converting synchronous code to asynchronous

2019-11-18

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~5 min read

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819 words

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.1

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.2

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.3

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.

Bonus: Error Handling With Async/Await

Async functions return a promise. As a result, they’re chain-able.

While most of the time, we simply await for the async function to resolve and assign the results to a variable, what about if there’s an error? The most common way to handle error handling with Async/Await is a try/catch block.

const server = http.createServer(async (req, res) => {
    /* … */
    try {
        if (routes[route]) {
            const assets = await findAssetAsync(routes[route])
            res.write(assets)
            /* … */
        } else {
            /* … */
        }
    } catch (error) {
        throw new Error(error)
    }
})

This is not that dissimilar to a single catch in a large chain of promises.

But what if we want a little more fine control? We could have multiple try/catch blocks. Or, we could use a .catch:

const server = http.createServer(async (req, res) => {
    /* … */
    if (routes[route]) {
        const assets = await findAssetAsync(routes[route]).catch((error) => {
            throw new Error(error)
        })
        res.write(assets)
        /* … */
    } else {
        /* … */
    }
})

I’ve historically used the try/catch, but having learned about this new approach, I imagine I’ll be adopting it much more going forward!

Footnotes


Hi there and thanks for reading! My name's Stephen. I live in Chicago with my wife, Kate, and dog, Finn. Want more? See about and get in touch!