Javascript: Unshift() is causing an infinite loop but can't see why

Question

I am trying to make a function that takes an array and returns the array with the even numbers before the odd numbers. I figured I'd iterate through the array and check the divisibility of each number with modulo. If it's an even number, I unshift that number to the front of the array and keep going.

I've removed unshift to log that it iterates through so I know the issue isn't with my loop.

/// A = [3,5,6,3,4,2,1]

var sortArrayByParity = function(A) {
    for (var x =0; x< A.length;x++) {
        if (A[x] % 2 == 0) {
           A.unshift(A[x])
        } 
    }
    return A;
};

Maximum stack error is occuring.

Answer 1

Because unshift adds an element to the array, thereby increasing its length . Furthermore, once the unshift adds an element to the beginning of the array, the next A[x] is the same as the previous one, as all elements get moved to the right, including x . So the loop never finishes, because the index x never reaches the ever increasing length .

If you need to add elements to an array through which you are iterating with a loop, it's better to do it with push in a loop that counts downwards from length to 0 . That way, the loop will never iterate through added elements, and the elements that it will iterate through will not move.

Answer 2

When you unshift an array an element is added to the start of that array and increase the length.

Lets try

 let arr = [1]; console.log('arr = ',arr, 'length =', arr.length); arr.unshift(0); console.log('arr = ',arr, 'length =', arr.length); 

As a result when you find a even number you unshift it to the front but it will also be available in x + 1 position. and as you iterate you will find the same even number in next iteration.

correct implementation would be.

 /// A = [3,5,6,3,4,2,1] var sortArrayByParity = function(A) { for (var x =0; x< A.length;x++) { if (A[x] % 2 == 0) { A.unshift(A[x]); A.splice(x+1, 1); } } return A; }; console.log(sortArrayByParity([3,5,6,3,4,2,1])); 

You should use both unshift and splice .