Recursion Base Case and Recursive Case in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Factorial Function

What is the output of this TypeScript code when calling factorial(4)?

DSA Typescript

function factorial(n: number): number {
  if (n === 0) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}
console.log(factorial(4));

A 120

B 10

C 24

D Error: Maximum call stack size exceeded

💡 Hint

Remember factorial of 0 is 1, and factorial(n) = n * factorial(n-1).

The function calculates factorial recursively. factorial(4) = 4 * 3 * 2 * 1 = 24.

❓ Predict Output

intermediate

Output of Recursive Sum Function

What is printed when this code runs: sumTo(5)?

DSA Typescript

function sumTo(n: number): number {
  if (n === 1) {
    return 1;
  } else {
    return n + sumTo(n - 1);
  }
}
console.log(sumTo(5));

A Error: Maximum call stack size exceeded

B 10

C 5

D 15

💡 Hint

Sum from 1 to n is n*(n+1)/2.

sumTo(5) = 5 + 4 + 3 + 2 + 1 = 15.

❓ Predict Output

advanced

Output of Recursive Fibonacci Function

What is the output of console.log(fibonacci(6)) for this code?

DSA Typescript

function fibonacci(n: number): number {
  if (n <= 1) {
    return n;
  } else {
    return fibonacci(n - 1) + fibonacci(n - 2);
  }
}
console.log(fibonacci(6));

A 13

B Error: Maximum call stack size exceeded

C 21

D 8

💡 Hint

Fibonacci sequence starts 0,1,1,2,3,5,8,...

fibonacci(6) = fibonacci(5) + fibonacci(4) = 8 + 5 = 13.

🔧 Debug

advanced

Identify the Error in Recursive Function

What error will this code cause when calling countdown(3)?

DSA Typescript

function countdown(n: number): void {
  if (n === 0) {
    console.log("Done!");
  } else {
    console.log(n);
    countdown(n);
  }
}
countdown(3);

A TypeError: countdown is not a function

B Infinite recursion causing stack overflow

C SyntaxError: Missing return statement

D Prints 3, 2, 1, Done!

💡 Hint

Check if the recursive call changes the argument.

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

🧠 Conceptual

expert

Understanding Base Case Importance

Why is the base case essential in a recursive function?

A It stops the recursion to prevent infinite calls

B It makes the function run faster by skipping steps

C It allows the function to call itself multiple times

D It initializes variables before recursion starts

💡 Hint

Think about what happens if recursion never stops.

Without a base case, recursion never ends and causes errors.