promises #2259
Description
now that promises are going mainstream, i'm trying to think of how to make express more async friendly. an idea is to use promises.
next()now returns a promise- if middleware returns a promise, that promise is resolved and propagated up
next()s
app.use(function (req, res, next) {
// a promise must be returned,
// otherwise the function will be assumed to be synchronous
return User.get(req.session.userid).then(function (user) {
req.user = user
})
.then(next) // execute all downstream middleware
.then(function () {
// send a response after all downstream middleware have executed
// this is equivalent to koa's "upstream"
res.send(user)
})
})Error handlers are now more koa-like:
app.use(function (req, res, next) {
return next().catch(function (err) {
// a custom error handler
if (res.headerSent) return
res.statusCode = err.status || 500
res.send(err.message)
})
})
app.use(function (req, res) {
throw new Error('hahahah')
})Pros:
- it should be backwards compatible since you don't have to resolve the promise returned from
next() - much easier error handling including
throwing - solves issues shown in Second middleware stack #2255
- no more
fn.lengthchecking_ - could probably easily upgrade to es7 async functions
Cons:
- promises
- upgrading middleware and supporting both signatures might be a pain in the ass
- probably a lot slower