Recursive function javascript

recursive.function.js

const data = [1, [2, [3], 4], 5];

const sum = (data) => {
  return data.reduce((res, el) => {
    if (el.length) {
      return res + sum(el);
    }
    return res + el;
  }, 0);
}

alert(sum(data)); // 15
// https://jsfiddle.net/2ary59t0/146/