Recursion Concept and Call Stack Visualization in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the function call itself recursively.

DSA Javascript

function countdown(n) {
  if (n <= 0) {
    return;
  }
  console.log(n);
  countdown([1]);
}

Drag options to blanks, or click blank then click option'

An + 1

Bn - 1

Cn * 2

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n without change causes infinite recursion.

The function calls itself with n - 1 to count down until it reaches zero.

2fill in blank

medium

Complete the code to calculate factorial using recursion.

DSA Javascript

function factorial(n) {
  if (n === 0) {
    return 1;
  }
  return n * factorial([1]);
}

Drag options to blanks, or click blank then click option'

A0

Bn + 1

Cn

Dn - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n causes infinite recursion.

The factorial of n is n times factorial of n-1, so the recursive call uses n - 1.

3fill in blank

hard

Fix the error in the recursive Fibonacci function call.

DSA Javascript

function fibonacci(n) {
  if (n <= 1) {
    return n;
  }
  return fibonacci([1]) + fibonacci(n - 2);
}

Drag options to blanks, or click blank then click option'

An

Bn + 1

Cn - 1

Dn - 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n or n - 2 twice causes wrong results.

The Fibonacci function calls itself with n - 1 and n - 2. The first recursive call must be n - 1.

4fill in blank

hard

Fill both blanks to create a recursive function that sums numbers from 1 to n.

DSA Javascript

function sumTo(n) {
  if (n === 1) {
    return 1;
  }
  return n + sumTo([1]);
}

console.log(sumTo([2]));

Drag options to blanks, or click blank then click option'

An - 1

Bn + 1

C5

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using 1 as the initial call sums only 1.

The recursive call reduces n by 1 each time, so n - 1 is used. The function is called with 5 to sum numbers from 1 to 5.

5fill in blank

hard

Fill all three blanks to create a recursive function that reverses a string.

DSA Javascript

function reverseString(str) {
  if (str.length === 0) {
    return '';
  }
  return str.charAt([1]) + reverseString(str.[2]([3]));
}

console.log(reverseString('code'));

Drag options to blanks, or click blank then click option'

Astr.length - 1

Bsubstring

C0, str.length - 1

Dslice

Attempts:

3 left

💡 Hint

Common Mistakes

Using substring instead of slice causes wrong substring.

Using 0 instead of length - 1 for charAt causes wrong character.

The function takes the last character str.charAt(str.length - 1) and adds the reverse of the string without the last character using slice(0, str.length - 1).

Practice

1. What happens when a recursive function calls itself without a base case?

easy

A. It returns immediately without doing anything

B. It causes an infinite loop leading to a stack overflow error

C. It executes only once and stops

D. It throws a syntax error

Solution

1. Step 1: Understand the role of base case in recursion

   The base case stops the recursion by providing a condition to end calls.

2. Step 2: Analyze what happens without a base case

   Without a base case, the function keeps calling itself endlessly, filling the call stack.

3. Final Answer:

   It causes an infinite loop leading to a stack overflow error -> Option B

4. Quick Check:

   No base case = infinite recursion = stack overflow [OK]

Hint: Always check for a base case to stop recursion [OK]

Common Mistakes:

• Thinking recursion stops automatically
• Confusing base case with recursive case
• Assuming syntax error occurs without base case

2. Which of the following is the correct syntax to define a recursive function countDown in JavaScript?

easy

A. function countDown(n) { if(n <= 0) return; countDown(n - 1); }

B. function countDown(n) { while(n > 0) countDown(n - 1); }

C. function countDown(n) { for(let i=0; i

D. function countDown(n) { if(n < 0) countDown(n + 1); }

Solution

1. Step 1: Identify correct base case and recursive call

   function countDown(n) { if(n <= 0) return; countDown(n - 1); } has a base case (if n <= 0 return) and calls itself with n-1.

2. Step 2: Check other options for syntax or logic errors

   function countDown(n) { while(n > 0) countDown(n - 1); } uses while loop causing infinite recursion; C uses for loop incorrectly; D lacks proper base case.

3. Final Answer:

   function countDown(n) { if(n <= 0) return; countDown(n - 1); } -> Option A

4. Quick Check:

   Base case + recursive call = correct syntax [OK]

Hint: Look for base case and self-call in function [OK]

Common Mistakes:

• Using loops instead of recursion
• Missing base case
• Wrong recursive call argument

3. What is the output of the following code?

function factorial(n) {
  if (n === 0) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(3));

medium

A. 9

B. 1

C. 3

D. 6

Solution

1. Step 1: Trace factorial calls for n=3

   factorial(3) = 3 * factorial(2); factorial(2) = 2 * factorial(1); factorial(1) = 1 * factorial(0); factorial(0) = 1.

2. Step 2: Calculate return values step-by-step

   factorial(1) = 1 * 1 = 1; factorial(2) = 2 * 1 = 2; factorial(3) = 3 * 2 = 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   3! = 6 [OK]

Hint: Factorial n! = n * (n-1)! [OK]

Common Mistakes:

• Confusing multiplication order
• Forgetting base case returns 1
• Calculating factorial(0) incorrectly

4. Find the error in this recursive function that calculates the sum of numbers from 1 to n:

function sum(n) {
  if (n === 1) return 1;
  return n + sum(n);
}

medium

A. The base case is missing

B. The function should return n - sum(n - 1)

C. The recursive call uses the same argument causing infinite recursion

D. The function should use a loop instead

Solution

1. Step 1: Check the recursive call argument

   The function calls sum(n) again without reducing n, causing infinite recursion.

2. Step 2: Identify correct recursive call

   It should call sum(n - 1) to reduce n and reach base case.

3. Final Answer:

   The recursive call uses the same argument causing infinite recursion -> Option C

4. Quick Check:

   Recursive call argument must change to reach base case [OK]

Hint: Recursive call argument must move toward base case [OK]

Common Mistakes:

• Calling function with same argument
• Missing base case
• Using wrong operator in return

5. Consider this recursive function that reverses a string:

function reverseStr(str) {
  if (str === "") return "";
  return reverseStr(str.substring(1)) + str[0];
}
console.log(reverseStr("code"));

What is the output and why?

hard

A. "edoc" because the function moves first character to the end recursively

B. "code" because substring does not change the string

C. "" because base case returns empty string immediately

D. SyntaxError because substring is used incorrectly

Solution

1. Step 1: Understand recursion on string substring

   The function calls itself with the string minus the first character each time.

2. Step 2: Trace the concatenation on return

   Each call adds the first character of the current string to the end of the reversed substring, reversing the string.

3. Final Answer:

   "edoc" because the function moves first character to the end recursively -> Option A

4. Quick Check:

   Recursive substring + concat reverses string [OK]

Hint: Recursion with substring(1) + first char reverses string [OK]

Common Mistakes:

• Thinking substring does not change string
• Confusing base case effect
• Assuming syntax error with substring