Native flatten in ES5 and ES6

FlattenArray

   
//ES5 example
var flattenES5 = function (arr) {

  return [].concat.apply([], arr.map(function (element) {
    return Array.isArray(element) ? flatten(element) : element;
  }))
}

//ES6 example
let flattenES6 = arr => [].concat.apply([], arr.map(element => Array.isArray(element) ? flatten(element) : element))


//e.g. [[1,2,[3]],4] -> [1,2,3,4]. 
var arr = [[1,2,[3]],4];
flattenES5(arr);
flattenES6(arr)

//output
//[1,2,3,4]
//[1,2,3,4]

Here is another take on the ES6 version:

let flatten = array => Array.isArray(array)
  ? [].concat(...array.map(flatten))
  : array