Return undefined on empty nested arrays with flat

I’m using flat() to count the number of empty or undefined items inside a nested array.

[[undefined, 4], [4]]

Using flat on this array will return 3 items:

1. undefined
2. 4
3. 4

However on this array:

[[], [], []]

using flat will return an empty array.

Is there a way to use it so it returns 3 undefined items?

>Solution :

You have to use flatMap for this reason:

[[], [], []].flatMap(item => item.length ? item : undefined)

returns what you need