Recursion Base Case and Recursive Case in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the base case value when n is 0.

DSA Javascript

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

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

Anull

B1

Cn

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 instead of 1 for the base case.

Not defining a base case causing infinite recursion.

The base case for factorial of 0 is 1, so the function returns 1 when n is 0.

2fill in blank

medium

Complete the code to call the recursive case correctly.

DSA Javascript

function sumToN(n) {
  if (n === 0) {
    return 0;
  }
  return n + [1];
}

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

AsumToN(n - 1)

BsumToN(n)

CsumToN(n + 1)

DsumToN(0)

Attempts:

3 left

💡 Hint

Common Mistakes

Calling sumToN with n + 1 causing infinite recursion.

Calling sumToN with the same n causing infinite recursion.

The recursive call should reduce n by 1 each time to reach the base case.

3fill in blank

hard

Fix the error in the base case condition to stop recursion correctly.

DSA Javascript

function countDown(n) {
  if (n [1] 0) {
    return;
  }
  console.log(n);
  countDown(n - 1);
}

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

A>

B==

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using n <= 0 which stops too early or causes wrong behavior.

Using n == 0 which may miss negative numbers.

The base case should stop when n is not greater than 0, so the condition is n > 0.

4fill in blank

hard

Fill both blanks to complete the recursive function that reverses a string.

DSA Javascript

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

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

A===

B-

C>

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of === in base case condition.

Using + instead of - in recursive call causing errors.

The base case checks if string length is 0 using ===, and the recursive call uses str.slice(0, str.length - 1) to reduce the string.

5fill in blank

hard

Fill all three blanks to complete the recursive function that calculates Fibonacci numbers.

DSA Javascript

function fibonacci(n) {
  if (n [1] 0 || n [2] 1) {
    return n;
  }
  return fibonacci(n [3] 1) + fibonacci(n - 2);
}

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

A===

B!==

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== instead of === in base case causing wrong base case.

Using + instead of - in recursive call causing infinite recursion.

The base case checks if n is 0 or 1 using ===, and the recursive call subtracts 1 from n to calculate Fibonacci.

Practice

1. What is the purpose of the base case in a recursive function?

easy

A. To call the function again with a smaller input

B. To increase the input size for the next call

C. To stop the recursion and prevent infinite calls

D. To print the result of the recursion

Solution

1. Step 1: Understand what recursion does

   Recursion calls the same function repeatedly with smaller inputs.

2. Step 2: Identify the role of the base case

   The base case stops the recursion by providing a condition where no further calls happen.

3. Final Answer:

   To stop the recursion and prevent infinite calls -> Option C

4. Quick Check:

   Base case = stop recursion [OK]

Hint: Base case stops recursion to avoid infinite loops [OK]

Common Mistakes:

• Confusing base case with recursive case
• Thinking base case calls recursion again
• Ignoring base case leads to infinite recursion

2. Which of the following is the correct syntax for a base case in this recursive function to calculate factorial?

function factorial(n) {
  if (___) {
    return 1;
  }
  return n * factorial(n - 1);
}

easy

A. n > 0

B. n === 0

C. n === 1

D. n < 0

Solution

1. Step 1: Recall factorial base case

   Factorial of 0 is 1, so base case is when n equals 0.

2. Step 2: Match condition to code

   The condition n === 0 correctly stops recursion and returns 1.

3. Final Answer:

   n === 0 -> Option B

4. Quick Check:

   Base case for factorial = n === 0 [OK]

Hint: Factorial base case is n equals zero [OK]

Common Mistakes:

• Using n > 0 as base case causes infinite recursion
• Using n < 0 which is invalid for factorial
• Using n === 1 misses factorial(0) base case

3. What will be the output of this recursive function call?

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

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace recursive calls

   sumTo(3) calls 3 + sumTo(2), sumTo(2) calls 2 + sumTo(1), sumTo(1) calls 1 + sumTo(0), sumTo(0) returns 0.

2. Step 2: Calculate total sum

   sumTo(3) = 3 + 2 + 1 + 0 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   3+2+1+0 = 6 [OK]

Hint: Sum from n down to 0 adds all numbers [OK]

Common Mistakes:

• Forgetting to add current n before recursive call
• Confusing base case return value
• Assuming output is just n instead of sum

4. Find the error in this recursive function to compute the nth Fibonacci number:

function fib(n) {
  if (n === 0) return 0;
  if (n === 1) return 1;
  return fib(n - 1) + fib(n - 2);
}
console.log(fib(-1));

medium

A. Missing base case for negative n causing infinite recursion

B. Incorrect base case values for fib(0) and fib(1)

C. Wrong recursive calls should be fib(n + 1) and fib(n + 2)

D. No error, output is correct

Solution

1. Step 1: Analyze base cases and input

   Base cases cover n=0 and n=1, but input is n=-1 which is not handled.

2. Step 2: Understand recursion with negative input

   Calling fib(-1) leads to fib(-2) and fib(-3) calls infinitely, no stop condition.

3. Final Answer:

   Missing base case for negative n causing infinite recursion -> Option A

4. Quick Check:

   Negative input needs base case [OK]

Hint: Check base cases cover all input ranges [OK]

Common Mistakes:

• Ignoring negative inputs in recursion
• Assuming fib(-1) is valid without base case
• Changing recursive calls incorrectly

5. You want to write a recursive function to reverse a string. Which base case and recursive case correctly achieve this?

function reverse(str) {
  if (___) {
    return ___;
  }
  return reverse(___) + str[0];
}

hard

A. str.length === 0 / str / str.slice(1)

B. str.length === 1 / str / str.slice(0, -1)

C. str === null / null / str.slice(1)

D. str === '' / str[0] / str.slice(1)

Solution

1. Step 1: Identify base case for empty string

   When string is empty (''), return the empty string to stop recursion.

2. Step 2: Define recursive call correctly

   Call reverse on substring from index 1 to end, then add first character at the end.

3. Final Answer:

   str === '' / str / str.slice(1) -> Option D

4. Quick Check:

   Empty string base case and slice(1) recursive call [OK]

Hint: Base case empty string, recursive call slice(1) [OK]

Common Mistakes:

• Using null instead of empty string as base case
• Returning wrong substring in recursive call
• Incorrect base case condition