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

flatten.test.js

/**
 * Flatten an array of arbitrarily nested arrays of integers into a flat array of integers
 * @param {Array} arr The array of arbitrarily nested arrays of integers
 * @returns {Array[number]} Flat array of integers
 */
const flatten = (arr = []) =>
  arr.reduce((acc, val) => (Array.isArray(val) ? acc.concat(flatten(val)) : acc.concat(val)), []);

it("works with no input", () => {
  expect(flatten()).toEqual([]);
});

it("works with the example input", () => {
  expect(flatten([[1, 2, [3]], 4])).toEqual([1, 2, 3, 4]);
});

it("works with empty nested array", () => {
  expect(flatten([[1, 2, []], 4])).toEqual([1, 2, 4]);
});

it("works with multiple nested array", () => {
  expect(flatten([1, 2, [[[3]]]])).toEqual([1, 2, 3]);
});