Find the missing letter in range

missingLetter.js

const strings = ['abcd', 'cdefg', 'pqrs', 'abde'];

const missingLetter = string => {
  return [...string]
    .map(ltr => ltr.charCodeAt())
    .map((code, index, all) => {
      const prev = all[index - 1];
      const expected = code - 1;

      if (prev && prev !== expected) {
        return String.fromCharCode(expected);
      }
    })
    .filter(Boolean)
    .pop();
};

console.log(strings.map(missingLetter));