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

```javascript
  else if (string.substring(0, b.length) == b) {
    return a + replace(string.substring(b.length), a, b);
  }
    
  else {
    return string[0] + replace(string.substring(1), a, b);
  }
```

```javascript
  else {
    return string[0] + replace(string.substring(1), a, b);
  }
```

```javascript
replace(string, a - 1, b - 1);
```

```javascript
  else if (string == b) {
    return replace(string.substring(b.length), a, b);
  }
    
  else {
    return string[0] + replace(string.substring(1), a, b);
  }
```

The code below replaces all occurrences of substring `b` with substring `a` in `string`. This process is done recursively.
```javascript
function replace(string, a, b) {
  // Base case
  if (string.length == 0) {
    return "";
  }
  
  // Recursive case
  ______________________________________________
}
```
What should the **recursive case** of the following code be?

The code below replaces all occurrences of substring <code>b</code> with substring <code>a</code> in <code>string</code>. This process is done recursively.
<pre><code class="language-javascript">function replace(string, a, b) {
 // Base case
 if (string.length == 0) {
 return &quot;&quot;;
 }
 
 // Recursive case
 ______________________________________________
}
</code></pre>
What should the recursive case of the following code be?

```javascript
helperFunction(string);
```

```javascript
helperFunction(string, low + 1, high - 1);
```

```javascript
helperFunction(string, low + 1, high);
```

```javascript
helperFunction(string, low, high - 1);
```

Write the **recursive case** for the following function that checks whether or not a string is a **Palindrome**.
```javascript
function isPalindrome(string) {
 return helperFunction(string, 0, string.length - 1);
}

function helperFunction(string, low, high) {
 // Base case1
 if (high <= low) { 
 return true;
 }
 
 // Base case2
 else if (string[low] != string[high]) {
 return false;
 }
 
 // Recursive case
 else {
 return ____________________________
 }
}
```

Write the recursive case for the following function that checks whether or not a string is a Palindrome.
<pre><code class="language-javascript">function isPalindrome(string) {
 return helperFunction(string, 0, string.length - 1);
}

function helperFunction(string, low, high) {
 // Base case1
 if (high &lt;= low) { 
 return true;
 }
 
 // Base case2
 else if (string[low] != string[high]) {
 return false;
 }
 
 // Recursive case
 else {
 return ____________________________
 }
}
</code></pre>

The code below calculates the length of a string recursively. Select the **line number** of the erroneous code line

```javascript
1.	function recursiveLength(testVariable) {
2.		if (testVariable === "") {
3.			return 1;
4.		}
5.
6.		else {
7.			return 1 + recursiveLength(testVariable.substring(1));
8.		}
9.	}
```

The code below calculates the length of a string recursively. Select the line number of the erroneous code line
<pre><code class="language-javascript">1.	function recursiveLength(testVariable) {
2.		if (testVariable === &quot;&quot;) {
3.			return 1;
4.		}
5.
6.		else {
7.			return 1 + recursiveLength(testVariable.substring(1));
8.		}
9.	}
</code></pre>

What is the output of the following code?
```javascript
function foo(string) {
  if (string.length === 0) {
    return 0;
  } 
  else {
    return ("aeiouAEIOU".indexOf(string[0]) != -1) + foo(string.slice(1));
  }
}

// Driver Code
console.log(foo("thisisrecursion"));
```

What is the output of the following code?
<pre><code class="language-javascript">function foo(string) {
 if (string.length === 0) {
 return 0;
 } 
 else {
 return (&quot;aeiouAEIOU&quot;.indexOf(string[0]) != -1) + foo(string.slice(1));
 }
}

// Driver Code
console.log(foo(&quot;thisisrecursion&quot;));
</code></pre>

This Quiz will take approximately 10 minutes.

This Quiz will take approximately 10 minutes.

---
In the next chapter, we will learn how to process arrays using recursion.


Recursion Fundamentals

Iteration Vs Recursion

Recursion with Numbers

Recursion with Strings

Recursion with Arrays

Recursion with Data Structures

Conclusion

Test Your Knowledge 4