Itās 6 PM. Iām tired and after a long day at work. Iāve arrived at the interview site and met the team leader whom I will be working with. we start with the basic āTell me about yourself questionsā, and go straight to the JavaScript coding test.

Iāve been handed a piece of paper, 30 minutes to solve the test, Go!

Most of the questions were pretty straight forward, explain the event loop, a few programs that I need to evaluate their results, and then I saw the recursion question.

Question: Write a recursive function that prints the Fibonacci Sequence in ascending order. The function receives as parameters the last and second last numbers of the sequence. Example: printFibonacci(55,34) -> 1 1 2 3 5 8 13 21 34 55

*If you donāt already know the answer, take a moment to think about the solution.*

At first I thought āItās easy, Iāve got this!ā, but then I started to struggle, Iāve already spent most of my time solving the other questions, I had around five minutes left to solve the question, but I couldnāt come up with a solution. All I could think of was the basic Fibonacci recursion that prints in descending order. I couldnāt figure out how to print the results in ascending order, and I ended up failing miserably.

# Fibonacci Descending Order

// The code will print: 55 34 21 13 8 5 3 2 1function printFibonacci(last, secondLast) {if (last == 1 && secondLast == 0) {return;}console.log(last);// Moving to the next fibonacci numbersreturn printFibonacci(secondLast, last - secondLast);}printFibonacci(55, 34);

In the above solution Iāve made two mistakes:

Fibonacci sequence for this question starts with the values: 1 1 ā¦ My stopping condition was correct, but it would only print ā1ā³ one time.

Iāve used āreturnā, therefore any code written after that return statement will not be executed. This was the error that prevented me from solving the question.

After the time was up, I sat down with the team leader to go over the test. Apparently this question was the most important one for him, because itās meant to show him my thought process, therefore I believe this was the question that cost me the job offer. I havenāt encountered such a recursion since my first year at college studying computer science.

We started discussing options to solve the question, and Iām trying to show him my thought process, but still I was not able to arrive at a solution.

Lets go over the solution together so you wonāt fail at this question like I did.

# Solution

// The code will print: 1 1 2 3 5 8 13 21 34 55function printFibonacci(last, secondLast) {// The code above the recursive function call will be in descending order.if (last == 1 && secondLast == 0) {console.log(1);return;}// Recursive function call, Moving to the next fibonacci numbersprintFibonacci(secondLast, last - secondLast);// The code below the recursive function call will be in ascending order.console.log(last);}printFibonacci(55, 34);

Any code above the printFibonacci recursive function call will run in descending order.

Any code below printFibonacci recursive function call will run in ascending order.

Lets look at the call stack to truly understand what happens:

Every time we reach the recursive function call (PrintFibonacci) we go one level deeper inside the recursion. When we reach the last function call in the recursion we start running each of the calls weāve put in our call stack.

We start from the last call (1,0), as seen in the image above, therefore printing the first Fibonacci value of the sequence (1) by hitting the stopping condition. Next, we go to the second function call (1,1) and print the second value of the sequence and so forth.

After grasping the idea that the order in which I put the recursive call shifts the order of code execution, I finally understood how it truly works.

I hope this will help you with your next interview.