You are creating an infinite loop here because of

for (let j = 0; j < arr[i].length; j *= product);

Here, j is always 0.

If you want to multiply all the nested value then you should do as:

function multiplyAll(arr) {
  let product = 1;
  for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr[i].length; ++j)
      product *= arr[i][j];
  }
  return product;
}

console.log(multiplyAll([[1], [2], [3]]));

If you just want to multiple all the nested value then you can simply do as:

function multiplyAll(arr) {
  return arr.flat().reduce((acc, curr) => acc * curr, 1);
  // If you want to get numbers at any depth then you can flat at any level
  // using Infinity as
  // return arr.flat(Infinity).reduce((acc, curr) => acc * curr, 1);
}

console.log(multiplyAll([[1], [2], [3]]));

Well, it looks like you have multiple issues, two of them are already as following:

1. In line 2 you wrote once Let with a big letter at the beginning.
2. Your second array needs an {} otherwise it will error out
3. Can you please explain to me j*= product? Can you maybe include a few examples of how you would call this Code and what kind of response you would get? depending on that I will create you a Clear answer!