Change value of element inside nested array, array created with Array().fill()

Question

Consider the code below. I want to change only the second elements value in the second list in matrix. The problem is, that somehow all elements are changed. How can this be?

let matrix = Array(2).fill(Array(2).fill(0));
// [[0, 0], [0, 0]]
matrix[1][1] = 8
console.log(matrix)
// [[0, 8], [0, 8]]

score 0 · Answer 1 · answered Nov 05 '21 at 23:17

0

This is because all elements inside matrix are pointing to or referencing the same list. By wanting to change one instance of the list, you change them all because you thereby change the list being pointed to. To avoid this, create an empty nested array using for-loops instead:

let matr = [];
for (let i = 0; i < 2; i++) {
  matr.push(Array(2).fill(0))
}
matr[1][1] = 8
console.log(matr)

answered Nov 05 '21 at 23:17

karl

71
1
8

No need to create a loop. `const matrix = Array.from({ length: 2 }, () => Array.from({ length: 2 }).fill(0));`. – Sebastian Simon Nov 05 '21 at 23:21
Thanks @SebastianSimon – karl Nov 05 '21 at 23:26

Change value of element inside nested array, array created with Array().fill()

1 Answers1