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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →You can’t get a timer callback’s return value from setTimeout(): the call returns a timer ID immediately, while the callback runs later. To use a value produced by delayed code, pass it to another callback or return a Promise that resolves with the value.
Why the callback’s return value is unavailable
setTimeout() schedules a function to run after a delay; it does not pause the current code until that function finishes. MDN describes it as an asynchronous function that returns immediately after scheduling its callback. The value returned by setTimeout() is a timer identifier, not the result of the callback. See MDN’s setTimeout() reference.
const timerId = setTimeout(() => {
return getValue(); // Returned only from this callback
}, 1000);
console.log(timerId); // Timer identifier, not getValue()'s resultReturning the timer call from another function does not change that:
function getValueLater() {
return setTimeout(() => getValue(), 1000);
}getValueLater() returns the timer identifier. The callback’s return happens in a later, separate function call and does not travel back to the caller.
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Use a callback to pass the eventual value
If the surrounding code already uses callbacks, call the supplied function with the result when the timer fires:
function getValueLater(callback) {
setTimeout(() => {
callback(getValue());
}, 1000);
}
getValueLater((value) => {
console.log(value);
});Put work that depends on value inside the callback. Code immediately after getValueLater(...) runs before the timer callback.
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Return a Promise when callers need to await the value
A Promise represents a value that may become available later, or a failure. Resolve it inside the timer callback, then let the caller use await or .then(). MDN demonstrates this timer-wrapping pattern in its guide to using promises.
function getValueLater() {
return new Promise((resolve) => {
setTimeout(() => {
resolve(getValue());
}, 1000);
});
}
async function showValue() {
const value = await getValueLater();
console.log(value);
}
showValue();Here, getValueLater() returns the Promise—not the eventual value synchronously. The await expression inside the async function produces the value after the Promise resolves. await must be used in an async function or a supported module context; MDN documents its use in async functions.
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You can also consume the Promise with .then():
getValueLater().then((value) => {
console.log(value);
});Use the Promise approach when callers need to compose asynchronous work or control what happens after completion. A Promise returned by .then() can be chained, with each handler’s result determining the next Promise’s value; see MDN’s then() reference.
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Choose the pattern that fits the surrounding code
| Pattern | How the value arrives | Best fit |
|---|---|---|
| Callback | Invoke a supplied function with the eventual value. | Existing callback-oriented APIs or a small event handler. |
| Promise | Resolve with the eventual value; consume it with .then() or await. |
Composable asynchronous functions and code already using Promises. |
Common mistakes
- Returning from the timer callback: that return belongs to the callback invocation, not the function that scheduled it.
- Writing
return setTimeout(...): this returns the timer identifier. - Writing
await setTimeout(...):setTimeout()does not return a Promise for the callback result. Wrap the timer in a Promise first. - Reading a variable right after scheduling: the callback has not run yet. Move dependent work into the callback or after the Promise resolves.
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