The purpose of `await` is to allow synchronous execution within an async function.
The purpose of `then` is to make it easier to manipulate the promises as if they were normal values. It is much closer to `Promise.prototype.then` than to `await`, since it does not prevent the statements following it from execution. For this reason it doesn't need to be in an async function. The synchronous nature of `await` can be annoying when you have multiple promise-creating expressions that you want to run in parallel. You have to run the expressions first, keep track of the promises they evaluate to, and then use `await` on those promises. `Promise.all` helps in this endeavor, but it prevents you from putting expressions exactly where you want them to be. Consider the following: ```js const pets = (await getDogs()).concat(await getCats()) ``` This would not be ideal if you want `getDogs` and `getCats` to start off in parallel. So you need to do something like this: ```js const [dogs, cats] = await Promise.all([getDogs(), getCats()]) const pets = dogs.concat(cats) ``` With `then` it could be done like so: ```js const pets = await (then getDogs()).concat(then getCats()) ``` On Sat, Mar 3, 2018 at 12:12 AM, Peter Jaszkowiak <p.jasz...@gmail.com> wrote: > Why not just use async/await then? Seems like if you were to replace `then` > with `await` in your top example it would work exactly as you want (as long > as it's in an async function). > > On Sat, Mar 3, 2018 at 1:06 AM, Bryant Petersen <petersen...@gmail.com> > wrote: >> >> I've put the details here: >> https://github.com/bwpetersen/proposal-then-operator >> >> The basic idea is that the `then` operator would allow you to >> manipulate a promise as if it were its resolved value. Expressions >> containing `then` become similar to an `onFulfillment` handler. >> >> Here is an example: >> >> ```js >> const greeting = `Hello, ${(then getUser()).name}. You have ${(then >> getMessages()).length} messages.` >> // This would behave like: >> const greeting = async () => { >> const user = getUser() >> const messages = getMessages() >> return `Hello, ${(await user).name}. You have ${(await >> messages).length} messages.` >> }() >> ``` >> >> I'm looking for feedback on this idea. >> _______________________________________________ >> es-discuss mailing list >> es-discuss@mozilla.org >> https://mail.mozilla.org/listinfo/es-discuss > > > > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
