🔥 Async JavaScript – Interview Questions

🟢 Basic Level (1–7)

1. What is Asynchronous JavaScript?

👉 JavaScript that allows tasks to run in the background without blocking the main thread.

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2. Difference between synchronous and asynchronous?

• Synchronous → Executes line by line (blocking)
• Asynchronous → Executes without waiting (non-blocking)

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3. What is setTimeout()?

👉 Runs a function once after a specified delay.

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4. What is setInterval()?

👉 Runs a function repeatedly after a fixed interval.

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5. What is a callback function?

👉 A function passed as an argument to another function and executed later.

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6. What is callback hell?

👉 Nested callbacks that make code unreadable and hard to maintain.

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7. Is JavaScript synchronous or asynchronous?

👉 Single-threaded but supports asynchronous behavior via event loop + Web APIs.

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🟡 Intermediate Level (8–14)

8. What is a Promise?

👉 An object representing the eventual completion or failure of an async operation.

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9. What are Promise states?

• Pending
• Fulfilled (Resolved)
• Rejected

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10. Difference between .then() and .catch()?

• .then() → handles success
• .catch() → handles errors

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11. What is finally() in Promise?

👉 Runs after success or failure (cleanup code).

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12. What is async/await?

👉 A cleaner way to handle Promises using synchronous-like syntax.

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13. Can we use await without async?

👉 ❌ No, only inside async functions.

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14. What happens if a Promise is rejected and not handled?

👉 It throws an Unhandled Promise Rejection error.

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🔵 Advanced Level (15–20)

15. What is the Event Loop?

👉 It manages execution of asynchronous code (handles call stack + callback queue).

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16. What is the Call Stack?

👉 A stack where JS executes function calls.

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17. What is the difference between microtask and macrotask?

• Microtask → Promise callbacks, queueMicrotask
• Macrotask → setTimeout, setInterval, DOM events

👉 Microtasks execute first.

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18. Output-based Question:

console.log("A");setTimeout(() => console.log("B"), 0);Promise.resolve().then(() => console.log("C"));console.log("D");

👉 Output:

A
D
C
B

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19. What is Promise.all()?

👉 Runs multiple promises in parallel and resolves when all succeed.

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20. Difference between Promise.all() and Promise.race()?

• Promise.all() → waits for all
• Promise.race() → returns first completed promise

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💡 Bonus Tip (Important for Interviews)

👉 Most asked topics:

• Event Loop
• Promise vs async/await
• Output questions
• Error handling