async function
==============

Baseline Widely available
-------------------------

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2017.

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The **`async function`** declaration creates a binding of a new async function to a given name. The `await` keyword is permitted within the function body, enabling asynchronous, promise-based behavior to be written in a cleaner style and avoiding the need to explicitly configure promise chains.

You can also define async functions using the `async function` expression.

Try it
------

Syntax
------


```js
async function name(param0) {
  statements
}
async function name(param0, param1) {
  statements
}
async function name(param0, param1, /\* …, \*/ paramN) {
  statements
}
```


**Note:** There cannot be a line terminator between `async` and `function`, otherwise a semicolon is automatically inserted, causing `async` to become an identifier and the rest to become a `function` declaration.

### Parameters

`name`

The function's name.

`param` Optional

The name of a formal parameter for the function. For the parameters' syntax, see the Functions reference.

`statements` Optional

 The statements comprising the body of the function. The `await`
 mechanism may be used.


Description
-----------

An `async function` declaration creates an `AsyncFunction` object. Each time when an async function is called, it returns a new `Promise` which will be resolved with the value returned by the async function, or rejected with an exception uncaught within the async function.

Async functions can contain zero or more `await` expressions. Await expressions make promise-returning functions behave as though they're synchronous by suspending execution until the returned promise is fulfilled or rejected. The resolved value of the promise is treated as the return value of the await expression. Use of `async` and `await` enables the use of ordinary `try` / `catch` blocks around asynchronous code.

**Note:** The `await` keyword is only valid inside async functions within regular JavaScript code. If you use it outside of an async function's body, you will get a `SyntaxError`.

`await` can be used on its own with JavaScript modules.


**Note:** The purpose of `async`/`await` is to simplify the syntax
 necessary to consume promise-based APIs. The behavior
 of `async`/`await` is similar to combining generators and
 promises.


 Async functions always return a promise. If the return value of an async function is
 not explicitly a promise, it will be implicitly wrapped in a promise.

For example, consider the following code:


```js
async function foo() {
  return 1;
}
```


It is similar to:


```js
function foo() {
  return Promise.resolve(1);
}
```


**Note:**

Even though the return value of an async function behaves as if it's wrapped in a `Promise.resolve`, they are not equivalent.

An async function will return a different *reference*, whereas `Promise.resolve` returns the same reference if the given value is a promise.

It can be a problem when you want to check the equality of a promise and a return value of an async function.


```js
const p = new Promise((res, rej) => {
  res(1);
});

async function asyncReturn() {
  return p;
}

function basicReturn() {
  return Promise.resolve(p);
}

console.log(p === basicReturn()); // true
console.log(p === asyncReturn()); // false
```


 The body of an async function can be thought of as being split by zero or more await
 expressions. Top-level code, up to and including the first await expression (if there is
 one), is run synchronously. In this way, an async function without an await expression
 will run synchronously. If there is an await expression inside the function body,
 however, the async function will always complete asynchronously.

For example:


```js
async function foo() {
  await 1;
}
```


It is also equivalent to:


```js
function foo() {
  return Promise.resolve(1).then(() => undefined);
}
```


 Code after each await expression can be thought of as existing in a `.then`
 callback. In this way a promise chain is progressively constructed with each reentrant
 step through the function. The return value forms the final link in the chain.

 In the following example, we successively await two promises. Progress moves through
 function `foo` in three stages.

1. The first line of the body of function `foo` is executed synchronously,
 with the await expression configured with the pending promise. Progress through
 `foo` is then suspended and control is yielded back to the function that
 called `foo`.
2. Some time later, when the first promise has either been fulfilled or rejected,
 control moves back into `foo`. The result of the first promise fulfillment
 (if it was not rejected) is returned from the await expression. Here `1` is
 assigned to `result1`. Progress continues, and the second await expression
 is evaluated. Again, progress through `foo` is suspended and control is
 yielded.
3. Some time later, when the second promise has either been fulfilled or rejected,
 control re-enters `foo`. The result of the second promise resolution is
 returned from the second await expression. Here `2` is assigned to
 `result2`. Control moves to the return expression (if any). The default
 return value of `undefined` is returned as the resolution value of the
 current promise.


```js
async function foo() {
  const result1 = await new Promise((resolve) =>
    setTimeout(() => resolve("1")),
  );
  const result2 = await new Promise((resolve) =>
    setTimeout(() => resolve("2")),
  );
}
foo();
```


 Note how the promise chain is not built-up in one go. Instead, the promise chain is
 constructed in stages as control is successively yielded from and returned to the async
 function. As a result, we must be mindful of error handling behavior when dealing with
 concurrent asynchronous operations.

 For example, in the following code an unhandled promise rejection error will be thrown,
 even if a `.catch` handler has been configured further along the promise
 chain. This is because `p2` will not be "wired into" the promise chain until
 control returns from `p1`.


```js
async function foo() {
  const p1 = new Promise((resolve) => setTimeout(() => resolve("1"), 1000));
  const p2 = new Promise((\_, reject) => setTimeout(() => reject("2"), 500));
  const results = [await p1, await p2]; // Do not do this! Use Promise.all or Promise.allSettled instead.
}
foo().catch(() => {}); // Attempt to swallow all errors...
```


`async function` declarations behave similar to `function` declarations — they are hoisted to the top of their scope and can be called anywhere in their scope, and they can be redeclared only in certain contexts.

Examples
--------

### Async functions and execution order


```js
function resolveAfter2Seconds() {
  console.log("starting slow promise");
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("slow");
      console.log("slow promise is done");
    }, 2000);
  });
}

function resolveAfter1Second() {
  console.log("starting fast promise");
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("fast");
      console.log("fast promise is done");
    }, 1000);
  });
}

async function sequentialStart() {
  console.log("== sequentialStart starts ==");

  // 1. Start a timer, log after it's done
  const slow = resolveAfter2Seconds();
  console.log(await slow);

  // 2. Start the next timer after waiting for the previous one
  const fast = resolveAfter1Second();
  console.log(await fast);

  console.log("== sequentialStart done ==");
}

async function sequentialWait() {
  console.log("== sequentialWait starts ==");

  // 1. Start two timers without waiting for each other
  const slow = resolveAfter2Seconds();
  const fast = resolveAfter1Second();

  // 2. Wait for the slow timer to complete, and then log the result
  console.log(await slow);
  // 3. Wait for the fast timer to complete, and then log the result
  console.log(await fast);

  console.log("== sequentialWait done ==");
}

async function concurrent1() {
  console.log("== concurrent1 starts ==");

  // 1. Start two timers concurrently and wait for both to complete
  const results = await Promise.all([
    resolveAfter2Seconds(),
    resolveAfter1Second(),
  ]);
  // 2. Log the results together
  console.log(results[0]);
  console.log(results[1]);

  console.log("== concurrent1 done ==");
}

async function concurrent2() {
  console.log("== concurrent2 starts ==");

  // 1. Start two timers concurrently, log immediately after each one is done
  await Promise.all([
    (async () => console.log(await resolveAfter2Seconds()))(),
    (async () => console.log(await resolveAfter1Second()))(),
  ]);
  console.log("== concurrent2 done ==");
}

sequentialStart(); // after 2 seconds, logs "slow", then after 1 more second, "fast"

// wait above to finish
setTimeout(sequentialWait, 4000); // after 2 seconds, logs "slow" and then "fast"

// wait again
setTimeout(concurrent1, 7000); // same as sequentialWait

// wait again
setTimeout(concurrent2, 10000); // after 1 second, logs "fast", then after 1 more second, "slow"
```


#### await and concurrency

 In `sequentialStart`, execution suspends 2 seconds for the first
 `await`, and then another second for the second `await`. The
 second timer is not created until the first has already fired, so the code finishes
 after 3 seconds.

 In `sequentialWait`, both timers are created and then `await`ed.
 The timers run concurrently, which means the code finishes in 2 rather than 3 seconds,
 i.e. the slowest timer.
 However, the `await` calls still run in series, which means the second
 `await` will wait for the first one to finish. In this case, the result of
 the fastest timer is processed after the slowest.

 If you wish to safely perform other jobs after two or more jobs run concurrently and are complete, you must await a call
 to `Promise.all()` or `Promise.allSettled()` before that job.


**Warning:** The functions `sequentialWait` and `concurrent1`
 are not functionally equivalent.


 In `sequentialWait`, if promise `fast` rejects before promise
 `slow` is fulfilled, then an unhandled promise rejection error will be
 raised, regardless of whether the caller has configured a catch clause.


 In `concurrent1`, `Promise.all` wires up the promise
 chain in one go, meaning that the operation will fail-fast regardless of the order of
 rejection of the promises, and the error will always occur within the configured
 promise chain, enabling it to be caught in the normal way.


### Rewriting a Promise chain with an async function

 An API that returns a `Promise` will result in a promise chain, and it
 splits the function into many parts. Consider the following code:


```js
function getProcessedData(url) {
  return downloadData(url) // returns a promise
    .catch((e) => downloadFallbackData(url)) // returns a promise
    .then((v) => processDataInWorker(v)); // returns a promise
}
```


it can be rewritten with a single async function as follows:


```js
async function getProcessedData(url) {
  let v;
  try {
    v = await downloadData(url);
  } catch (e) {
    v = await downloadFallbackData(url);
  }
  return processDataInWorker(v);
}
```


Alternatively, you can chain the promise with `catch()`:


```js
async function getProcessedData(url) {
  const v = await downloadData(url).catch((e) => downloadFallbackData(url));
  return processDataInWorker(v);
}
```


 In the two rewritten versions, notice there is no `await` statement after the
 `return` keyword, although that would be valid too: The return value of an
 async function is implicitly wrapped in `Promise.resolve` - if
 it's not already a promise itself (as in the examples).

Specifications
--------------

| Specification |
| --- |
| ECMAScript Language Specification # sec-async-function-definitions |

Browser compatibility
---------------------

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See also
--------

* Functions guide
* Using promises guide
* Functions
* `AsyncFunction`
* `async function` expression
* `function`
* `function*`
* `async function*`
* `await`
* `Promise`
* Decorating async JavaScript functions on innolitics.com (2016)