Async / Await

async functions

The async keyword before a function declaration makes it an async function. Async functions always return a Promise — even if you return a plain value, it is automatically wrapped in Promise.resolve(value).

async function greet() {
  return 'hello';
}

greet().then(function(v) { console.log(v); }); // hello

await

Inside an async function, await pauses execution until the Promise settles, then resumes with the resolved value. The rest of the program keeps running while awaiting.

async function loadUser() {
  var user = await fetchUser(42);   // pauses here until fetchUser resolves
  var posts = await fetchPosts(user.id);
  return posts;
}

Error handling

Wrap await expressions in try/catch to handle rejections:

async function run() {
  try {
    var data = await fetchData();
    console.log(data);
  } catch (err) {
    console.error('Failed:', err.message);
  }
}

Sequential vs concurrent

Two sequential await calls run one after the other — the second starts only after the first completes:

var a = await fetchA();  // waits
var b = await fetchB();  // then waits again

To run both concurrently, start the Promises first and await them together:

var [a, b] = await Promise.all([fetchA(), fetchB()]);

Runnable demo

The snippet below uses a fakeFetch helper (setTimeout-based) to simulate async calls. It first fetches sequentially, then fetches both in parallel with Promise.all.

JavaScript

function fakeFetch(label) {
  return new Promise(function(resolve) {
    setTimeout(function() { resolve(label); }, 0);
  });
}

async function runSequential() {
  var a = await fakeFetch('A');
  var b = await fakeFetch('B');
  console.log('Sequential: ' + a + ' then ' + b);
}

async function runParallel() {
  var results = await Promise.all([fakeFetch('A'), fakeFetch('B')]);
  console.log('Parallel: ' + results[0] + ' and ' + results[1]);
}

async function main() {
  await runSequential();
  await runParallel();
}

main();

Output: Sequential: A then B, then Parallel: A and B.