expression

A Promise, a thenable object, or any value to wait for.

The fulfillment value of the promise or thenable object, or, if the expression is not thenable, the expression's own value.

Throws the rejection reason if the promise or thenable object is rejected.

await is usually used to unwrap promises by passing a Promise as the expression. Using await pauses the execution of its surrounding async function until the promise is settled (that is, fulfilled or rejected). When execution resumes, the value of the await expression becomes that of the fulfilled promise.

If the promise is rejected, the await expression throws the rejected value. The function containing the await expression will appear in the stack trace of the error. Otherwise, if the rejected promise is not awaited or is immediately returned, the caller function will not appear in the stack trace.

The expression is resolved in the same way as Promise.resolve(): it's always converted to a native Promise and then awaited. If the expression is a:

• Native Promise (which means expression belongs to Promise or a subclass, and expression.constructor === Promise): The promise is directly used and awaited natively, without calling then().
• Thenable object (including non-native promises, polyfill, proxy, child class, etc.): A new promise is constructed with the native Promise() constructor by calling the object's then() method and passing in a handler that calls the resolve callback.
• Non-thenable value: An already-fulfilled Promise is constructed and used.

Even when the used promise is already fulfilled, the async function's execution still pauses until the next tick. In the meantime, the caller of the async function resumes execution. See example below.

Because await is only valid inside async functions and modules, which themselves are asynchronous and return promises, the await expression never blocks the main thread and only defers execution of code that actually depends on the result, i.e. anything after the await expression.

If a Promise is passed to an await expression, it waits for the Promise to be fulfilled and returns the fulfilled value.

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  const x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}

f1();

Thenable objects are resolved just the same as actual Promise objects.

async function f() {
  const thenable = {
    then(resolve, _reject) {
      resolve("resolved!");
    },
  };
  console.log(await thenable); // "resolved!"
}

f();

They can also be rejected:

async function f() {
  const thenable = {
    then(resolve, reject) {
      reject(new Error("rejected!"));
    },
  };
  await thenable; // Throws Error: rejected!
}

f();

If the value is not a Promise, await converts the value to a resolved Promise, and waits for it. The awaited value's identity doesn't change as long as it doesn't have a then property that's callable.

async function f3() {
  const y = await 20;
  console.log(y); // 20

  const obj = {};
  console.log((await obj) === obj); // true
}

f3();

If the Promise is rejected, the rejected value is thrown.

async function f4() {
  try {
    const z = await Promise.reject(30);
  } catch (e) {
    console.error(e); // 30
  }
}

f4();

You can handle rejected promises without a try block by chaining a catch() handler before awaiting the promise.

const response = await promisedFunction().catch((err) => {
  console.error(err);
  return "default response";
});
// response will be "default response" if the promise is rejected

This is built on the assumption that promisedFunction() never synchronously throws an error, but always returns a rejected promise. This is the case for most properly-designed promise-based functions, which usually look like:

function promisedFunction() {
  // Immediately return a promise to minimize chance of an error being thrown
  return new Promise((resolve, reject) => {
    // do something async
  });
}

However, if promisedFunction() does throw an error synchronously, the error won't be caught by the catch() handler. In this case, the try...catch statement is necessary.

You can use the await keyword on its own (outside of an async function) at the top level of a module. This means that modules with child modules that use await will wait for the child modules to execute before they themselves run, all while not blocking other child modules from loading.

Here is an example of a simple module using the Fetch API and specifying await within the export statement. Any modules that include this will wait for the fetch to resolve before running any code.

// fetch request
const colors = fetch("../data/colors.json").then((response) => response.json());

export default await colors;

When an await is encountered in code (either in an async function or in a module), the awaited expression is executed, while all code that depends on the expression's value is paused and pushed into the microtask queue. The main thread is then freed for the next task in the event loop. This happens even if the awaited value is an already-resolved promise or not a promise. For example, consider the following code:

async function foo(name) {
  console.log(name, "start");
  console.log(name, "middle");
  console.log(name, "end");
}

foo("First");
foo("Second");

// First start
// First middle
// First end
// Second start
// Second middle
// Second end

In this case, the two async functions are synchronous in effect, because they don't contain any await expression. The three statements happen in the same tick. In promise terms, the function corresponds to:

function foo(name) {
  return new Promise((resolve) => {
    console.log(name, "start");
    console.log(name, "middle");
    console.log(name, "end");
    resolve();
  });
}

However, as soon as there's one await, the function becomes asynchronous, and execution of following statements is deferred to the next tick.

async function foo(name) {
  console.log(name, "start");
  await console.log(name, "middle");
  console.log(name, "end");
}

foo("First");
foo("Second");

// First start
// First middle
// Second start
// Second middle
// First end
// Second end

This corresponds to:

function foo(name) {
  return new Promise((resolve) => {
    console.log(name, "start");
    resolve(console.log(name, "middle"));
  }).then(() => {
    console.log(name, "end");
  });
}

While the extra then() handler is not necessary, and the handler can be merged with the executor passed to the constructor, the then() handler's existence means the code will take one extra tick to complete. The same happens for await. Therefore, make sure to use await only when necessary (to unwrap promises into their values).

Other microtasks can execute before the async function resumes. This example uses queueMicrotask() to demonstrate how the microtask queue is processed when each await expression is encountered.

let i = 0;

queueMicrotask(function test() {
  i++;
  console.log("microtask", i);
  if (i < 3) {
    queueMicrotask(test);
  }
});

(async () => {
  console.log("async function start");
  for (let i = 1; i < 3; i++) {
    await null;
    console.log("async function resume", i);
  }
  await null;
  console.log("async function end");
})();

queueMicrotask(() => {
  console.log("queueMicrotask() after calling async function");
});

console.log("script sync part end");

// Logs:
// async function start
// script sync part end
// microtask 1
// async function resume 1
// queueMicrotask() after calling async function
// microtask 2
// async function resume 2
// microtask 3
// async function end

In this example, the test() function is always called before the async function resumes, so the microtasks they each schedule are always executed in an intertwined fashion. On the other hand, because both await and queueMicrotask() schedule microtasks, the order of execution is always based on the order of scheduling. This is why the "queueMicrotask() after calling async function" log happens after the async function resumes for the first time.

Sometimes, the await is omitted when a promise is directly returned from an async function.

async function noAwait() {
  // Some actions...

  return /* await */ lastAsyncTask();
}

However, consider the case where lastAsyncTask asynchronously throws an error.

async function lastAsyncTask() {
  await null;
  throw new Error("failed");
}

async function noAwait() {
  return lastAsyncTask();
}

noAwait();

// Error: failed
//    at lastAsyncTask

Only lastAsyncTask appears in the stack trace, because the promise is rejected after it has already been returned from noAwait — in some sense, the promise is unrelated to noAwait. To improve the stack trace, you can use await to unwrap the promise, so that the exception gets thrown into the current function. The exception will then be immediately wrapped into a new rejected promise, but during error creation, the caller will appear in the stack trace.

async function lastAsyncTask() {
  await null;
  throw new Error("failed");
}

async function withAwait() {
  return await lastAsyncTask();
}

withAwait();

// Error: failed
//    at lastAsyncTask
//    at async withAwait

However, there's a little performance penalty coming with return await because the promise has to be unwrapped and wrapped again.

• async function
• async function expression
• AsyncFunction
• Top-level await on v8.dev (2019)