Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flatten.js

/**
 * Code was written in Javascript (ES6)
 * Tests will work only in Node.js environment
 */

/**
 * Flatten an array of arbitrarily nested arrays of integers into
 * a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].
 *
 * @param {Array} input
 */
function flatten(input) {
  if (!Array.isArray(input)) {
    throw TypeError(`An Array is expected, ${typeof input} was given.`);
  }

  let result = [];

  input.forEach(element => {
    result.push(...(Array.isArray(element) ? flatten(element) : [element]));
  });

  return result;
}

/** Tests */
const assert = require('assert');

assert.throws(() => flatten(), TypeError);
assert.throws(() => flatten(null), TypeError);
assert.throws(() => flatten(1), TypeError);
assert.throws(() => flatten('string'), TypeError);
assert.throws(() => flatten(false), TypeError);
assert.throws(() => flatten({}), TypeError);
assert.throws(() => flatten(NaN), TypeError);
assert.throws(() => flatten(Symbol()), TypeError);
assert.throws(() => flatten(() => {}), TypeError);

assert.deepEqual(flatten([]), []);
assert.deepEqual(flatten([1, 2, 3]), [1, 2, 3]);
assert.deepEqual(flatten([1, 2, [3]]), [1, 2, 3]);
assert.deepEqual(flatten([1, 2, [3, [4]]]), [1, 2, 3, 4]);
assert.deepEqual(flatten([[1, 2, [3]], 4]), [1, 2, 3, 4]);
assert.deepEqual(flatten([[1, 2, 3, 4]]), [1, 2, 3, 4]);
assert.deepEqual(flatten([[[[[1]]]]]), [1]);