Call stack behavior in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of nested function calls?

Consider the following JavaScript code. What will be printed to the console when start() is called?

Javascript

function first() {
  console.log('first');
}

function second() {
  first();
  console.log('second');
}

function start() {
  second();
  console.log('start');
}

start();

A "first" "second" "start"

B "start" "second" "first"

C "second" "first" "start"

D "first" "start" "second"

💡 Hint

Think about the order functions are called and when console.log runs inside each.

The call stack runs start(), which calls second(), which calls first(). So first() logs first, then second() logs, then start() logs last.

❓ Predict Output

intermediate

What happens with recursive calls?

Look at this recursive function. What will be the last number printed before the function stops?

Javascript

function countdown(n) {
  if (n <= 0) {
    console.log('Done');
    return;
  }
  console.log(n);
  countdown(n - 1);
}

countdown(3);

A "3"

B "1"

C "Done"

D "0"

💡 Hint

Think about when the base case is reached and what it prints.

The function prints numbers 3, 2, 1, then when n reaches 0 it prints 'Done' and stops.

❓ Predict Output

advanced

What is the output with asynchronous calls and call stack?

What will be the order of console logs when this code runs?

Javascript

console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
console.log('End');

A "Start" "Timeout" "End"

B "Timeout" "Start" "End"

C "End" "Start" "Timeout"

D "Start" "End" "Timeout"

💡 Hint

Remember that setTimeout callback runs after the current call stack is empty.

The synchronous logs run first: 'Start' then 'End'. The 'Timeout' runs later after the call stack clears.

❓ Predict Output

advanced

What error occurs with infinite recursion?

What error will this code produce when run?

Javascript

function recurse() {
  recurse();
}

recurse();

A ReferenceError: recurse is not defined

B RangeError: Maximum call stack size exceeded

C SyntaxError: Unexpected token

D TypeError: recurse is not a function

💡 Hint

Think about what happens when a function calls itself endlessly without stopping.

The function calls itself endlessly, filling the call stack until it overflows, causing a RangeError.

🧠 Conceptual

expert

How does the call stack handle nested asynchronous functions?

Given the code below, what is the order of console logs printed?

Javascript

async function first() {
  console.log('first start');
  await second();
  console.log('first end');
}

async function second() {
  console.log('second start');
  await Promise.resolve();
  console.log('second end');
}

first();
console.log('script end');

A "first start" "second start" "script end" "second end" "first end"

B "script end" "first start" "second start" "second end" "first end"

C "first start" "second start" "second end" "first end" "script end"

D "first start" "script end" "second start" "second end" "first end"

💡 Hint

Remember that await pauses the async function and lets the event loop run other code.

The synchronous logs run first: 'first start', then 'second start'. Then the code hits await and returns control to the event loop, so 'script end' prints next. Then the promises resolve and 'second end' and 'first end' print in order.

Practice

1. What does the call stack do in JavaScript?

easy

A. Keeps track of function calls in the order they are called

B. Stores variables and their values

C. Manages asynchronous events

D. Handles network requests

Solution

1. Step 1: Understand the role of the call stack

   The call stack tracks which functions are currently running and in what order.

2. Step 2: Differentiate from other JavaScript components

   Variables are stored in memory, asynchronous events are handled by the event loop, and network requests are managed separately.

3. Final Answer:

   Keeps track of function calls in the order they are called -> Option A

4. Quick Check:

   Call stack = function order tracking [OK]

Hint: Call stack = function call order tracker [OK]

Common Mistakes:

• Confusing call stack with event loop
• Thinking call stack stores variables
• Mixing call stack with network handling

2. Which of the following is the correct way to call a function named foo in JavaScript?

easy

A. call foo();

B. function foo();

C. execute foo();

D. foo();

Solution

1. Step 1: Recall JavaScript function call syntax

   Functions are called by writing their name followed by parentheses, like foo();.

2. Step 2: Identify incorrect options

   "call foo();" and "execute foo();" are not valid JavaScript syntax. "function foo();" is a function declaration, not a call.

3. Final Answer:

   foo(); -> Option D

4. Quick Check:

   Function call = name + () [OK]

Hint: Call functions with name and parentheses () [OK]

Common Mistakes:

• Using 'call' keyword incorrectly
• Confusing declaration with call
• Adding extra keywords like 'execute'

3. What will be the output of the following code?

function first() {
  console.log('first');
  second();
  console.log('first again');
}

function second() {
  console.log('second');
}

first();

medium

A. second\nfirst\nfirst again

B. first\nsecond\nfirst again

C. first\nfirst again\nsecond

D. first again\nsecond\nfirst

Solution

1. Step 1: Trace the call stack execution order

   Calling first() prints 'first', then calls second() which prints 'second'. After second() finishes, control returns to first() and prints 'first again'.

2. Step 2: Confirm output sequence

   The output lines are: 'first', 'second', 'first again' in that order.

3. Final Answer:

   first second first again -> Option B

4. Quick Check:

   Call stack order = first, second, first again [OK]

Hint: Follow function calls step-by-step [OK]

Common Mistakes:

• Assuming all logs run before second()
• Mixing order of console logs
• Ignoring return to first() after second()

4. Identify the error in the following code related to call stack behavior:

function a() {
  b();
}

function b() {
  a();
}

a();

medium

A. Function b is not defined

B. Syntax error due to missing semicolons

C. Infinite recursion causing stack overflow

D. No error, code runs fine

Solution

1. Step 1: Analyze function calls and recursion

   Function a() calls b(), and b() calls a() again, creating an endless loop.

2. Step 2: Understand call stack impact

   This infinite mutual recursion causes the call stack to grow until it overflows, leading to a runtime error.

3. Final Answer:

   Infinite recursion causing stack overflow -> Option C

4. Quick Check:

   Mutual recursion without stop = stack overflow [OK]

Hint: Watch for functions calling each other endlessly [OK]

Common Mistakes:

• Thinking missing semicolons cause error
• Assuming functions are undefined
• Believing code runs without error

5. Consider this code:

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

console.log(factorial(3));

How many times is the factorial function added to the call stack during factorial(3) execution?

hard

A. 4 times

B. 6 times

C. 1 time

D. 3 times

Solution

1. Step 1: Trace recursive calls for factorial(3)

   Calls happen for n=3, n=2, n=1, and n=0. Each call adds a new frame to the call stack.

2. Step 2: Count total calls added to the stack

   There are 4 calls total before reaching the base case (n=0), so 4 stack frames are added.

3. Final Answer:

   4 times -> Option A

4. Quick Check:

   Recursive calls = n+1 stack frames [OK]

Hint: Count calls including base case for recursion [OK]

Common Mistakes:

• Counting only n times, missing base case call
• Confusing total calls with return steps
• Assuming single call for recursion