I often find myself looking up patterns for converting synchronous code into async variants in Javascript. Whether it’s remembering how exactly Promise chains work or what I need to do to create a Promise - there always seems to be one part that trips me up.
I wanted to document a simple, but I think representative, example of how to take a synchronous function and convert it to be asynchronous in Javascript.
I’ll be using a node function that is reading a file from the file system.
The original function is:
const findAssetSync = (name) => {
const assetPath = path.join(__dirname, 'assets', name)
return fs.readFileSync(assetPath, {encoding: 'utf-8'}).toString()
}
The first step is to make this function return a promise instead.
const findAssetAsync = (name) => {
const assetPath = path.join(__dirname, 'assets', name)
return new Promise((resolve, reject) => {
fs.readFile(assetPath, {encoding: 'utf-8'}, (err, data) => {
if (err) reject(err);
return resolve(data);
})
})
}
Now, let’s look at how this would actually be used. I’ll start with the synchronous version.1
const server = http.createServer((req, res) => {
const route = url.parse(req.url).pathname
if (routes[route]) {
const assets = findAssetSync(routes[route])
res.write(assets)
res.end()
} else {
res.writeHead(404, ‘Not Found’)
res.end()
}
})
To use the asynchronous version, however, we either need to convert the callback within createServer
into an Async/Await
function or now use a promise chain.
The point, however, is that now, instead of returning the string itself as we do in findAssetSync
, findAssetAsync
returns a promise.
Using Promise Chain
Promise chains create some complexity. Because we want to to make sure we resolve before moving onto the writing of the server response - we can’t do this:
const server = http.createServer(req, res) => {
/* ... */
if (routes[route]) {
let assets = findAssetAsync(routes[route]).then(results => {
assets = results;
}).catch(err => console.error(err))
res.write(assets)
res.end()
} else {
/* ... */
}
})
This would error, because while the promise is resolving, node would just continue moving along and reading the file — so we’d write assets (which would be undefined at the time) and then end the response.
To handle this - we place the response inside the .then
block:
const server = http.createServer(req, res) => {
/* ... */
if (routes[route]) {
findAssetAsync(routes[route]).then(results => {
res.write(results)
res.end()
}).catch(err => console.error(err))
} else {
/* ... */
}
})
It’s easy to see how, if this were to get much more complicated - and/or you wanted to carry variables forward (instead of just using the “response” variable from a Promise) how this can quickly get messy.2
Using Async/Await
The async await syntax is much simpler to reason through. While it’s not creating synchronous code - it reads as if it is. And underneath, it’s all just using Promises.
const server = http.createServer( async (req, res) => {
/* ... */
if (routes[route]) {
const assets = await findAssetAsync(routes[route])
res.write(assets)
/* ... */
} else {
/* ... */
}
})
That’s it. We’re now waiting for the Async function to resolve before preceeding - all while not blocking other requests.
Conclusion
Converting from synchronous to asynchronous javascript code is not particularly difficult. It’s a matter of understanding what is actually happening with the event loop and then pattern recognition.
Footnotes
-
1 I have simplified the code here a bit. For example, we’re missing the
routes
object that I’m looking for theroute
. - 2 I found Danny Moerkeke’s article on Async Programming in JavaScript particularly helpful in highlighting this challenge.
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