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async

JavaScript Basics

In one line

The async keyword declares a function that always returns a promise, enabling await inside it.

In simple words

The async keyword marks a function as asynchronous. An async function always returns a promise — if you return a plain value, JavaScript wraps it in a resolved promise automatically. The real power of async is that it lets you use await inside the function body.

Before async/await, asynchronous code used nested callbacks or promise chains. With async, you can write async code that reads top-to-bottom like synchronous code, while still being non-blocking. The event loop keeps running while your function waits.

Marking a function async does not make it run on a separate thread — JavaScript is single-threaded. It only changes how the function's return value is wrapped and enables await. The actual concurrency comes from the underlying APIs (fetch, timers, etc.).

Example

javascript
async function getUser(id) {
  const response = await fetch(`/api/users/${id}`);
  const user = await response.json();
  return user;
}

getUser(42).then((user) => console.log(user));

async lets you use await inside. The function returns a promise that resolves with the user.

Common confusions

  • Confused with: await

    The difference: async declares a function that returns a promise; await pauses execution inside an async function until a promise settles. You use them together: async enables await.

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