dwait

dwait

Deffered async operation made easy in Javascript

What is it?

dwait is an alternative way of handling async tasks. The name stands for Deferred Await and it basically lets you defer multiple await operations to the end of the function chain, So you can stop writing things like this:

  const resp = await getUserAsync();
  const json = await resp.body.toJson();
  const username = json.username.trim();

And instead, write more readable(in my opinion) code like this:

  const username = await dwait(getUserAsync())
    .body
    .toJson()
    .username
    .trim()
    .await; // or .toPromise();

Or you can pass the function itself to the dwait function and get a deferred function you can use directly!

  const getUserDeferred = dwait(getUserAsync);
  const username = await getUserDeferred()
    .body
    .toJson()
    .username
    .trim()
    .await; // or .toPromise();

Why?

If you have ever seen any async code from Rust language you can immediately see the source of inspiration for the dwait library. Most languages have opted-in for writing the await keyword before the expression, This way it will read more naturally. For example something like this:

await asyncTask();

It will read as await async task which is nice, But whenever you get knee-deep into the async/await pattern, this style of awaiting can get a little bit annoying in even some trivial cases. Take this code as an example:

const fileNames = (await (await getFilesRepository()).getFileNames()).map((f) => f.trim().split("."));

Well to be honest nobody writes code like this, So let's break it into multiple lines to make it readbly.

const fileRepo = await getFilesRepository();
const files = await fileRepo.getFileNames();
const fileNames = files.map((fullname) => fullname.trim().split("."));

Now that's much better, But wouldn't it be nice if we could get rid of extra variables and just chain our operations? Well, that is exactly what the right-hand side awaiting solves, Just imagine if we could write code with await as a function on the Promise instead of a keyword, Or even better what if we could just write .await at the end of an awaitable expression and it would just get awaited? Unfortunately, that's not possible without language-level support (either via typescript/babel or ECMAScript specification). So what's the next best thing? If we want to get the result from a Promise we have to either use then callback or await the expression. So what if we could wrap the promise inside an object and use it to dispatch the subsequent function calls and/or property accesses inside a new Promise? This way we can defer the await keyword to the last expression and we can just await that last promise to get the result of whole function chain. So let's rewrite the code above using dwait.

const fileNames = await dwait(getFilesRepository())
  .getFileNames()
  .map((fullname) => fullname.trim().split("."))
  .await;

Much cleaner, isn't it?

How does it work?

So as mentioned earlier dwait works in the user space and doesn't need any new language feature to provide an appropriately typed solution for this situation. But how does it work? Whenever you want to defer awaiting a chain of async operation you have to wrap the first Promise<T> inside a DeferredPromise<T>, It can be done by passing the promise into the dwait function.

const deferredPromise = dwait(promise);

DeferredPromise extends the javascript underlying Promise type, It has anything existing on the type T in DeferredPromise<T>, But it will return another DeferredPromise for the result instead of the actual return type.

const r1: string = getStringSync().trim();
const r2: DeferredPromise<string> = dwait(getStringAsync()).trim();

As you can see, calling trim on a DeferredPromise<string will result in a DeferreedPromise<string> instead of the string result. This way we can start to chain these DeferredAsync operations and at the end, we either call the toPromise function or read the value of the await field.

const result = await dwait(getStringAsync()).trim().slice(3).split(" ").toPromise();
// or
const result = await dwait(getStringAsync()).trim().slice(3).split(" ").await;

Both toPromise and await will return a native Promise object which can be used to await the final result of the operation chain.