Unexpected behavior using Array Map on an Array Initialized with Array Fill

Question

I\'ve ran into an issue with the new Array.prototype.fill method due to unexpected output when using it with Array.prototype.map. For example:

Answer 1

Your code is equivalent to:

let inner = Array(3).fill(0);
let M = Array(3).fill(inner);

When you pass inner to .fill(), it doesn't make copies of it, the M array contains 3 references to the same array. So anything you do to one element of M happens to them all.

You need to make new arrays for each element of M:

let M = [];
for (var i = 0; i < 3; i++) {
    M.push(Array(3).fill(0));
}