Javascript array flattening

flatten.js

/**
 * Flattens an array of arbitrarily nested arrays into
 * a flat array. e.g. [[1,2,[3]],4] -> [1,2,3,4]
 *
 * @param items - source array
 * @return - flat array
 */

function flatten(items) {
  const flat = [];

  items.forEach(item => {
    if (Array.isArray(item)) {
      flat.push(...flatten(item));
    } else {
      flat.push(item);
    }
  });

  return flat;
}

/** @TEST */

var arr = [[1,2,[3]],4];

console.log('before: ' + JSON.stringify(arr));
console.log('after: ' + JSON.stringify(flatten(arr)));