Await

In computer programming, await is a feature found in C# 5.0, Python 3.5, Hack, Dart, Kotlin 1.1, in an experimental extension for Scala,^[1] and more recently JavaScript that allows an asynchronous, non-blocking method call to be performed in a similar way to an ordinary synchronous method call.

While a casual reading of the code would suggest that the method call blocks until the requested data is available, in fact it does not.

In C#

In C# versions before C# 7, async methods are required to return either void, Task, or Task<T>. This has been expanded in C# 7 to include certain other types such as ValueTask<T>. Async methods that return void are intended for event handlers; in most cases where a synchronous method would return void, returning Task instead is recommended, as it allows for more intuitive exception handling.^[2]

Methods that make use of await must be declared with the async keyword. In methods that have a return value of type Task<T>, methods declared with async must have a return statement of type assignable to T instead of Task<T>; the compiler wraps the value in the Task<T> generic. It is also possible to await methods that have a return type of Task or Task<T> that are declared without async.

The following async method downloads data from a URL using await.

public async Task<int> SumPageSizesAsync(IList<Uri> uris) 
{
    int total = 0;
    foreach (var uri in uris) {
        statusText.Text = string.Format("Found {0} bytes ...", total);
        var data = await new WebClient().DownloadDataTaskAsync(uri);
        total += data.Length;
    }
    statusText.Text = string.Format("Found {0} bytes total", total);
    return total;
}

In Scala

In the experimental Scala-async extension to Scala, await is a "method", although it does not operate like an ordinary method. Furthermore, unlike in C# 5.0 in which a method must be marked as async, in Scala-async, a block of code is surrounded by an async "call".

How it works

In Scala-async, async is actually implemented using a Scala macro, which causes the compiler to emit different code, and produce a finite state machine implementation (which is considered to be more efficient than a monadic implementation, but less convenient to write by hand).

There are plans for Scala-async to support a variety of different implementations, including non-asynchronous ones.

In Python

Python 3.5 has added support for Async/Await as described in PEP0492 (https://www.python.org/dev/peps/pep-0492/).

In JavaScript

The await operator in JavaScript can only be used from inside an async function. If the parameter is a promise, execution of the async function will resume when the promise is resolved (unless the promise is rejected, in which case an error will be thrown that can be handled with normal JavaScript exception handling.) If the parameter is not a promise, the parameter itself will be returned immediately.^[3]

Many libraries provide promise objects that can also be used with await, as long as they match the specification for native JavaScript promises. However, promises from the jQuery library were not Promises/A+ compatible until jQuery 3.0.^[4]

Here's an example (modified from this^[5] article):

async function createNewDoc() {
  let response = await db.post({}); // post a new doc
  return await db.get(response.id); // find by id
}

~async function main() {
  try {
    let doc = await createNewDoc();
    console.log(doc);
  } catch (err) {
    console.log(err);
  }
}()

Node.js version 8 includes a utility that enables using the standard library callback-based methods as promises.^[6]

See also

• Coroutines
• Continuation-passing style

References