More recent additions to the JavaScript language are async functions and the
await keyword, part of the
so-called ECMAScript 2017 JavaScript edition (see ECMAScript Next support in
Mozilla). These features basically act as syntactic sugar on top of promises, making asynchronous code easier to
write and to read afterwards. They make async code look more like old-school synchronous code, so they're well worth
learning. This article gives you what you need to know.
| Prerequisites: | Basic computer literacy, a reasonable understanding of JavaScript fundamentals, an understanding of async code in general and promises. |
|---|---|
| Objective: | To understand promises and how to use them. |
There are two parts to using async/await in your code.
First of all we have the async keyword, which you put in front of a function declaration to turn it into
an async function. An async
function is a function that knows how to expect the possibility of the await keyword being used to invoke
asynchronous code.
Try typing the following lines into your browser's JS console:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">function hello() { return "Hello" }; hello();The function returns "Hello" — nothing special, right?
But what if we turn this into an async function? Try the following:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">async function hello() { return "Hello" }; hello();Ah. Invoking the function now returns a promise. This is one of the traits of async functions — their return values are guaranteed to be converted to promises.
You can also create an async function expression, like so:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">let hello = async function() { return "Hello" }; hello();And you can use arrow functions:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">let hello = async () => { return "Hello" };These all do basically the same thing.
To actually consume the value returned when the promise fulfills, since it is returning a promise, we could use a .then() block:
or even just shorthand such as
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">hello().then(console.log)Like we saw in the last article.
So the async keyword is added to functions to tell them to return a promise rather than directly returning the value.
The real advantage of async functions becomes apparent when you combine it with the await keyword — in fact, await
only works inside async functions. This can be put in front of any async promise-based function to pause
your code on that line until the promise fulfills, then return the resulting value.
You can use await when calling any function that returns a Promise, including web API functions.
Here is a trivial example:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">async function hello() { return greeting = await Promise.resolve("Hello"); }; hello().then(alert);Of course, the above example is not very useful, although it does serve to illustrate the syntax. Let's move on and look at a real example.
Let's look back at a simple fetch example that we saw in the previous article:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">fetch('coffee.jpg') .then(response => { if (!response.ok) { throw new Error(`HTTP error! status: ${response.status}`); } else { return response.blob(); } }) .then(myBlob => { let objectURL = URL.createObjectURL(myBlob); let image = document.createElement('img'); image.src = objectURL; document.body.appendChild(image); }) .catch(e => { console.log('There has been a problem with your fetch operation: ' + e.message); });By now, you should have a reasonable understanding of promises and how they work, but let's convert this to use async/await to see how much simpler it makes things:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">async function myFetch() { let response = await fetch('coffee.jpg'); if (!response.ok) { throw new Error(`HTTP error! status: ${response.status}`); } else { let myBlob = await response.blob(); let objectURL = URL.createObjectURL(myBlob); let image = document.createElement('img'); image.src = objectURL; document.body.appendChild(image); } } myFetch() .catch(e => { console.log('There has been a problem with your fetch operation: ' + e.message); });It makes code much simpler and easier to understand — no more .then() blocks everywhere!
Since an async keyword turns a function into a promise, you could refactor your code to use a hybrid approach of promises and await, bringing the second half of the function out into a new block to make it more flexible:
You can try typing in the example yourself, or running our live example (see also the source code).
You'll note that we've wrapped the code inside a function, and we've included the async keyword before the function keyword. This is necessary — you have to create an async function to define a block of code in which you'll run your async code; as we said earlier, await only works inside of async functions.
Inside the myFetch() function definition you can see that the code closely resembles the previous promise version, but there are some differences. Instead of needing to chain a .then() block on to the end of each promise-based method, you just need to add an await keyword before the method call, and then assign the result to a variable. The await keyword causes the JavaScript runtime to pause your code on this line, not allowing further code to execute in the meantime until the async function call has returned its result — very useful if subsequent code relies on that result!
Once that's complete, your code continues to execute starting on the next line. For example:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">let response = await fetch('coffee.jpg');The response returned by the fulfilled fetch() promise is assigned to the response variable
when that response becomes available, and the parser pauses on this line until that occurs. Once the response is
available, the parser moves to the next line, which creates a
Blob out of it. This line also invokes an async
promise-based method, so we use await here as well. When the result of operation returns, we return it
out of the myFetch() function.
This means that when we call the myFetch() function, it returns a promise, so we can chain a .then() onto the end of it inside which we handle displaying the blob onscreen.
You are probably already thinking "this is really cool!", and you are right — fewer .then() blocks to wrap around code, and it mostly just looks like synchronous code, so it is really intuitive.
And if you want to add error handling, you've got a couple of options.
You can use a synchronous
try...catch
structure with async/await. This example expands on the first version of the code we showed
above:
The catch() {} block is passed an error object, which we've called e; we can now log that to the console, and it will give us a detailed error message showing where in the code the error was thrown.
If you wanted to use the second (refactored) version of the code that we showed above, you would be better off just continuing the hybrid approach and chaining a .catch() block onto the end of the .then() call, like this:
This is because the .catch() block will catch errors occurring in both the async function call and the promise chain. If you used the try/catch block here, you might still get unhandled errors in the myFetch() function when it's called.
You can find both of these examples on GitHub:
async/await is built on top of promises, so it's compatible with all the
features offered by promises. This includes
Promise.all() —
you can quite happily await a Promise.all() call to get all the results returned into a variable in a way
that looks like simple synchronous code. Again, let's return to an example
we saw in our previous article. Keep it open in a separate tab so you can compare and contrast with the new
version shown below.
Converting this to async/await (see live demo and source code), this now looks like so:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">async function fetchAndDecode(url, type) { let response = await fetch(url); let content; if (!response.ok) { throw new Error(`HTTP error! status: ${response.status}`); } else { if(type === 'blob') { content = await response.blob(); } else if(type === 'text') { content = await response.text(); } return content; } } async function displayContent() { let coffee = fetchAndDecode('coffee.jpg', 'blob'); let tea = fetchAndDecode('tea.jpg', 'blob'); let description = fetchAndDecode('description.txt', 'text'); let values = await Promise.all([coffee, tea, description]); let objectURL1 = URL.createObjectURL(values[0]); let objectURL2 = URL.createObjectURL(values[1]); let descText = values[2]; let image1 = document.createElement('img'); let image2 = document.createElement('img'); image1.src = objectURL1; image2.src = objectURL2; document.body.appendChild(image1); document.body.appendChild(image2); let para = document.createElement('p'); para.textContent = descText; document.body.appendChild(para); } displayContent() .catch((e) => console.log(e) );You'll see that the fetchAndDecode() function has been converted easily into an async function with just a few changes. See the Promise.all() line:
By using await here we are able to get all the results of the three promises returned into the values array, when they are all available, in a way that looks very much like sync code. We've had to wrap all the code in a new async function, displayContent(), and we've not reduced the code by a lot of lines, but being able to move the bulk of the code out of the .then() block provides a nice, useful simplification, leaving us with a much more readable program.
For error handling, we've included a .catch() block on our displayContent() call; this will handle errors ocurring in both functions.
Note: It is also possible to use a sync
finally
block within an async function, in place of a
.finally()
async block, to show a final report on how the operation went — you can see this in action in our live
example (see also the source
code).
Async/await is really useful to know about, but there are a couple of downsides to consider.
Async/await makes your code look synchronous, and in a way it makes it behave more synchronously. The await keyword blocks execution of all the code that follows it until the promise fulfills, exactly as it would with a synchronous operation. It does allow other tasks to continue to run in the meantime, but the awaited code is blocked.
This means that your code could be slowed down by a significant number of awaited promises happening straight after one another. Each await will wait for the previous one to finish, whereas actually what you want is for the promises to begin processing simultaneously, like they would do if we weren't using async/await.
There is a pattern that can mitigate this problem — setting off all the promise processes by storing the Promise objects in variables, and then awaiting them all afterwards. Let's have a look at some examples that prove the concept.
We've got two examples available — slow-async-await.html
(see source
code) and fast-async-await.html
(see source
code). Both of them start off with a custom promise function that fakes an async process with a
setTimeout() call:
Then each one includes a timeTest() async function that awaits three timeoutPromise() calls:
Each one ends by recording a start time, seeing how long the timeTest() promise takes to fulfill, then recording an end time and reporting how long the operation took in total:
It is the timeTest() function that differs in each case.
In the slow-async-await.html example, timeTest() looks like this:
Here we await all three timeoutPromise() calls directly, making each one alert for 3 seconds. Each subsequent one is forced to wait until the last one finished — if you run the first example, you'll see the alert box reporting a total run time of around 9 seconds.
In the fast-async-await.html example, timeTest() looks like this:
Here we store the three Promise objects in variables, which has the effect of setting off their associated processes all running simultaneously.
Next, we await their results — because the promises all started processing at essentially the same time, the promises will all fulfill at the same time; when you run the second example, you'll see the alert box reporting a total run time of just over 3 seconds!
You'll have to test your code carefully, and bear this in mind if performance starts to suffer.
Another minor inconvenience is that you have to wrap your awaited promises inside an async function.
As a final note before we move on, you can even add async in front of class/object methods to make them
return promises, and await promises inside them. Take a look at the ES class code we saw in our
object-oriented JavaScript article, and then look at our modified version with an async method:
The first class method could now be used something like this:
pre data-role="codeBlock" data-info="js" class="language-javascript" class="brush: js notranslate">han.greeting().then(console.log);One consideration when deciding whether to use async/await is support for older browsers. They are available in modern versions of most browsers, the same as promises; the main support problems come with Internet Explorer and Opera Mini.
If you want to use async/await but are concerned about older browser support, you could consider using the BabelJS library — this allows you to write your applications using the latest JavaScript and let Babel figure out what changes if any are needed for your user’s browsers. On encountering a browser that does not support async/await, Babel's polyfill can automatically provide fallbacks that work in older browsers.
And there you have it — async/await provide a nice, simplified way to write async code that is simpler to read and maintain. Even with browser support being more limited than other async code mechanisms at the time of writing, it is well worth learning and considering for use, both for now and in the future.
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