JavaScript Interview Code Challenge Solution

findMissingLetters.js

// A "pangram" is a sentence that includes every letter of the alphabet.
// Write a function that will return what letters of the alphabet are
// missing from a sentence (thus meaning it is not a pangram).
//   "A quick brown fox jumps over the lazy dog" includes every letter, returning ""
//   "Lions, and tigers, and bears, oh my!" does not include every letter, returning "cfjkpquvwxz"

function findMissingLetters(str) {
  const alphabet = 'abcdefghijklmnopqrstuvwzyz';
  if (str.length) {
    const reducer = (memo, letter) => {
      if (!str.includes(letter) && !memo.includes(letter)) {
        return [...memo, letter];
      }
      return memo;
    };
    return alphabet
      .split('')
      .reduce(reducer, [])
      .join('');
  }
  return alphabet;
}

console.log(findMissingLetters('A quick brown fox jumps over the lazy dog')); // ==> ""
console.log(findMissingLetters('A slow yellow fox crawls under the proactive dog')); // ==> "bjkmqz"
console.log(findMissingLetters('Lions, and tigers, and bears, oh my!')); // ==> "cfjkpquvwxz"
console.log(findMissingLetters('')); // ==> "abcdefghijklmnopqrstuvwxyz"