Flatten array exercise (JS)

flatten.js

// See live demo: https://repl.it/DCWZ

// Flatten array exercise
// Hubert Orlik–Grzesik
// hubert@orlikarnia.com
// 2016C


// Of course, in real-life environment I would write:
//
// import _ from 'lodash';
// _.flattenDeep(array);
//


// We use a separate subroutine so that the main method
// takes only the expected arguments.


// Subroutine:
// Iterate through all the leaf elements
// and push them to the target array.
const flattenTo = (target, source) => {
  source.forEach(el => {
    if(Array.isArray(el))           // Found an array, iterate through children
      flattenTo(target, el);
    else                            // Not an array, push to target flat array
      target.push(el);
  });
  return target;                    // Return the target so that the main procedure
                                    // can be a one-liner :-)
};

// Main method:
// Push all the leaf elements to a new empty array.
const flatten = array => flattenTo([], array);


// Some examples
console.log(flatten([2,3,4,5,6,7,8,9]));
console.log(flatten([[2,3], [4,5], [6,7], [8,9]]));
console.log(flatten([[[[2,3]]], [4,[5]], [6,7, [[8],9]]]));
console.log(flatten([[[[2,[[[3]]]]]], [4,[5]], [[6,7], [[8],9]]]));