freeCodeCamp Recursion Example Explained

/* Note that the below recursion challenge can be found here: https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/basic-javascript/use-recursion-to-create-a-countdown. But it's the *example* provided, not the challenge itself. */ function countup(n) { if (n < 1) { // the base case return [] } else { const countArray = countup(n - 1) // remember that, on the way "down", each call is added to the stack countArray.push(n) // console.log(countArray) // just a helpful line return countArray } } console.log(countup(5)) // [1, 2, 3, 4, 5] /* fCC: "At first, this seems counterintuitive since the value of n decreases but the values in the final array are increasing. This happens because the push happens last, after the recursive call has returned. At the point where n is pushed into the array, countup(n - 1) has already been evaluated and returned [1, 2, ..., n - 1]." */ // The way "down": // countup(5 - 1) // added to stack first // countup(4 - 1) // added to stack second // countup(3 - 1) // added to stack third // countup(2 - 1) // added to stack fourth // countup(1 - 1) // added to stack fifth // So the stack looks like this, top to bottom: // countup(1 - 1) // countup(2 - 1) // countup(3 - 1) // countup(4 - 1) // countup(5 - 1) // The base case is reached: // countup(1 - 1) evaluates to countup(0) // ...which means n is 0 // ...which means n < 1 is true // ...which means [] is returned to countup() // ...which means countArray = [] // The way "back up": // It is now that 'control' moves on to the 'countArray.push(n)' line // countArray = [1] // the 1 comes from the top of the stack (and then the 2 is at the top of the stack) // countArray = [1, 2] // the 2 comes from the top of the stack (and then the 3 is at the top of the stack) // countArray = [1, 2, 3] // the 3 comes from the top of the stack (and then the 4 is at the top of the stack) // countArray = [1, 2, 3, 4] // the 4 comes from the top of the stack (and then the 5 is at the top of the stack) // countArray = [1, 2, 3, 4, 5] // the 5 comes from the top of the stack (and then the stack is empty and countArray is returned)