Flatten nested arrays using recursion in JavaScript

Question

I'm trying to flatten nested arrays while preserving the order, e.g. [[1, 2], 3, [4, [[5]]]] should be converted to [1, 2, 3, 4, 5].

I'm trying to use recursion in order to do so, but the code below does not work and I don't understand why. I know there are other methods to do it, but I'd like to know what's wrong with this.

function flatten (arr) {
  var newArr = [];
  for (var i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      flatten(arr);
    } else {
      newArr.push(arr[i]);
    }
  }
  return newArr;
}

flatten([[1, 2], 3, [4, [[5]]]]);

Thanks

Answer 1

When calling flatten recursively, you need to pass arr[i] to it and then concat the result with newArr. So replace this line:

flatten(arr);

with:

newArr = newArr.concat(flatten(arr[i]));

Answer 2

Here's a common pattern that I regularly use for flattening nested arrays, and which I find a bit more clean due to its functional programming nature:

var flatten = (arrayOfArrays) =>
    arrayOfArrays.reduce((flattened, item) =>
        flattened.concat(Array.isArray(item) ? flatten(item) : [item]), []);

Or for those who like a shorter, less readable version for code golf or so:

var flatten=a=>a.reduce((f,i)=>f.concat(Array.isArray(i)?flatten(i):[i]),[]);