Array.prototype.flatMap ponyfill

flatMap.js

const flatMap = (xs, fn, thisArg) => {
  if (typeof Array.prototype.flatMap === 'function') {
    return Array.prototype.flatMap.call(xs, fn, thisArg);
  }

  const bFn = fn.bind(thisArg);
  return xs.reduce((acc, x) => {
    const r = bFn(x);
    if (Array.isArray(r)) return [...acc, ...r];
    return [...acc, r];
  }, []);
};

// Usage
flatMap([1,2,3], x => [x, x, x]); // [ 1, 1, 1, 2, 2, 2, 3, 3, 3 ]

flatMap([1,2,3], x => x); // [ 1, 2, 3 ]