Callback vs Await Performance in Express

Question

People say that async/await is just another way to write callbacks for promises. However, I feel as though they are not interchangeable; in fact, callbacks would be better in Express because it returns the result to the user right away. Is this correct?

In the following example, let's say we don't care about the result of doSomethingAsync(), but want to redirect right away. Here, we use await.

router.get('/register', async (req, res) => {
    res.redirect('/')           
    await doSomethingAsync()
    console.log(1)
}

router.get('/', (req, res) => {
    console.log(2)
    ...
}

This would print out 1, then 2, meaning that we won't be able to exit the /register handler and redirect unless the async function finishes.

In the below version, we use a promise with a callback in then.

router.get('/register', (req, res) => {
    res.redirect('/')           
    doSomethingAsync().then(() => {
        console.log(1)
    })
}

router.get('/', (req, res) => {
   console.log(2)
   ...
}

This would print out 2, then 1, meaning that we can exit immediately and redirect the user. Isn't the second way better for performance? Or are they actually the same?

Answer 1

People say that async/await is just another way to write callbacks for promises. However, I feel as though they are not interchangeable;

That is incorrect, and they are indeed interchangeable. In fact, async/await allows you to do things more easily than with plain promises, since an await expression can be inserted into virtually any control flow logic seamlessly, while you cannot do so with plain promises, and are required to chain them using sequential .then() calls.

In fact, callbacks would be better in Express because it returns the result to the user right away. Is this correct?

No. A callback is a different mechanism for delivering data asynchronously; it is not "right away", nor is it significantly faster, and the benefits of programming with promises far outweigh any negligible performance implications of resorting to callback hell. In addition, your second example is not a true callback, it is a promise chain.

This would print out 1, then 2, meaning that we won't be able to exit the /register handler and redirect unless the async function finishes.

That is incorrect. await is non-blocking. The async function returns to the caller as soon as doSomethingAsync() is invoked, and the async function's return value is a promise that is resolved after console.log(1) has been invoked.

This would print out 2, then 1, meaning that we can exit immediately and redirect the user. Isn't the second way better for performance? Or are they actually the same?

The two snippets are algorithmically the same, and since the console.log(2) is dependent on the client's browser performing the HTTP request indicated by res.redirect('/') which is independent of doSomethingAsync(), you actually have a race condition, and neither order of logs is guaranteed. Its performance is also no different than the example using await.