What is the difference between synchronous and asynchronous code?

Answer

JavaScript runs synchronous work on a call stack and schedules asynchronous continuations through the event loop. Promise reactions run as microtasks, ahead of the next task, so ordering and cancellation must be designed explicitly.

For What is the difference between synchronous and asynchronous code?, start with the rule or behaviour, then anchor it in a small realistic example. Distinguish the default approach from the exceptions and name the observable outcome: correctness, maintainability, accessibility, performance, or security.

Example

console.log("start")
Promise.resolve().then(() => console.log("microtask"))
setTimeout(() => console.log("task"), 0)
console.log("end")
// start, end, microtask, task

Promise handlers use the microtask queue, which runs before the next timer task.

How to structure your answer

1. Define the concept in one or two sentences.
2. Explain when you would use it and when you would choose an alternative.
3. Walk through a small example, including an edge case.
4. Close with how you would test or measure the result.

Common mistakes

• Repeating a definition without connecting it to real code.
• Treating an optimization or abstraction as a default rather than a trade-off.
• Omitting lifecycle, error, cleanup, accessibility, or testing considerations when they apply.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

Interview tip

For this hard-level question, narrate your assumptions before coding. Interviewers can assess reasoning from a clear, bounded example much better than from a list of APIs.