Basic JavaScript - Use Recursion to Create a Countdown

JavaScript
1

Tell us what’s happening:
Describe your issue in detail here.

I wrote this code without any assistance and it runs perfectly. But i honestly just patterned it after the example in the exercise, by my logic the console.log should be returning the reverse of the countdown i.e 1,2,3,4,5 and i should not be passing this exercise.

Here’s how i see it -

The first " countArray.unshift(n) " adds the intial value of n into the beginning of the array … lets say n = 5.
[5, …]

next time the loop runs " countArray.unshift(n) " should add 4 to the beginning of the array.

[ 4, 5, …]

until we get 1,2,3,4,5 instead i get 5,4,3,2,1 .

can someone dumb down what is happening here for me ?
Your code so far

function countdown(n){
if (n < 1){
return ;
} else {
const countArray = countdown(n - 1);
countArray.unshift(n);
return countArray;
}
}
console.log(countdown(5));

// Only change code below this line
function countdown(n){
if (n < 1){
  return [];
} else {
  const countArray = countdown(n - 1);
  countArray.unshift(n);
  return countArray;
}
}
console.log(countdown(5));
// Only change code above this line

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36

Challenge: Basic JavaScript - Use Recursion to Create a Countdown

Link to the challenge:

2

Yes, it does, but what happens on the line just before that?

1 Like
3

We assign the value of the function “countdown” -1 to the variable called “countArray”.

Its not clear to me why that would even give us an array in decreasing order, as each decreasing value of n is ultimately shifted to the beginning of the array.

4

We assign the results of the function call countdown(n-1) to the variable countArray.

Yes, but the important point to realize is when that happens. If you call the function as:

countdown(5);

You are correct, the 5 is ultimately going to be added to the beginning of the countArray. But not until you do the following:

const countArray = countdown(n - 1);

You can’t just write this line off. It does something. It returns a value. And then you are adding 5 to the value it returned.

So don’t start with countdown(5), that’s too many numbers. Let’s start with countdown(2). When you first call it, you hit the else. You can’t add 2 to countArray until the line above it returns a value which is assigned to countArray. Start plugging in the values. If we call your function as:

countdown(2);

Fill in the values for:

const countArray = countdown(n - 1);

What is n - 1. What does countdown(n-1) return? You have to do that before you can add 2 to the beginning of it.

2 Likes
5

I worry that i’m not following this properly, here’s my understanding of your explanation.

In a scenario where n is equal to 2.
// we hit the else statement -

const countArray = countdown(n - 1);

// n = 2, but before we can “push” n to “countArray” the function “countdown (1)” has to run and we will store the result of that in countArray.

//countdown(1) runs and also hits the “else statement” and we’d be at (1-1).

At this point the logic falls apart in my head because then we’d be running countdown(0) which would return an empty array.

6

That’s exactly right. So what does countdown(1) return? First you do:

const countArray = countdown(0);

You said yourself that countdown(0) returns an empty array, so that means that countArray will be set to an empty array. Then we can do the next line:

countArray.unshift(1);

So countArray now equals [1] and that’s what will be returned for countdown(1).

So now you know what countdown(1) returns. Let’s go back to countdown(2). The first thing we do is:

const countArray = countdown(1);

But you know what countdown(1) returns, right? So we can replace it with:

const countArray = [1];

And then we go on to the next line:

countArray.unshift(2);

Which makes countArray equal to [2, 1].

1 Like
7

Ouuuuuuuu !
I’ve got tears in my eyes, this makes so much sense.
Thank you so much !

8

This topic was automatically closed 182 days after the last reply. New replies are no longer allowed.