Basic JavaScript - Use Recursion to Create a Countdown

JavaScript
1

Tell us what’s happening:
Below I have the solution text but I am struggling to understand it.

I’m trying to understand the order in which the code executes. As far as I can see, (because we are counting down from n) the very last thing that is happening is the introduction of the square array brackets.

For example, if n=6, we skip the if statement (because n !< 1) and go straight to the else statement that is creating the variable arr and unshifting values into the array that does not yet exist. Only after 6 recursions does the if statement trigger, creating the array.

I know that I am incorrect but I can’t fathom how. Any help would be appreciated aid my understanding of this concept.

Your code so far

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

// Only change code above this line

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Challenge: Basic JavaScript - Use Recursion to Create a Countdown

Link to the challenge:

2

Don’t use countdown(6) as an example. Make it easy on yourself and use countdown(1). When n is 1 then you will hit the else statement:

let countArray = countdown(n - 1);

Now remember, the function can’t move on from that line until it gets a value from the recursive call countdown(n - 1). So tell me, what will countArray equal after that recursive call returns?

3

Thanks for your response. countArray will equal countdown(0), which will return . But once this has been returned in the if statement, how then can the else statement be triggered to unshift the values into the array?

4

Yes, this is correct. So that means that after this line is executed:

let countArray = countdown(n - 1);

Then countArray will have the value of [ ].

So now we can move on to the next two lines:

arr.unshift(n);
return arr;

So what is the function going to return?

Remember, we never left the else statement of the original function call for countdown(1). We were just waiting at

let countArray = countdown(n - 1)

Now that we have the return value for countdown(n - 1) we merely move to the next line.

5

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