converting synchronous code to asynchronous

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

• ¹ I found André Staltz’s article on Promises interesting, and learned about the eagerness of promises!
• ² I have simplified the code here a bit. For example, we’re missing the routes object that I’m looking for the route.
• ³ Danny Moerkeke’s article on Async Programming in JavaScript was particularly informative in highlighting this challenge.