I was trying to convert a binary tree to array and I am confused in it. This is the code I am writing:

var maxDepth = function(root) {
    var rc=rc??[];
    if(root==null)
        return ;
    rc.push(root.val);
    maxDepth(root.left);
    maxDepth(root.right);
    return ((rc.length-1)/2)+1;
};

So the problem is that my array always contains 1 element. But according to my understanding , won't after pushing the first element in rc , the nullish coalescing operator will assign the value of rc to the array? Why is it essentially resetting the array?