Async/await was introduced in NodeJS 7.6 and is currently supported in all modern browsers. I believe it has been the single greatest addition to JS since 2017. If you are not convinced, here are a bunch of reasons with examples why you should adopt it immediately and never look back.
For those who have never heard of this topic before, here’s a quick intro
Assuming a function getJSON
that returns a promise, and that promise resolves with some JSON object. We just want to call it and log that JSON, then return "done"
.
This is how you would implement it using promises
const makeRequest = () => getJSON() .then(data => { console.log(data) return "done" })makeRequest()
And this is how it looks with async/await
const makeRequest = async () => {
console.log(await getJSON())
return “done”
}makeRequest()
There are a few differences here
async
before it. The await
keyword can only be used inside functions defined with async
. Any async
function returns a promise implicitly, and the resolve value of the promise will be whatever you return
from the function (which is the string “done”
in our case).await
at the top level of our code since that is not inside an async
function.// this will not work in top level
// await makeRequest()// this will work
makeRequest().then((result) => {
// do something
})
await getJSON()
means that the console.log
call will wait until getJSON()
promise resolves and print its value.1 - Concise and clean
Look at how much code we didn’t write! Even in the contrived example above, it’s clear we saved a decent amount of code. We didn’t have to write .then
, create an anonymous function to handle the response, or give a name data
to a variable that we don’t need to use. We also avoided nesting our code. These small advantages add up quickly, which will become more obvious in the following code examples.
2 - Error handling
Async/await makes it finally possible to handle both synchronous and asynchronous errors with the same construct, good old try/catch
. In the example below with promises, the try/catch
will not handle if JSON.parse
fails because it’s happening inside a promise. We need to call .catch
on the promise and duplicate our error handling code, which will (hopefully) be more sophisticated than console.log
in your production-ready code.
const makeRequest = () => {
try {
getJSON()
.then(result => {
// this parse may fail
const data = JSON.parse(result)
console.log(data)
})
// uncomment this block to handle asynchronous errors
// .catch((err) => {
// console.log(err)
// })
} catch (err) {
console.log(err)
}
}
Now look at the same code with async/await. The catch
block now will handle parsing errors.
const makeRequest = async () => {
try {
// this parse may fail
const data = JSON.parse(await getJSON())
console.log(data)
} catch (err) {
console.log(err)
}
}
3 - Conditionals
Imagine something like the code below which fetches some data and decides whether it should return that or get more details based on some value in the data.
const makeRequest = () => {
return getJSON()
.then(data => {
if (data.needsAnotherRequest) {
return makeAnotherRequest(data)
.then(moreData => {
console.log(moreData)
return moreData
})
} else {
console.log(data)
return data
}
})
}
Just looking at this gives you a headache. It’s easy to get lost in all that nesting (6 levels), braces, and return statements that are only needed to propagate the final result up to the main promise.
This example becomes way more readable when rewritten with async/await.
const makeRequest = async () => {
const data = await getJSON()
if (data.needsAnotherRequest) {
const moreData = await makeAnotherRequest(data);
console.log(moreData)
return moreData
} else {
console.log(data)
return data
}
}
4 - Intermediate values
You have probably found yourself in a situation where you call a promise1
and then use what it returns to call promise2
, then use the results of both promises to call a promise3
. Your code most likely looked like this
const makeRequest = () => {
return promise1()
.then(value1 => {
// do something
return promise2(value1)
.then(value2 => {
// do something
return promise3(value1, value2)
})
})
}
If promise3
didn’t require value1
it would be easy to flatten the promise nesting a bit. If you are the kind of person who couldn’t live with this, you could wrap both values 1 & 2 in a Promise.all
and avoid deeper nesting, like this
const makeRequest = () => {
return promise1()
.then(value1 => {
// do something
return Promise.all([value1, promise2(value1)])
})
.then(([value1, value2]) => {
// do something
return promise3(value1, value2)
})
}
This approach sacrifices semantics for the sake of readability. There is no reason for value1
& value2
to belong in an array together, except to avoid nesting promises.
This same logic becomes ridiculously simple and intuitive with async/await. It makes you wonder about all the things you could have done in the time that you spent struggling to make promises look less hideous.
const makeRequest = async () => {
const value1 = await promise1()
const value2 = await promise2(value1)
return promise3(value1, value2)
}
5 - Error stacks
Imagine a piece of code that calls multiple promises in a chain, and somewhere down the chain, an error is thrown.
const makeRequest = () => {
return callAPromise()
.then(() => callAPromise())
.then(() => callAPromise())
.then(() => callAPromise())
.then(() => callAPromise())
.then(() => {
throw new Error(“oops”);
})
}makeRequest()
.catch(err => {
console.log(err);
// output
// Error: oops at callAPromise.then.then.then.then.then (index.js:8:13)
})
The error stack returned from a promise chain gives no clue of where the error happened. Even worse, it’s misleading; the only function name it contains is callAPromise
which is totally innocent of this error (the file and line number are still useful though).
However, the error stack from async/await points to the function that contains the error
const makeRequest = async () => {
await callAPromise()
await callAPromise()
await callAPromise()
await callAPromise()
await callAPromise()
throw new Error(“oops”);
}makeRequest()
.catch(err => {
console.log(err);
// output
// Error: oops at makeRequest (index.js:7:9)
})
This is not a huge plus when you’re developing on your local environment and have the file open in an editor, but it’s quite useful when you’re trying to make sense of error logs coming from your production server. In such cases, knowing the error happened in makeRequest
is better than knowing that the error came from a then
after a then
after a then
…
6 - Debugging
A killer advantage when using async/await is that it’s much easier to debug. Debugging promises has always been such a pain for 2 reasons
Try setting a breakpoint anywhere here
.then
block and use debug shortcuts like step-over, the debugger will not move to the following .then
because it only “steps” through synchronous code.7 - You can await
anything
Last but not least, await
can be used for both synchronous and asynchronous expressions. For example, you can write await 5
, which is equivalent to Promise.resolve(5)
. This might not seem very useful at first, but it’s actually a great advantage when writing a library or a utility function where you don’t know whether the input will be sync or async.
Imagine you want to record the time taken to execute some API calls in your application, and you decide to create a generic function for this purpose. Here’s how it would look with promises
const recordTime = (makeRequest) => {
const timeStart = Date.now();
makeRequest().then(() => { // throws error for sync functions (.then is not a function)
const timeEnd = Date.now();
console.log(‘time take:’, timeEnd - timeStart);
})
}
You know that all API calls are going to return promises, but what happens if you use the same function to record the time taken in a synchronous function? It will throw an error because the sync function does not return a promise. The usual way to avoid this is wrapping makeRequest()
in Promise.resolve()
If you use async/await, you won’t have to worry about these cases because await allows you to work safely with any value, promise or not.
const recordTime = async (makeRequest) => {
const timeStart = Date.now();
await makeRequest(); // works for any sync or async function
const timeEnd = Date.now();
console.log(‘time take:’, timeEnd - timeStart);
}
Async/await is one of the most revolutionary features that have been added to JavaScript in the past few years. It makes you realize what a syntactical mess promises are, and provides an intuitive replacement.
Thanks for reading ❤
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