Function that takes an array of arbitrarily nested arrays and returns a flat array

flatten.js

// this function takes an array and return a flattened array
function flatten(array) {
	// Iterates over the given array to return just one flatten array
	return array.reduce((newArray, item) => {
  	// Check if the item to add is an array itself to call the function again to make the item a flatten array
  	if (item.constructor === Array) return newArray.concat(flatten(item));
    // If is not an array just adds the item to the new array
    return newArray.concat(item);
  }, []);
}

// Some tests
const a = flatten([1,2,[3,4,5],[6,7,[8,[9,10,11,12,13,14,[15,16,17,[18,19,[20]]]]]]]);
const b = flatten([[1,2,[3]],4]);
const c = flatten([["1","2",["3"]],"4"]);
const d = flatten([[{"1":1},{"2":2},[{"3":3}]],{"4":4}]);
const e = flatten([1,2,[3,4,5],[6,7,[8,[9,10,11,12,13,14,[15,16,17,[18,19,[20,[["1","2",["3"]],"4",[[{"1":1},{"2":2},[{"3":3}]],{"4":4}]]]]]]]]])

console.log(a);
console.log(b);
console.log(c);
console.log(d);
console.log(e);

//Copy and paste on some browsers console or <script> tag and see the results