All 15 Async Patterns questions
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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 an async generator, and how does 'for await...of' let you consume asynchronously produced data? What is the difference between an unhandled promise rejection and an uncaught exception in Node.js, and how does Node handle each by default? What is a common pitfall when using async/await inside Array.prototype.forEach(), and how do you fix it? How would you implement a concurrency-limited task queue in Node.js to process many async tasks without overwhelming a downstream resource? What is 'fire-and-forget' async code, and why is it generally discouraged in production Node.js applications?
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What is the difference between micro tasks and macro tasks in Node.js's event loop, and how does this affect Promise callback ordering?

Intermediate

Answer

Microtasks (Promise .then()/.catch()/.finally() callbacks, and process.nextTick() callbacks which have their own even-higher-priority queue) are processed completely -- draining the entire microtask queue -- after each single macrotask, before the event loop moves on to the next phase. Macrotasks include timers (setTimeout/setInterval), I/O callbacks, and setImmediate() callbacks, each processed one at a time per event loop phase. This is why a resolved promise's .then() callback always runs before a setTimeout(fn, 0) callback, even though both appear to be scheduled for 'immediately'.
console.log('1');
setTimeout(() => console.log('2 (macrotask)'), 0);
Promise.resolve().then(() => console.log('3 (microtask)'));
console.log('4');
// Output order: 1, 4, 3, 2 -- microtasks always drain before the next macrotask
Real-world example A developer debugging unexpected ordering in a logging system realizes that a Promise-based logger's output was appearing before a setTimeout-scheduled cleanup task's output specifically because of this microtask/macrotask distinction, not because of any bug in either piece of code.

Common follow-ups: Where does process.nextTick() fit relative to regular Promise microtasks in terms of priority?;Can an application accidentally starve macrotasks by continuously scheduling new microtasks?

Event Loop & Non-blocking IO;Advanced Node.js
All 15 Async Patterns questions