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Async/Await, Promises and Error Handling in Node.js
A complete guide to Promise internal mechanics, microtask queue scheduling, async/await desugaring, unhandled rejections, and architecting bulletproof error boundaries in production Node.js applications.
Every asynchronous function in JavaScript always returns a Promise. When an async function throws an error, it is converted into a rejected Promise, never a synchronous exception.
Use `Promise.allSettled()` instead of `Promise.all()` whenever fetching data from multiple optional downstream dependencies where a single failure should not abort the entire request.
- Understand Promise states, internal reactions, and microtask queue placement
- Explain how async/await transforms into Promise chains and generator continuations
- Avoid silent swallowed errors and process crashes caused by unhandled rejections
- Choose accurately between Promise.all, Promise.allSettled, and Promise.race in production
1. The Anatomy of a Promise
Free Chapter PreviewA Promise in JavaScript is a state machine with three mutually exclusive states:
- Pending: Initial state; asynchronous work is in progress.
- Fulfilled: Operation completed successfully with a resolved
value. - Rejected: Operation failed with a rejection
reason(usually anErrorinstance).
Once a Promise transitions to Fulfilled or Rejected, it becomes settled and its state is permanently immutable.
const promise = new Promise((resolve, reject) => {
// Synchronous executor runs immediately
setTimeout(() => resolve("success"), 100);
});When .then(onFulfilled, onRejected) is called, the callbacks are attached to the Promise's internal reaction lists. Even if the Promise is already settled, the callback is always scheduled on the Microtask Queue and never runs synchronously.
2. How async/await Works Under the Hood
In this section, we examine the production failure mechanics, internal socket buffers, and kernel-level trace diagnostics...
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