Call stack behavior in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the function named greet.

Javascript

function greet() {
  console.log('Hello!');
}

greet[1];

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

A()

B[]

C{}

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets instead of parentheses.

Forgetting to add parentheses, so the function is not called.

To call a function in JavaScript, you use parentheses () after the function name.

2fill in blank

medium

Complete the code to log the value returned by the function add.

Javascript

function add(a, b) {
  return a + b;
}

console.log(add[1]);

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

A{2, 3}

B(2, 3)

C[2, 3]

D2, 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets or curly braces instead of parentheses.

Passing arguments without parentheses.

Functions are called with parentheses and arguments inside, separated by commas.

3fill in blank

hard

Fix the error in the recursive function call to avoid infinite recursion.

Javascript

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

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

A(n - 1)

B(n + 1)

Cn - 1

Dn + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Calling with n + 1 causing infinite recursion.

Forgetting parentheses around the argument.

To count down, the function must call itself with n - 1 inside parentheses.

4fill in blank

hard

Fill both blanks to create a function that returns the sum of squares of numbers in an array.

Javascript

function sumSquares(arr) {
  return arr.reduce((acc, num) => acc [1] num[2] 2, 0);
}

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

A+

B*

C**

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of addition in reduce.

Using wrong operator for squaring.

The reduce function adds (+) the square (** 2) of each number.

5fill in blank

hard

Fill all three blanks to create a recursive function that calculates factorial of n.

Javascript

function factorial([1]) {
  if ([2] <= 1) {
    return 1;
  }
  return [3] * factorial(n - 1);
}

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

An

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causing errors.

Missing base case or wrong condition.

The function uses n as parameter, checks if n <= 1, and returns n * factorial(n - 1).

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