Well, one of the ways we "sold" async/await it to Google was by showing how we could improve Promise-based tests.

I recall that one of our test suites was tens of thousands of lines of code using `then()`. The code was complicated enough that these lines were by and large considered write-only, partly because async loops were really annoying to write, partly because error-handling was non-trivial.

I rewrote that test suite using `Task.spawn` (our prototype for async/await). I don't have the exact numbers in mind, but this decreased the number of LoC by a factor of 2-3 and suddenly people could see the familiar uses of loops and `try`/`catch`.

Well, consider the difference between

  a().then(() =>
         b())
     .then(() =>
         c())

Compared to

  await a()
  await b() 
  await c() 

Even for this simple case I think it's much clearer. Then look at a more complex case:

  for( i=0; i<n; i++) {
    await a(i) ;
  } 

Now try re-writing this with then() and see the difference.

For the first one, it really feels like only a matter of how you break lines. Sure there is also the matter of anonymous function syntax, but

The latter is more fair, here is a possible solution:

  const gen = (function* () {
    for (let i = 0; i < n; i++) yield a(i);
  })();

  const run = next => !next.done && next.value.then(() => run(gen.next()));
  run(gen.next());

Or something similar using reduce. But in both cases, it illustrates the point, I guess.

But if we are at point we can introduce new keywords/syntax in the language, it would just as well possible to come with something like

  a.chain(b, c)

In case you need to pass parameters

  a.chain([b, p1, p2], c)

And for the latter case

  const indexes = (function* () {
    for (let i = 0; i < n; i++) yield i;
  })
  a.through(indexes)

Well, sure, if you have `yield`, you pretty much have `await` already, as `await` is thin syntactic sugar on top of `yield` in all languages other than OCaml.