Adding padding around a 2D JavaScript array

I am attempting to create a function that pads a 2D array with zeros. I have made the following function:

function addPadding(arr){
    var a = new Array(arr.length + 2).fill(0)

    //left and right padding
    arr.forEach(el => {
        el.push(0)
        el.unshift(0)
    })

    //top padding
    arr.unshift(a)

    //bottom padding
    arr.push(a)

    return arr;
}

console.table(addPadding(addPadding([[1,2],[3,4]])));

The function works fine if I only call it once, but if I call it twice, like in this example, I get the following table:

My function has an unintended result, it has added extra zeroes for 2 rows. Does anyone know why this is happening?

The first time you are padding at top and bottom the same array a. This means the second time that same array gets padded (left & right) twice.

Avoid adding the same array by taking a copy.

Change:

arr.push(a)

to:

arr.push([...a])