flatten.js

/* flatten an array of integers
 * @param arr {Array} - An array of integers to flatten
 * @return {Array} - A flattened array of integers
 */
function flatten(arr) {
  return arr.reduce((res, _arr) => {
    // typecast to array if not already (for next step)
    // otherwise, ensure provided array is flat
    _arr = Array.isArray(_arr) ? flatten(_arr) : [_arr];

    // use push (instead of concat) for memory conservation
    for (var i = 0; i < _arr.length; i++) {
      res.push(_arr[i]);
    }

    return res;
  }, []);
}

// quick assertion [turbo naive approach]
function flatEqualityCheck(a, b) {
  return JSON.stringify(a) === JSON.stringify(b) ? 'PASS' : 'FAIL';
}

const tests = [
  { input: [[1,2,[3]],4], expect: [1,2,3,4] },
  { input: [[1,2,[3],[[]]],4], expect: [1,2,3,4] },
  { input: [[1,2,[3],[['b']]],4], expect: [1,2,3,'b',4] }
];
tests.forEach(test => {
  const { input, expect } = test;
  const result = flatten(input);
  const equality = flatEqualityCheck(result, expect).toString().toUpperCase();
  console.log(equality, ':::', expect, '\t<===', input);
});