I don't think the Deferred pattern is a good primitive to have in the language, and it's a pretty trivial primitive to write yourself if you need it.
On Thu, Jul 19, 2018 at 6:13 PM, Isiah Meadows <[email protected]> wrote: > Sometimes, it's *very* convenient to have those `resolve`/`reject` > functions as separate functions. However, when logic gets complex > enough and you need to send them elsewhere, save a continuation, etc., > it'd be much more convenient to just have a capability object exposed > more directly rather than go through the overhead and boilerplate of > going through the constructor with all its callback stuff and > everything. > > It's surprisingly not as uncommon as you'd expect for me to do this: > > ```js > let resolve, reject > let promise = new Promise((res, rej) => { > resolve = res > reject = rej > }) > ``` > > But doing this repeatedly gets *old*, especially when you've had to > write it several dozen times already. And it comes up frequently when > you're writing lower-level async utilities that require saving promise > state and resolving it in a way that's decoupled from the promise > itself. > > ----- > > So here's what I propose: > > - `Promise.newCapability()` - This basically returns the result of > [this][1], just wrapped in a suitable object whose prototype is > %PromiseCapabilityPrototype% (internal, no direct constructor). It's > subclass-safe, so you can do it with subclasses as appropriate, too. > - `capability.resolve(value)` - This invokes the implicit resolver > created for it, spec'd as [[Resolve]]. > - `capability.reject(value)` - This invokes the implicit rejector > created for it, spec'd as [[Reject]]. > - `capability.promise` - This returns the newly created promise. > > Yes, this is effectively a deferred API, but revealing constructors > are a bit too rigid and wasteful for some use cases. > > [1]: https://tc39.github.io/ecma262/#sec-newpromisecapability > > ----- > > Isiah Meadows > [email protected] > www.isiahmeadows.com > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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