Synchronous vs asynchronous execution in Javascript - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this synchronous code?

Consider the following JavaScript code snippet. What will be printed to the console?

Javascript

console.log('Start');
console.log('Middle');
console.log('End');

A End\nMiddle\nStart

B Middle\nStart\nEnd

C Start\nEnd\nMiddle

D Start\nMiddle\nEnd

💡 Hint

Synchronous code runs line by line in order.

All console.log statements run one after another in the order they appear, so the output is Start, then Middle, then End.

❓ Predict Output

intermediate

What is the output of this asynchronous code with setTimeout?

Look at this JavaScript code using setTimeout. What will be printed to the console and in what order?

Javascript

console.log('First');
setTimeout(() => console.log('Second'), 0);
console.log('Third');

A First\nSecond\nThird

B First\nThird\nSecond

C Second\nFirst\nThird

D Third\nFirst\nSecond

💡 Hint

setTimeout with 0 delay runs after the current call stack is empty.

The synchronous console.log('First') runs first, then console.log('Third'). The setTimeout callback runs last, even with 0 delay.

❓ Predict Output

advanced

What is the output of this Promise-based asynchronous code?

Analyze this JavaScript code using Promises. What will be printed to the console and in what order?

Javascript

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

A A\nC\nB

B A\nB\nC

C B\nA\nC

D C\nA\nB

💡 Hint

Promise.then callbacks run after the current synchronous code but before setTimeout callbacks.

The synchronous logs 'A' and 'C' run first. The Promise.then callback runs after, printing 'B'.

❓ Predict Output

advanced

What error does this asynchronous code cause?

What error will this JavaScript code produce when run?

Javascript

setTimeout(() => {
  console.log(x);
}, 10);

let x = 5;

A TypeError: x is not a function

B undefined

C 5

D ReferenceError: Cannot access 'x' before initialization

💡 Hint

Variables declared with let are hoisted but not initialized until their declaration line.

The setTimeout callback runs after 10ms, by which time x is initialized to 5, so it prints 5.

🧠 Conceptual

expert

Which option best describes the JavaScript event loop behavior?

Choose the correct statement about how JavaScript handles synchronous and asynchronous code execution with the event loop.

A Synchronous code runs first, then microtasks, then macrotasks in order.

B Macrotasks run before synchronous code and microtasks.

C Microtasks run only after all macrotasks are completed.

D Asynchronous code runs before any synchronous code.

💡 Hint

Remember the order: synchronous code, then microtasks, then macrotasks.

JavaScript runs all synchronous code first, then microtasks (like Promise callbacks), then macrotasks (like setTimeout callbacks).