Array Flattener in JS

arrayFlattener.js

var a = [[1,2,[3]],4, [5, [6, 7, [8, [9]]]]];

function flattenNestedArrays(arr){
  var flattenedArray = [];
  
  (function flattenArray(arr){
    arr.forEach(function(item){
      if(Array.isArray(item)){
        flattenArray(item);
      }else{
        flattenedArray.push(item);
      }
    });
  })(arr);
  return flattenedArray;
}

console.log(flattenNestedArrays(a)); // [1,2,3,4,5,6,7,8,9]

Or simpler:

function flatMapDeep(arr, flat = []){
    return arr.reduce((accum, el) => {
        if(Array.isArray(el)){
          return flatMapDeep(el, flat);
        } else {
          accum.push(el);
          return accum;
        }
    }, flat);
}

var b = [[2,3, [33]], 9, 8, [2,3]];

var flattened = flatMapDeep(b);

console.log('flattened:    ', flattened);