JavaScript execution flow - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding event loop with setTimeout

What will be the output of the following code snippet?

Javascript

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

A "Start" "Timeout" "End"

B "End" "Start" "Timeout"

C "Start" "End" "Timeout"

D "Timeout" "Start" "End"

💡 Hint

Remember that setTimeout callbacks run after the current call stack is empty.

The synchronous console.log statements run first, printing "Start" then "End". The setTimeout callback runs last, printing "Timeout".

❓ Predict Output

intermediate

Variable hoisting with var

What will be the output of this code?

Javascript

console.log(x);
var x = 5;
console.log(x);

A undefined undefined

B ReferenceError 5

C 5 5

D undefined 5

💡 Hint

Variables declared with var are hoisted but not initialized.

The declaration of x is hoisted, so the first console.log prints undefined. Then x is assigned 5, so the second console.log prints 5.

❓ Predict Output

advanced

Promise execution order

What is the output order of this code?

Javascript

console.log('A');
Promise.resolve().then(() => console.log('B'));
console.log('C');

A "C" "A" "B"

B "A" "C" "B"

C "B" "A" "C"

D "A" "B" "C"

💡 Hint

Promises callbacks run after the current synchronous code but before setTimeout.

"A" and "C" are logged synchronously. The Promise callback logs "B" after the synchronous code.

❓ Predict Output

advanced

Async function execution flow

What will this code print?

Javascript

async function f() {
  console.log('1');
  await null;
  console.log('2');
}
f();
console.log('3');

A "1" "3" "2"

B "1" "2" "3"

C "3" "1" "2"

D "3" "2" "1"

💡 Hint

Await pauses the async function after the current synchronous code.

"1" is logged immediately. Then the function pauses at await. "3" logs next. Finally, "2" logs after the await.

🧠 Conceptual

expert

Call stack and recursion depth

Consider this recursive function. What will happen when calling recurse(3)?

Javascript

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

A "Done123"

B "123Done"

C TypeError

D RangeError

💡 Hint

Think about how the recursion returns and concatenates strings.

The base case returns "Done". Then each recursive call adds the current number to the string, resulting in "Done123".

Practice

1. What happens when JavaScript encounters a function declaration in the code?

easy

A. The function runs immediately.

B. The function is stored but not run until called.

C. The function is ignored.

D. The function causes an error.

Solution

1. Step 1: Understand function declaration behavior

   When JavaScript sees a function declared, it saves it in memory but does not execute it immediately.

2. Step 2: Recognize when functions run

   Functions only run when they are explicitly called later in the code.

3. Final Answer:

   The function is stored but not run until called. -> Option B

4. Quick Check:

   Functions run only when called = A [OK]

Hint: Functions run only when called, not on declaration [OK]

Common Mistakes:

• Thinking functions run immediately on declaration
• Confusing declaration with invocation
• Assuming functions are ignored until called

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

easy

A. greet();

B. greet;

C. call greet();

D. function greet();

Solution

1. Step 1: Identify function call syntax

   In JavaScript, calling a function requires the function name followed by parentheses.

2. Step 2: Check each option

   greet(); uses greet(); which correctly calls the function. Others are incorrect syntax.

3. Final Answer:

   greet(); -> Option A

4. Quick Check:

   Function call needs parentheses = A [OK]

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

Common Mistakes:

• Forgetting parentheses when calling functions
• Using 'call' keyword incorrectly
• Confusing declaration syntax with call

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

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

medium

A. StartEndTimeout

B. EndStartTimeout

C. TimeoutStartEnd

D. StartTimeoutEnd

Solution

1. Step 1: Understand synchronous and asynchronous code

   console.log('Start') runs first and prints 'Start'. setTimeout schedules a function to run later, after current code finishes.

2. Step 2: Trace the execution order

   console.log('End') runs immediately after 'Start'. Then the setTimeout callback runs, printing 'Timeout'.

3. Final Answer:

   Start
   End
   Timeout -> Option A

4. Quick Check:

   Async runs after sync code = D [OK]

Hint: setTimeout runs after current code finishes [OK]

Common Mistakes:

• Assuming setTimeout runs immediately
• Mixing order of console logs
• Ignoring asynchronous behavior

4. Find the error in this code snippet:

function sayHi() {
  console.log('Hi');
}
sayHi

medium

A. console.log syntax error

B. Function name is incorrect

C. Missing parentheses to call the function

D. No error, code runs fine

Solution

1. Step 1: Check function call syntax

   The code writes sayHi without parentheses, so the function is not called.

2. Step 2: Identify the fix

   Adding parentheses sayHi() calls the function and prints 'Hi'.

3. Final Answer:

   Missing parentheses to call the function -> Option C

4. Quick Check:

   Function call needs () = C [OK]

Hint: Always use () to call functions [OK]

Common Mistakes:

• Forgetting parentheses to invoke functions
• Thinking function name alone calls it
• Assuming no error when function not called

5. Consider this code:

console.log('1');
setTimeout(() => console.log('2'), 100);
Promise.resolve().then(() => console.log('3'));
console.log('4');

What is the correct order of output?

hard

A. 1342

B. 4132

C. 1234

D. 1432

Solution

1. Step 1: Understand synchronous, microtask, and macrotask queues

   console.log('1') and console.log('4') run immediately (sync). Promise.then callbacks run next (microtask). setTimeout callbacks run last (macrotask).

2. Step 2: Trace output order

   Output order is '1' (sync), '4' (sync), '3' (Promise microtask), then '2' (setTimeout macrotask).

3. Final Answer:

   1
   4
   3
   2 -> Option D

4. Quick Check:

   Promise microtasks before setTimeout macrotasks = B [OK]

Hint: Promise.then runs before setTimeout even with 0 delay [OK]

Common Mistakes:

• Thinking setTimeout runs before Promise.then
• Ignoring microtask queue priority
• Assuming all async run in order scheduled