Question

Can someone visually explain what’s going on here please.

var stack = [];

function countDown(int) {
    stack.push(int);
    if (int === 1) {
        return 1;
    }
    return countDown(int - 1);
}

function multiplyEach() {
    // Remove the last value of the stack
    // and assign it to the variable int
    int = stack.pop();
    x = stack.length;
    // Base case
    if (x === 0) {
        return int;
    }
    // Recursive case
    else {
        stack[x - 1] = int * stack[x - 1];
        return multiplyEach();
    }
}

// Call the function

 countDown(7);

// And then print out the value returned by multiplyEach()

console.log(multiplyEach());

I sorta understand that’s it’s creating a stack and them multiplying everything together but I can’t visualize it.

Then:

Stack[x-1] is getting me

Answer 1

This is a two part algorithm.

First countDown(n) fills the array stack with values from n down to 1, so stack = [n, n-1, n-2, ..., 3, 2, 1]. The return value of countDown is never used and so the return statement can be ignored. The only thing that matters is that it fills the stack array as explained.

Second, multiplyEach repeatedly takes the last element of the stack, removes it, and multiplies it with the next last element in the array:

[n, n-1, n-2, ..., 6, 5, 4, 3, 2, 1]
[n, n-1, n-2, ..., 6, 5, 4, 3, 2]
[n, n-1, n-2, ..., 6, 5, 4, 6]
[n, n-1, n-2, ..., 6, 5, 24]
[n, n-1, n-2, ..., 6, 120]
...
[n!]

In other words, the algorithm calculates the factorial of the number n given to countDown.