Arbitrarily nested arrays of integers into a flat array of integers

nested_to_flatten.js

/*
Write some code, that will flatten an array of arbitrarily nested arrays
of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]
I use JS in ES6 Syntax
Tested in Chrome, Firefox and Safari
*/
let flatten = []
evaluate = (el=[0,[[1, 2],[3]],[4,[[5,6]]]])=>{
 el.map(n=>n instanceof Array ? evaluate(n) : flatten = [...flatten,n])
}
evaluate()
console.log('Flatten: ',flatten)//Flatten:  [ 1, 2, 3, 4, 5, 6 ]