Nested Arrays in Javascript have weird, undefined behavior

Question

When I make a nested array in Javascript using this:

let arr = Array(5).fill(Array(5).fill("O"))

It properly shows a 5x5 array of "O"

However, when I do:

arr[1][0] = "S"

It shows a result of:

["S", "O", "O", "O", "O"],
["S", "O", "O", "O", "O"],
["S", "O", "O", "O", "O"],
["S", "O", "O", "O", "O"],
["S", "O", "O", "O", "O"]

instead of just overwriting a single O.

`.fill` takes a single *value*, it doesn't call the methods over and over again. — jonrsharpe, Oct 15 '17 at 14:45
*"Nested Arrays in Javascript have weird, undefined behavior"* **Nothing** in JavaScript has undefined behavior. *Weird* sometimes, yes, but never *undefined*. :-) — T.J. Crowder, Oct 15 '17 at 14:46

score -2 · Accepted Answer · answered Oct 15 '17 at 14:46

-2

Array.fill returns the array, so the same array reference exists on each "row".

answered Oct 15 '17 at 14:46

Evan Trimboli

1 Answers1