Some code, that will flatten an array of arbitrarily nested arrays

es6-flatten.js

// ECMAScript 6

const flatten = (arr) =>
  // Reducer concat array items to accumulated result
  // or recursively calls flatten again if the item is an array.
  arr.reduce((acc, item) =>
    acc.concat(Array.isArray(item) ? flatten(item) // recursively flatten nested array
                                   : item),        // concat ordinary item
    []); // init empty accumulator

console.log(flatten( [[1,2,[3]],4] ));