Password generator for use in Browser devtools

password_gen.js

/**
 * KeePass-style password generator
 * Designed for browser usage (DevTools snippets)
 */

const charSets = {
  a: "abcdefghijklmnopqrstuvwxyz0123456789",
  A: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789",
  U: "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
  d: "0123456789",
  h: "0123456789abcdef",
  H: "0123456789ABCDEF",
  l: "abcdefghijklmnopqrstuvwxyz",
  L: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz",
  u: "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
  p: ".,:;",
  b: "()[]{}<>",
  s: "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
};

charSets.S = charSets.A + charSets.s;

const MAX_REPEAT = 1000;

/**
 * Secure random character (no modulo bias)
 */
function getRandomChar(set) {
  const arr = new Uint32Array(1);
  const max = 0xffffffff;
  const limit = max - (max % set.length);

  do {
    crypto.getRandomValues(arr);
  } while (arr[0] >= limit);

  return set[arr[0] % set.length];
}

/**
 * Generate password from pattern
 */
function generatePassword(pattern) {

  let result = "";

  for (let i = 0; i < pattern.length; i++) {

    let token = pattern[i];
    let isLiteral = false;

    // escape handling
    if (token === "\\" && i + 1 < pattern.length) {
      token = pattern[++i];
      isLiteral = true;
    }

    // repeat handling
    let repeat = 1;

    if (i + 1 < pattern.length && pattern[i + 1] === "{") {

      const end = pattern.indexOf("}", i + 2);

      if (end !== -1) {

        const num = parseInt(pattern.slice(i + 2, end), 10);

        if (!Number.isNaN(num)) {
          repeat = num;
          i = end;
        }

      }
    }

    if (repeat > MAX_REPEAT) {
      throw new Error("Repeat value too large");
    }

    const set = isLiteral ? null : charSets[token];

    if (!isLiteral && !set) {
      throw new Error(`Unknown token: ${token}`);
    }

    for (let r = 0; r < repeat; r++) {
      result += set ? getRandomChar(set) : token;
    }

  }

  return result;
}