Flatten nested arrays to an array one level deep

flattenArray-Spec.js

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

describe('Flatten Nested Arrays', function () {
  it('should flatten a nested array', function () {
    var testArr = [[1,2,[3]],4];
    
    flattenArray(testArr).should.be.an.Array;
    flattenArray(testArr).should.equal([1,2,3,4]);
  });
  
  it('should return a blank array if non-array elements are passed in', function () {
    var testObj = {hello: 'world'};
    
    flattenArray(testObj).should.equal([]);
  });
});

flattenArray.js

module.exports = function flattenArray (arr, result) {
  result = result || [];
  
  for (var i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      flattenArray(arr[i], result);
    } else {
      result.push(arr[i]);
    }
  }
  
  return result;
};