error-handling
try / catch with await
Since await throws when the awaited promise rejects, standard try/catch is the idiomatic way to handle async errors inside an async function. A finally block runs on both success and failure paths.
try { await promise; } catch (err) { ... } finally { ... }Parameters
| Parameter | Purpose |
|---|---|
| try | Wrap the awaited call(s) to catch rejections |
| catch (err) | err is the rejection reason (typically an Error) |
| finally | Runs after try/catch — even on early return |
Examples
async function load() {
try {
const res = await fetch('/api');
return await res.json();
} catch (err) {
console.error(err);
return null;
}
}Uniform handling for fetch errors and JSON-parse errors.
async function withCleanup() {
const lock = await acquire();
try {
return await doWork(lock);
} finally {
await release(lock); // always runs
}
}finally handles cleanup on success, rejection, or thrown error.
// Common bug: not awaiting inside try
try {
return fetchData(); // rejection escapes the try!
} catch (e) { /* never runs */ }Without await, the promise rejection is not thrown synchronously and the catch is skipped.
Gotcha
You must await the promise inside the try — returning a promise without await lets its rejection escape the catch. try/catch cannot catch errors from unawaited fire-and-forget promises.
Related
await
Pauses execution of the surrounding async function until the awaited promise settles, then resumes with its resolved value. If the promise rejects, await throws the reason, letting you use try/catch.
.catch()
Attaches a rejection handler to a promise chain, equivalent to .then(undefined, onRejected). Returns a new promise that fulfills with the handler's return value, letting you recover from errors.
unhandledrejection event
Fires when a promise rejects and has no rejection handler attached by the end of the current microtask cycle. Provides a last-chance hook to log or report the error before the runtime warns or exits.
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