Learn !

 

1. function code (start, end, step = 1) {
2.   return new Array(Math.ceil((end.charCodeAt(0) - start.charCodeAt(0))/step))
3.   .fill(start.charCodeAt(0))
4.   .map((x, i) => String.fromCharCode(i*step + start.charCodeAt(0)));
5. }
6. console.log(code('a', 'z')); oh !
7. console.log(code('a', 'z', 3));
8. console.log(code('c', 'h', 2)); // oh ! 

function code(start, end, step = 1) {
  const res = [];
  const startCode = start.charCodeAt(0);
  const endCode = end.charCodeAt(0);

  for (let c = startCode; c <= endCode; c += step) {
    res.push(String.fromCharCode(c));
  }

  return res;
}

function code(start, end, step = 1) {
  const res = [];
  const startCode = start.charCodeAt(0);
  const endCode = end.charCodeAt(0);

  for (let c = startCode; c <= endCode; c += step) {
    res.push(String.fromCharCode(c));
  }

  return res;
}