Alternative to Array.prototype.reduce()

reduce-alternative.js

const arr = [1, [2], [3, [[4], [5]]]];
let x = [];

function recurse(arr) {
  let dest = [];

  if (Array.isArray(arr)) {
    arr.forEach(el => {
      dest = dest.concat(recurse(el));
    });
  } else {
    dest.push(arr);
  }

  return dest;
}

x = recurse(arr);
console.log(x);