hyphenize.js

function hyphenize(str) {
  // /y is to make sure that there are no characters between the matches
  // /g is so that .match() works the way we need here
  // /u is not strictly necessary here, but generally a good habit
  return str.match(/([a-z]{1,2})/uyg).join('-');
}

assert.equal(
  hyphenize('hello'), 'he-ll-o'
);
assert.equal(
  hyphenize('abcdefghijkl'), 'ab-cd-ef-gh-ij-kl'
);


// A simple `for` loop works quite well, too

function hyphenize2(str) {
  let result = '';
  for (let index = 0; index < str.length; index += 2) {
    if (index > 0) {
      result += '-';
    }
    // .slice() doesn’t mind if the second argument is too large
    result += str.slice(index, index+2);
  }
  return result;
}

@WebReflection Good point. Others did this on Twitter, too. I changed it.