async and await make Promise-based code easier to read, but they do not change the underlying rules: an async function returns a Promise, await pauses only that function’s continuation, and the way you start and combine operations determines whether they run in sequence or overlap. Use sequential awaits for dependent work, a Promise combinator for independent work, and cancellation signals when the underlying API supports cancellation.
What async and await do
An async function always returns a Promise. Its returned value becomes the Promise’s fulfillment value; an uncaught exception or rejection makes that Promise reject. This remains true when the function returns an ordinary value rather than a Promise.
await accepts a Promise, a thenable, or an ordinary value. If the awaited Promise is pending, JavaScript suspends the current async function at that point. When the Promise fulfills, the await expression evaluates to its value; when it rejects, the rejection reason is thrown at that point in the function. That is why ordinary try/catch works with awaited operations.
Await does not block the main thread while a Promise is pending. Other JavaScript work can proceed; only the part of this function that depends on the awaited result waits. Promise concurrency therefore lets asynchronous operations overlap, but it is not the same as running JavaScript code in parallel. At a given instant, one task executes on a JavaScript thread; worker threads can provide parallel execution.
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Choose sequence or concurrency based on dependencies
Keep dependent operations sequential
If the next operation needs a value from the first, await the first result before starting the second:
const user = await getUser();
const orders = await getOrders(user.id);getOrders needs user.id, so this sequence expresses the dependency directly.
Start independent operations before awaiting them
If operations do not depend on each other, call both first and await their combined result:
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const [profile, settings] = await Promise.all([
getProfile(),
getSettings(),
]);Both calls begin before the aggregate is awaited, allowing their asynchronous work to overlap. By contrast, writing await getProfile() and then await getSettings() makes the second call wait until the first settles. This does not guarantee faster completion in every application; it expresses that the operations are independent and may proceed concurrently.
Use branches when only some work applies
Put the conditional work inside an ordinary if statement. Await the branch’s operation when its result is needed before the function can continue:
async function loadDashboard(user, includeActivity) {
const profile = await getProfile(user.id);
if (includeActivity) {
const activity = await getRecentActivity(user.id);
return { profile, activity };
}
return { profile };
}The profile request is required in both paths. The activity request is made only when includeActivity is true.
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Pick a Promise combinator by its failure policy
Promise combinators are not interchangeable speed helpers. Choose according to whether the application needs every task to succeed, any task to succeed, the first settlement, or a record of every outcome.
| Method | When the aggregate settles | What happens on rejection | Useful when |
|---|---|---|---|
Promise.all |
Fulfills when all inputs fulfill; rejects as soon as an input rejects. | The aggregate rejects on an input rejection. It does not cancel the other underlying operations. | Every result is required for the next step. |
Promise.allSettled |
Waits until every input has fulfilled or rejected. | Individual rejections are returned as outcomes rather than making the aggregate reject. | You must inspect every result, including failures. |
Promise.any |
Fulfills with the first input to fulfill; rejects if all inputs reject. | A single rejection does not end the aggregate if another input may fulfill. | Any successful result is enough. |
Promise.race |
Settles as soon as the first input settles, whether by fulfillment or rejection. | The first rejection makes the aggregate reject. | The outcome of the earliest settlement is the one that matters. |
For example, a page that cannot render without both profile and settings data fits Promise.all. A reporting screen that must show which independent requests succeeded and failed fits Promise.allSettled. If several equivalent sources are being tried and the first successful response is sufficient, Promise.any matches that policy. Use Promise.race when the first settlement itself should determine the result, not as a shortcut for cancellation.
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An awaited rejection behaves like a thrown error at the await point. Catch it where the code can recover meaningfully or add context that helps the caller. If the current function cannot resolve the failure, let it reject so its caller can decide what to do.
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async function getData() {
try {
return await fetchData();
} catch (error) {
// Add context while preserving the original failure.
throw new Error("Could not load data", { cause: error });
}
}A catch block that only logs an error and then finishes normally changes the outcome: callers may receive undefined as if the function succeeded. Return a deliberate fallback only when it is valid for the application; otherwise rethrow or allow the rejection to propagate. The same Promise rejection-chain behavior applies when using .then()/.catch() instead of await/try/catch.
Cancellation must reach the underlying operation
A Promise has no universal built-in cancellation protocol. Cancellation depends on support in the operation being performed. Where an API supports it, pass its cancellation token or signal—commonly an AbortSignal created by an AbortController—and handle the resulting abort through the operation’s error flow.
A timeout built with Promise.race can stop your code from waiting for the result once the timeout wins, but it does not guarantee that the losing operation stops. To stop work, cancellation must be propagated to an underlying API that honors it.
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Check the module context for top-level await
In an ordinary script, await is valid inside an async function. Top-level await is available in modules, not in ordinary script context. A top-level await syntax error is therefore a parse-time context issue: use module context where appropriate, or put the work inside an async function.
Further reading
For a deeper treatment of Promise concurrency, cancellation, and top-level await, O’Reilly’s publisher-hosted preview for Modern Asynchronous JavaScript identifies the book by ISBN 9781680509267. Its preview establishes the subject coverage, not current price, format, stock, or regional availability. For broader design-pattern context, the publisher-hosted chapter “Asynchronous Programming Patterns” from Learning JavaScript Design Patterns, 2nd Edition includes async/await patterns and practical examples.
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