Function flattens an array of arbitrarily nested arrays of integers into a flat array of integers

flattenArray.js

/**
 * Flattens an array of arbitrarily nested arrays of integers into a flat array of integers.
 * Run `jest ./flattenArray.test.js` for tests.
 *
 * @param {*} arr - nested array of integers
 * @returns flattened array
 */
function flattenArray(arr) {
  const result = [];
  arr.forEach(elem => {
    if (Array.isArray(elem)) {
      result.push(...flattenArray(elem));
    } else {
      result.push(elem);
    }
  });
  return result;
}

module.exports = flattenArray;

flattenArray.test.js

const flattenArray = require("./flattenArray");

describe("flattenArray", () => {
  it("should map flat array to flat array", () => {
    expect(flattenArray([])).toEqual([]);
    expect(flattenArray([1, 2, 3])).toEqual([1, 2, 3]);
  });
  it("should flatten nested array to flat array", () => {
    expect(flattenArray([1, 2, [3, 4, [5, 6, 7], 8, 9]])).toEqual([
      1,
      2,
      3,
      4,
      5,
      6,
      7,
      8,
      9
    ]);
    expect(flattenArray([1, 2, [], 3, [4, [], 5], 6])).toEqual([
      1,
      2,
      3,
      4,
      5,
      6
    ]);
  });
});

jest.config.js

module.exports = { verbose: true };