Flatten multi-dimensional array. Run the tests here: http://codepen.io/dannycroft/full/wgrmQo/

flattenArray-test.js

var flattenArray = require('../flattenArray');

describe("flattenArray()", function(){
  it("should return an array", function() {
    var test = flattenArray([1,2,3]);

    expect(test).to.be.an('array');
    expect(test.length).to.equal(3);
  });
  
  it("should throw if passed a non array", function() {
    expect(function() { flattenArray("array"); }).to.throw(TypeError);
    expect(function() { flattenArray(1); }).to.throw(TypeError);
    expect(function() { flattenArray(true); }).to.throw(TypeError);
    expect(function() { flattenArray(false); }).to.throw(TypeError);
    expect(function() { flattenArray(Array); }).to.throw(TypeError);
  });
  
  it("should flatten a multi-dimensional array", function() {
    var array1 = [[1,2,[3]],4];
    var array2 = [[1, 2],[3, 4, 5], [6, 7, 8, 9]];
    var array3 = [1, [2,3], [4,[5,[6]]]];

    expect(flattenArray(array1)).to.deep.equal([1, 2, 3, 4]);
    expect(flattenArray(array2)).to.deep.equal([1, 2, 3, 4, 5, 6, 7, 8, 9]);
    expect(flattenArray(array3)).to.deep.equal([1, 2, 3, 4, 5, 6]);
  });
  
  it("should return the same item type as the origin array", function() {
    var array1 = [[1,2,[3]],4];
    var array2 = [['1', 2],[true, 4, 5], [6, 7, 8, '9']];
    var array3 = ['1', ['2',3], ['4',[5,[false]]]];

    expect(flattenArray(array1)).to.deep.equal([1, 2, 3, 4]);
    expect(flattenArray(array2)).to.deep.equal(['1', 2, true, 4, 5, 6, 7, 8, '9']);
    expect(flattenArray(array3)).to.deep.equal(['1', '2', 3, '4', 5, false]);
  });
  it("should handle deeply nested arrays", function() {
    var array = [[[[[[[[[[[[[[[['hello']]]]]], 'world']]]]]]]]]];

    expect(flattenArray(array).join(' ')).to.equal('hello world');
  });
});

flattenArray.js

'use strict';

module.exports = flattenArray;

/**
 * Flatten multi-dimensional array.
 *
 * @param  {Array} arr
 * @return {Array}
 */
function flattenArray(arr) {
  if (!Array.isArray(arr)) {
    throw new TypeError('Expected value to be an array');
  }
  return arr.reduce((accumulator, item) => {
    return Array.isArray(item)

      // item is array so flatten before adding to accumulator
      ? accumulator = accumulator.concat(flattenArray(item))

      // item not array so add into accumulator
      : accumulator.push(item) && accumulator;
  }, []);
}