Find missing elements in sorted set of characters

main.js

/**
 * Write a function that takes an array of consecutive, increasing letters as
 * input, and returns any missing letters in the array between the first and
 * last letter.
 *
 * Example:
 * ```
 * > missingLetters(['a','b','c','d','f'])
 * > ['e']
 *
 * > missingLetters(['a','b','c','d','e','h','i','j','k','l','m','n','o','p','q','r','s','t','u','w','x','y','z'])
 * > ['f','g','v']
 * ```
 */

// constant of all letters
const LETTERS = 'abcdefghijklmnopqrstuvwxyz'.split('');

/**
 * returns an array with the letters missing from the array `a`
 * @param {Array<string>} a an array of letters, assumed to be sorted
 */
export function findMissing(a) {
  let chunk = new Set(LETTERS.slice(
    LETTERS.indexOf(a[0]),
    LETTERS.indexOf(a[a.length-1])+1
  ));

  for (const c of a) {
    chunk.delete(c)
  }

  return Array.from(chunk)
}