Check for duplicate array values

unique.js

  // test if string contains any duplicate chars
  function isUniq(str) {
    let letters = {};
    str.split('').forEach(char => {
      if (letters[char]) {
        letters[char] = false;
      } else {
        letters[char] = true;
      }
    });

    // If the letters object has any false values, there's one or more duplicates
    return !Object.values(letters).includes(false);
  }