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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsCallbacks, Promises, and async/await are ways to arrange JavaScript code that responds to work completing later. A callback is a function invoked at completion; a Promise represents a future success or failure; and async/await provides readable syntax for working with Promises. None of them, by itself, makes an operation run in the background or blocks all JavaScript.
What is a callback in JavaScript?
A callback is a function passed to another function or API so it can be called when an event or operation completes. Callbacks are also used for ordinary synchronous tasks; the term alone does not mean that code is asynchronous.
For example, setTimeout registers a function for later execution:
setTimeout(() => {
console.log("finished later");
}, 0);
console.log("runs first");The current synchronous code finishes before the timer callback runs, even with a delay of zero. JavaScript processes one statement at a time; when an asynchronous action completes, the host schedules its callback as a job to run after the current execution stack is empty. The browser or server runtime coordinates the timer, network request, or other external work; the callback is the JavaScript code that handles completion. MDN’s event-loop guide explains this job-queue model.
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Why nested callbacks get difficult
When each operation depends on the previous one, callbacks can nest inside callbacks. This “pyramid of doom” makes it harder to follow the order of work and see which errors each step handles. Callback APIs also use different error conventions, such as an error-first argument or a separate error callback, so check the API’s documentation and make both success and failure paths explicit. Nesting is a readability problem; callbacks themselves do not create concurrency.
What is a Promise?
A Promise is an object representing the eventual completion or failure of an asynchronous operation and its resulting value, as MDN’s Promise reference puts it. It begins pending and eventually becomes fulfilled with a value or rejected with a reason. You handle its result with .then(), .catch(), and, when cleanup should happen regardless of outcome, .finally().
fetch("/data.json")
.then(response => {
if (!response.ok) throw new Error(`HTTP ${response.status}`);
return response.json();
})
.then(data => console.log(data))
.catch(error => console.error(error));Here, the first .then() checks the HTTP response and returns the Promise for parsing its body. The next handler receives the parsed data. Throwing an error rejects the chain, so the final .catch() can handle a failure from either step. A fulfilled fetch Promise does not necessarily mean the server returned a successful HTTP status; that is why the example checks response.ok.
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How Promise chains compose
Each .then() returns a new Promise. Returning a value passes it to the next handler; returning another Promise makes the chain wait for that Promise. Throwing in a handler rejects the Promise returned by that step. Return Promises from handlers when later steps need to wait for them—otherwise the chain may finish before the nested work does.
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How async/await works
An async function always returns a Promise. Returning a plain value fulfills that Promise with the value; throwing an error rejects it. Inside the function, await lets you write Promise-dependent steps in a top-to-bottom style:
async function loadData() {
try {
const response = await fetch("/data.json");
if (!response.ok) throw new Error(`HTTP ${response.status}`);
return await response.json();
} catch (error) {
console.error("Loading failed", error);
throw error;
}
}When an awaited Promise settles, the function continues with its result, or an error is thrown at the await expression if the Promise rejects. The try/catch handles either a rejected fetch or an error raised while checking or parsing the response. Rethrowing keeps the returned Promise rejected so the caller can decide how to respond. MDN notes that “Async functions can contain zero or more await expressions.” See MDN’s async function reference.
Does await block JavaScript?
No. await suspends the current async function while the awaited Promise settles; it does not freeze unrelated JavaScript work. Other code can run while the function is waiting. Once the Promise settles, the suspended function resumes through JavaScript’s scheduling model. await is valid inside an async function, and JavaScript modules also allow top-level await.
Async/await is Promise-based syntax, not a separate asynchronous mechanism. It makes dependent steps easier to read, but the underlying operation still relies on the host environment and Promise behavior.
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Callbacks vs. Promises vs. async/await
| Style | How completion is handled | Dependent steps | Error handling | Best fit |
|---|---|---|---|---|
| Callback | A function is invoked by the API on completion. | Often nested as each step starts the next. | Depends on the API’s documented convention. | APIs that provide callbacks, or simple event handlers. |
| Promise | Handlers observe eventual fulfillment or rejection. | Composed with chained .then() calls. |
Rejections can flow down the chain to .catch(). |
Composing asynchronous results and integrating Promise-returning APIs. |
| async/await | Uses Promises with a sequential-looking syntax. | Reads top to bottom when each step depends on the prior result. | Rejected awaits can be caught with try/catch; the async function still returns a Promise. |
Making Promise-based workflows easier to read and maintain. |
These styles are not three competing ways to make an operation run in the background. Callbacks and Promises both handle completion; async/await changes how Promise-based code is written. A callback-based API can be wrapped or adapted to work with Promises, but whether to do so depends on the API and application.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When to use sequential awaits or Promise.all
Use sequential await when a later operation needs the earlier result. For example, the orders request needs the user ID:
const user = await getUser();
const orders = await getOrders(user.id);When operations are independent, start them together and wait for their combined results with Promise.all():
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchconst [profile, recommendations] = await Promise.all([
getProfile(),
getRecommendations()
]);If you instead await independent operations one at a time, the second does not start until the first has settled, adding avoidable delay. Promise.all() fulfills with results in input order when every input fulfills; it rejects as soon as one input rejects. It does not make the underlying operations parallel by magic—the operations must be started before the combined wait, and the host environment determines how they proceed. MDN recommends Promise.all() for independent operations in its Promise guide.
How to handle errors in each style
- Callbacks: Follow the API’s documented error convention. If it passes an error argument, check it before using the success result; do not assume every callback API uses the same pattern.
- Promise chains: Return each Promise that downstream handlers must wait for, then attach
.catch()at a boundary that can handle or report the failure. A.catch()handler can itself throw or return a rejected Promise, so that failure also needs an appropriate handler. - async/await: Put related awaited work in
try/catchwhen the current function can recover or add useful context. Rethrow if the caller should make the recovery or presentation decision.
Async/await does not remove rejection handling. If a returned Promise rejects and no part of the application handles it, the application still needs a policy for that unhandled failure.
Compatibility to check
MDN labels Promise support broadly available and dates cross-browser availability to July 2015. Its current Promise reference notes that Opera Mini and Internet Explorer 11 and earlier are notable compatibility exceptions. Check the browser and runtime versions your application actually targets rather than treating broad availability as universal support.
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