index.ts

const LOWERCASE_ALPHA = "abcdefghijklmnopqrstuvwxyz",
  UPPERCASE_ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
  DIGITS = "0123456789",
  PUNCT1 = "!@#$%^&*()",
  PUNCT2 = "~`-_=+[]{}\\|;:'\",.<>?/";

// Calculate the size of the alphabet.
//
// This is a mostly back-of-the hand calculation of the alphabet.
// We group the a-z, A-Z and 0-9 together with the leftovers of the keys on an US keyboard.
// Characters outside ascii add one more to the alphabet. Meaning that the alphabet size of the word:
// "ümlout" will yield 27 characters. There is no scientific reasoning behind this, besides to
// err on the save side.
/**
 * @param {Str} str String to calculate the alphabet from
 * @returns {Number} n Size of the alphabet
 */
const alphabetSize = (str: string): number => {
  let c: string;
  let size = 0;

  const collect: Record<string, number> = {
    alcaps: 0,
    punct1: 0,
    digits: 0,
    alpha: 0,
    unicode: 0,
    size: 0,
  };

  let seen = "";

  for (var i = 0; i < str.length; i++) {
    c = str[i];

    // we only need to look at each character once
    if (str.indexOf(c) !== i) continue;
    if (LOWERCASE_ALPHA.indexOf(c) !== -1)
      collect.alpha = LOWERCASE_ALPHA.length;
    else if (UPPERCASE_ALPHA.indexOf(c) !== -1)
      collect.alcaps = UPPERCASE_ALPHA.length;
    else if (DIGITS.indexOf(c) !== -1) collect.digits = DIGITS.length;
    else if (PUNCT1.indexOf(c) !== -1) collect.punct1 = PUNCT1.length;
    else if (PUNCT2.indexOf(c) !== -1) collect.size = PUNCT2.length;
    // I can only guess the size of a non-western alphabet.
    // The choice here is to grant the size of the western alphabet, together
    // with an additional bonus for the character itself.
    //
    // Someone correct me if I'm wrong here.
    else if (c.charCodeAt(0) > 127) {
      collect.alpha = 26;
      collect.unicode += 1;
    }

    seen += c;
  }

  for (var k in collect) {
    size += collect[k];
  }

  return size;
};

// Calculate [information entropy](https://en.wikipedia.org/wiki/Password_strength#Entropy_as_a_measure_of_password_strength)
/**
 * @param {String} str String to calculate entropy for
 * @returns {Number} entropy
 */
export const entropy = (str: string): number => {
  if (!str) return 0;
  return Math.round(str.length * (Math.log(alphabetSize(str)) / Math.log(2)));
};