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]

var x=[1, 2, [3, 4, [5, 6,[7], 9],12, [12, 14]]];
var y=JSON.parse(`[${JSON.stringify(x).replace(/[|]/g,'')}]`);
console.log(y)

@rsantoshreddy interesting approach

@rsantoshreddy, yeah nice. But might need to escape those brackets ala (/\[|\]/g,'').

Here is another take on the ES6 version:

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