javascript random string generator with options

namer.js

export const namer = ({
  length = 5,
  useDigit = true,
  useAlpha = true,
  upperOnly = false,
  lowerOnly = false,
}) => {
  let str = [];

  let stock = [];
  if (useDigit) {
    stock = stock.concat(
      Array(10)
        .fill(0)
        .map((_, j) => j),
    );
  }

  if (useAlpha) {
    if (upperOnly) {
      stock = stock.concat(
        Array(26)
          .fill(0)
          .map((_, j) => String.fromCharCode(j + 65)),
      );
    } else if (lowerOnly) {
      stock = stock.concat(
        Array(26)
          .fill(0)
          .map((_, j) => String.fromCharCode(j + 97)),
      );
    } else {
      stock = stock.concat(
        Array(26)
          .fill(0)
          .map((_, j) => String.fromCharCode(j + 65)),
      );
      stock = stock.concat(
        Array(26)
          .fill(0)
          .map((_, j) => String.fromCharCode(j + 97)),
      );
    }
  }

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

  return str.join('');
};