Javascript nested loop not returns outer loop's index as expected

Question

const a = 3
const b = 4

const arr = new Array(a).fill([]);

for (let i = 0; i < a; i++) {
  for (let j = 0; j < b; j++) {
    arr[i][j] = i;
  }
}

console.log(JSON.stringify(arr))

I would expect to return

[[0,0,0,0],[1,1,1,1],[2,2,2,2]]

but it returns

[[2,2,2,2],[2,2,2,2],[2,2,2,2]]

I think this is not because of the closure things but please let me know what is the problem of this code not working as expected.

* edited -> sorry, I forgot to paste initializing arr code *

@Phil Oh I missed it. Would you check it again? – Dong Gyu Lee Jun 11 '20 at 00:25 — Dong Gyu Lee, Jun 11 '20 at 00:25

djcaesar9114 · Accepted Answer · 2020-06-11T00:28:30.963

3

Don't assign a value directly in a 2-dimension array. Initialize the first dimension first.

var arr = []
const a = 3
const b = 4

for(let i=0; i<a; i++){
  arr.push([])
  for(let j=0; j<b; j++){
      arr[i][j] = i;
  }
}

console.log(arr)

Don't initialize your array like this:

const arr = new Array(a).fill([]);

By doing this, you pass references and not values, as you could do with integers. The difference between references and values is important.

edited Jun 11 '20 at 00:28

answered Jun 11 '20 at 00:11

djcaesar9114

1,880
1
21
41

I do edit my code. can you check it again? Your code work properly but I cannot understand why my code is still returns the unexpected result.. – Dong Gyu Lee Jun 11 '20 at 00:25
I explained the mistake you made. – djcaesar9114 Jun 11 '20 at 00:29
Checked! really appreciate it :) – Dong Gyu Lee Jun 11 '20 at 00:35

score 0 · Answer 2 · answered Jun 11 '20 at 00:48

0

I found and post another question about Array.fill() for the other people having the same issue.

Array.fill(Array) creates copies by references not by value

answered Jun 11 '20 at 00:48

Dong Gyu Lee

31
5

Javascript nested loop not returns outer loop's index as expected

2 Answers2