Flat an array of arbitrarily nested arrays into a flat array (ES6)

flat-aray.js

function flatten(arr, accumulator = []) {
  if (arr.length === 0) {
    return accumulator;
  }

  const [head, ...tail] = arr;
  if (!Array.isArray(head)) {
    return flatten(tail, accumulator.concat(head));
  }

  return flatten(tail, accumulator.concat(flatten(head)));
}

export default function flatArray(arr) {
  if (!Array.isArray(arr)) {
    throw new Error('Argument Exception! Argument should be an array.');
  }

  return flatten(arr);
}

flat-array.test.js

import { expect } from "chai";
import flatArray from "../src/flat-array";

describe("Flat array tests", () => {
  describe("flatArray()", () => {
    it("Should flat array", () => {
      expect(flatArray([[1, 2, [3]], 4])).to.deep.equal([1, 2, 3, 4]);
    });

    it("Should flat array", () => {
      expect(flatArray([1, [2, 3, [4]]])).to.deep.equal([1, 2, 3, 4]);
    });

    it("Should throw if argument is not array", () => {
      expect(flatArray.bind({}, 'not array')).to.throw(Error);
    });
  });
});