async / await

                async function f() { const x = await somePromise; }
              

Purpose

An async function always returns a promise. Inside it, await pauses execution at that line until the awaited promise settles, then resumes with its resolved value (or throws, if it rejected) — all without blocking anything outside that function.

Syntax

async function getGreeting() {
  try {
    const name = await lookUpName();
    return `Hello, ${name}!`;
  } catch (error) {
    console.error('Could not look up name:', error.message);
    return 'Hello, stranger!';
  }
}

getGreeting().then((greeting) => console.log(greeting));

Behavior notes

• await can only be used directly inside an async function (or, in modern module code, at the top level of a module).
• A rejected awaited promise throws inside the async function — catch it with an ordinary try/catch, exactly as with a synchronous throw.
• An async function’s return value is automatically wrapped in a resolved promise, even if the function body doesn’t explicitly return a promise itself.
• Awaiting several independent promises one at a time (await a(); await b();) runs them sequentially, wasting time if they don’t depend on each other — use Promise.all (see Promise) to run them concurrently instead.

Common mistakes

• Assuming await blocks the entire program rather than just the function it’s in.
• Awaiting requests sequentially that could run concurrently with Promise.all.
• Forgetting await before a promise-returning call, which leaves you with an unresolved Promise object instead of its eventual value.

Runtime notes

Core ECMAScript (ES2017), identical behavior across every current browser and Node version.