A simple way to flatten an array

flatten.js

const flatten = (arr) => arr.reduce(
    (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);

// es5 version
var flattenES5 = function flatten(arr) {
    return arr.reduce(function (a, b) {
        return a.concat(Array.isArray(b) ? flatten(b) : b);
    }, []);
};

export default flatten;

flatten.test.js

import flatten from './flatten.js'
const chai = require('chai');
const {expect} = chai;

describe("flatten()", function(){
  it("should throw an error if the given a non array as an argument", function(){
    expect(() => flatten(""))
    .to.throw(Error, "flatten needs an Array as an argument. `string` given instead")
  });
  it("should flatten an array of nested arrays", function(){
    expect(flatten([[1,2,[3]],4])).to.eql([1,2,3,4]);
    expect(flatten([1, ["string"], [], [{obj:[]}]])).to.eql([1,"string",{obj:[]}]);
  })
});