flatten.js

// Try out by visiting: https://esnextb.in/?gist=e98c7a5295d3e940114ee87b7c273fe2


/*
 * Recursively flattens each item in the array.
 */
function flatten(array) {
  if (array.length === 0) return array;
  const [head, ...tail] = array;
  if (Array.isArray(head)) return flatten(head).concat(flatten(tail));
  // If the head is a single item, it just becomes the head of the result.
  return [head].concat(flatten(tail));
}

// TESTS

import test from 'tape';
import tapeDom from 'tape-dom'

// Show results in the DOM;
tapeDom(test);

test('empty array', function (t) {
  t.deepEqual(
    [], 
    flatten([]), 
    'an empty array should equal a flattened empty array',
  );
  t.end();
});

test('already flat array', function (t) {
  t.deepEqual(
    flatten([1, 2, 3]), 
    [1, 2, 3], 
    'a flat array should be the same after being flattened',
  );
  t.end();
});

test('nested array', function (t) {
  t.deepEqual(
    flatten([[[1], [[2, [3]]], 4, [[[5]]]]]), 
    [1, 2, 3, 4, 5], 
    'a nested array should be flattened',
  );
  t.end();
});