The ultimate guide to recursion interviews in JavaScript. Developed by FAANG engineers. Practice with real-world interview questions and get interview-ready in just a few hours.

If you’ve ever struggled with solving problems using recursion, or if you have to brush up your skills for an interview, this course is for you.

We'll start with the basics of what recursion is and why it’s important before diving into practicing solving actual questions. You’ll have access to detailed explanations and visualizations for each problem to help you along the way. 

By the time you have completed this course, you’ll be able to use all the concepts to solve complex real-world problems. We hope that through practicing recursion, you will be able to ace all recursion related questions in interviews at top tech companies.

Recursion for Coding Interviews in JavaScript

function fibonacci(testVariable) {
 var fn0 = 0;
 var fn1 = 1;
 
 for (let i = 0; i < testVariable; i++) {
 var temp = fn0 + fn1;

 // Setting variables for next iteration
 fn0 = fn1;
 fn1 = temp;
 }
 return fn0;
}

// Driver Code
var testVariable = 7;
console.log(fibonacci(testVariable));

### Explanation
In the iterative method, we keep track of the two previous elements using the variables `fn0` and `fn1`.
Initially, the values of the two variables are:
```javascript
fn0 = 0;
fn1 = 1;
```
However, in each iteration, the values are updated in the following way:

function fibonacci(testVariable) {
 // Base case
 if (testVariable <= 1) {
 return testVariable;
 }

 // Recursive case
 return(fibonacci(testVariable - 1) + fibonacci(testVariable - 2));
}

// Driver Code
var testVariable = 7;
console.log(fibonacci(testVariable));

### Explanation:

In the code above, the function `fibonacci()` is a recursive function as it calls itself in the function body.

The **base case** of the function **(line number 3)** deals with the two initial values, i.e, for index $0$ the value is $0$ and for the index, $1$ the value is $1$:
```javascript
 if (testVariable <= 1) {
 return testVariable;
 }
```
However, for the **recursive case**, the parent function calls two instances of itself recursively with $n-1$ and $n-2$. The input arguments correspond to the equation for finding a general term in the *Fibonacci sequence*: 

 $F$n $=F$n-2$+F$n-1

> This type of recursion is called **binary recursion** where the parent function makes two recursive calls to a different instance of the same function in each call. This is as opposed to **single recursion** where there is just one recursive call. There is also **multiple recursion** where a recursive function calls more than two versions of itself.


Let's look at the code flow:

## Explanation
In the iterative method, we keep track of the two previous elements using the variables `fn0` and `fn1`.
Initially, the values of the two variables are:
```javascript
fn0 = 0;
fn1 = 1;
```
However, in each iteration, the values are updated in the following way:

## Explanation:

In the code above, the function `fibonacci()` is a recursive function as it calls itself in the function body.

The **base case** of the function **(line number 3)** deals with the two initial values, i.e, for index $0$ the value is $0$ and for the index, $1$ the value is $1$:
```javascript
 if (testVariable <= 1) {
 return testVariable;
 }
```
However, for the **recursive case**, the parent function calls two instances of itself recursively with $n-1$ and $n-2$. The input arguments correspond to the equation for finding a general term in the *Fibonacci sequence*: 

 $F$n $=F$n-2$+F$n-1

> This type of recursion is called **binary recursion** where the parent function makes two recursive calls to a different instance of the same function in each call. This is as opposed to **single recursion** where there is just one recursive call. There is also **multiple recursion** where a recursive function calls more than two versions of itself.


Let's look at the code flow:

This review provides a detailed analysis of the ways to find the corresponding element at a given index in the Fibonacci series.

Recursion Fundamentals

Iteration Vs Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Conclusion

Solution Review: Corresponding Fibonacci Number

Solution #1: Iterative Method

Explanation

Solution #2: Recursive Method

Explanation: