7 Reasons Why JavaScript Async/Await Is Better Than Plain Promises

7 Reasons Why JavaScript Async/Await Is Better Than Plain Promises

In this article, you'll see 7 reasons why JavaScript Async/Await is better than Plain Promises

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.

Async/Await

For those who have never heard of this topic before, here’s a quick intro

  • Async/await is a new way to write asynchronous code. Previous alternatives for asynchronous code are callbacks and promises.
  • Async/await is actually just syntax sugar built on top of promises. It cannot be used with plain callbacks or node callbacks.
  • Async/await is, like promises, non-blocking.
  • Async/await makes asynchronous code look and behave a little more like synchronous code. This is where all its power lies.

Syntax

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

  • Our function has the keyword 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).
  • The above point implies that we can’t use 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.

Why Is It better?

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

  • You can’t set breakpoints in arrow functions that return expressions (no body).

Try setting a breakpoint anywhere here

  • If you set a breakpoint inside a .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.
  • With async/await you don’t need arrow functions as much, and you can step through await calls exactly as if they were normal synchronous calls.

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);
}

In Conclusion

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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