Return undefined on empty nested arrays with flat

sir-haver 0
Viewed 43

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?

javascript
3 Answers

You have to use flatMap for this reason:

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

returns what you need

Anton Marinenko0 
arr=[[],[],[]]

console.log(arr.reduce((acc, val) => acc.concat(val==''?undefined:val),[]));

according to js MDN docs combination of reducer and concat simmilar to the exact Array.prototype.flat function. therefor altering it you can get your expected answer.the code for that is as above.

YJR0

If your array is n-level nested then you can create recursive function to achieve the desired result:

const flat = arr => arr.flatMap(o=> o?.length ? flat(o) : Array.isArray(o) ? undefined : o );

console.log(flat([[], [], []]));
console.log(flat([[undefined, 4], [4]]));
console.log(flat([[undefined, [[],[],[]], 4], [4,[]]]));

gorak0
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