Weird Leetcode Bug: Array.fill() vs new Array.from()

Question

I was working on Leetcode Problem 1770 and ran into some weird behavior that I'm hoping someone can help me understand.

It's a dynamic programming problem that requires you to instantiate a 2-dimensional array of length m. Pretty straightforward.

What's weird is that my syntax for generating a 2D array causes the problem to fail, but an alternative syntax passes all tests. As far as I can tell the two lines output an identical result.

My syntax:

//  const m = 3

    let memo = new Array(m).fill(new Array(m).fill(-Infinity))
    console.log({memo})

//  stdout: {
//    memo: [
//      [ -Infinity, -Infinity, -Infinity ],
//      [ -Infinity, -Infinity, -Infinity ],
//      [ -Infinity, -Infinity, -Infinity ]
//    ]
//  }

Alternative syntax:

//  const m = 3

    let memo = Array.from(Array(m), () => Array(m).fill(-Infinity))
    console.log({memo})

//  stdout: {
//    memo: [
//      [ -Infinity, -Infinity, -Infinity ],
//      [ -Infinity, -Infinity, -Infinity ],
//      [ -Infinity, -Infinity, -Infinity ]
//    ]
//  }

To my eye the output of the two is exactly identical. Yet one passes all tests and what doesn't. What gives?

Guerric P · Accepted Answer · 2022-01-07T23:41:14.230

In the first case you are filling the array with the same Array instance. In the second case every row is a new Array because you provided a factory function.

const m = 3;

let memo = new Array(m).fill(new Array(m).fill(-Infinity));

console.log(memo[0] === memo[1]); // true

const m = 3

let memo = Array.from(Array(m), () => Array(m).fill(-Infinity));

console.log(memo[0] === memo[1]); // false

As a side note, you don't need to pass an Array as the first parameter of Array.from, an ArrayLike is enough:

const m = 3

let memo = Array.from({ length: m }, () => Array(m).fill(-Infinity));

console.log(memo);

Weird Leetcode Bug: Array.fill() vs new Array.from()

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