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What is a callback function in Node.js, and what is 'callback hell'? What is a Promise in JavaScript, and what are its three possible states? How do you convert a Node.js callback-based function into a Promise-based one manually, without using util.promisify()? What is the purpose of the 'node:timers/promises' module, and how does it simplify working with delays in async code? How does async/await relate to Promises under the hood, and what does 'await' actually do to program execution? What is the difference between running async operations sequentially with multiple awaits versus concurrently with Promise.all()? How do you implement a retry-with-exponential-backoff pattern for a flaky async operation in Node.js? What is util.promisify(), and when would you still need it in modern Node.js? What is the difference between micro tasks and macro tasks in Node.js's event loop, and how does this affect Promise callback ordering? What is an async generator, and how does 'for await...of' let you consume asynchronously produced data?
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What is the difference between Promise.all(), Promise.allSettled(), Promise.race(), and Promise.any()?

Intermediate

Answer

Promise.all() resolves when every promise in the array resolves, but rejects immediately as soon as any single promise rejects (short-circuiting the rest). Promise.allSettled() always waits for every promise to settle regardless of outcome, returning an array describing each result's status ('fulfilled' or 'rejected') rather than throwing -- useful when you need every result even if some fail. Promise.race() settles as soon as the first promise settles, whether fulfilled or rejected. Promise.any() resolves as soon as the first promise fulfills, only rejecting if every promise rejects.
const results = await Promise.allSettled([
  fetch('/api/a'), fetch('/api/b'), fetch('/api/c')
]);
results.forEach(r => r.status === 'fulfilled' ? console.log(r.value) : console.error(r.reason));

// Promise.race for a timeout pattern
const result = await Promise.race([fetchData(), timeout(5000)]);
Real-world example A dashboard fetching data from five independent microservices uses Promise.allSettled() rather than Promise.all(), so that if one service is temporarily down, the dashboard can still render data from the four services that succeeded instead of failing the entire page load.

Common follow-ups: How would you implement a request timeout using Promise.race()?;What's a scenario where Promise.any() is specifically the right choice over Promise.race()?

Async Patterns;Error Handling
All 15 Async Patterns questions