
  let rangeStart = 0;
  pool.forEach(({ weight }) => {
    const rangeEnd = rangeStart + weight / weightSum;
    ranges.push([ rangeStart, rangeEnd ]);
    rangeStart = rangeEnd;
  });
  return ranges;
};
const isValueInRange = (value, [ rangeStart, rangeEnd ]) => rangeStart <= value && value < rangeEnd;

console.clear();
console.log(generatePassword({
  length: 16,
  pool: [
    {
      characters: LETTERS,
      weight: 1
    },
    {
      characters: CAPITAL_LETTERS,
      weight: 1
    },
    {
      characters: DIGITS,
      weight: 1
    },
    {
      characters: SYMBOLS,
      weight: 1
    }
  ]
}));