Put dependent asynchronous steps in an async function and await each result where the next step needs it. Return the final value, then have the caller await or otherwise handle the promise returned by the function.
Rewrite nested .then() calls as a straight-line sequence
A nested callback often hides the order in which results are needed:
getUser(id).then((user) => {
getPermissions(user.id).then((permissions) => {
render(user, permissions);
});
});In an async function, each await makes the following statement wait for the preceding operation’s result:
async function showUser(id) {
const user = await getUser(id);
const permissions = await getPermissions(user.id);
return render(user, permissions);
}
const result = await showUser("123");This keeps the dependency visible: permissions are requested only after the user is available, and rendering receives both results. MDN recommends keeping simple promise chains flat rather than nesting them in callbacks; MDN’s promise guide also demonstrates the equivalent async/await style.
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Understand what await and async return
await gives code inside the async function the fulfilled value of a promise or thenable. If it rejects, the rejection reason is thrown at that await expression, where ordinary try/catch can handle it. The function is suspended at the await while other JavaScript work can continue; it does not block the main thread. See MDN’s await reference.
An async function call always returns a promise. Returning an ordinary value fulfills that promise with the value; returning another promise or thenable makes the outer promise adopt its eventual state. The caller still needs to await the function call or attach handlers to its promise:
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async function loadProfile(userId) {
const response = await fetch(`/users/${userId}`);
const user = await response.json();
const permissions = await fetchPermissions(user.id);
return { user, permissions };
}
const profile = await loadProfile("123");For a caller that is not itself async, use a promise handler instead:
loadProfile("123").then((profile) => {
renderProfile(profile);
}).catch((error) => {
showError(error);
});These behaviors are covered in MDN’s async function reference. Flattening the syntax does not make the operation synchronous or remove the need to handle the returned promise.
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Choose sequential awaits or concurrent work by dependency
Await in sequence when a later operation needs an earlier result. If two operations are independent, start both before awaiting their combined result:
const [user, settings] = await Promise.all([
getUser(id),
getSettings(id),
]);| Situation | Pattern | Why |
|---|---|---|
| The next operation needs the previous result | const user = await getUser(id);const permissions = await getPermissions(user.id); |
The second call cannot be formed until the first result is available. |
| Operations are independent and both results are needed | const [a, b] = await Promise.all([getA(), getB()]); |
Both calls are started before waiting for the aggregate promise. |
Promise.all fulfills with results in input order when all inputs fulfill, and rejects if an input rejects. If the task needs a different failure policy, such as recording every outcome rather than rejecting on the first failure, choose a Promise combinator whose settlement behavior fits that requirement. MDN documents the available helpers in the Promise reference.
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Decide where rejection handling belongs
Catch locally when this function can recover, add useful context, or perform cleanup that depends on the failure. Otherwise, let the async function’s promise reject and handle it at the caller’s boundary.
async function loadProfileSafely(userId) {
try {
const response = await fetch(`/users/${userId}`);
const user = await response.json();
return await fetchPermissions(user.id);
} catch (error) {
// Recover, add context, or let the error propagate.
throw error;
}
}Here, return await places the final promise’s rejection inside this try block. If there is no local catch or finally that must cover that settlement, returning fetchPermissions(user.id) directly is ordinarily simpler. See MDN’s guidance on await behavior and async functions.
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Know what promise flattening does—and does not do
Promise resolution assimilates returned promises and thenables: a promise fulfilled with another promise-like value adopts that value’s eventual state instead of exposing a promise as its fulfillment value. A thenable is an object with a callable then property; it does not have to be a native Promise. This is why returning a promise from a .then() callback or an async function keeps the chain connected. MDN describes this behavior in its Promise reference and the Promise constructor reference content.
This is not a recursive flattener for arrays or ordinary object properties. await resolves the promise-like value it receives; it does not traverse an array or object and await every nested property. Treat objects with custom then methods carefully, because awaiting them invokes thenable assimilation rather than treating them as inert data.
Quick Recap
Avoid common async/await mistakes
- Forgetting the outer promise:
const result = loadProfile(id)stores a promise, not the completed profile. Await it or handle its result and rejection. - Awaiting independent calls one by one: this delays starting the second call. If neither depends on the other and both results are required, start both and aggregate them.
- Adding
awaitmechanically: await when the current function needs the value, a local error boundary, or settlement-aware cleanup. Otherwise, directly returning the promise is often clearer.
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