Support Promise.resolve(null) to return next

I’ve made a library and I’m trying to support both express’ router and this one.

The value from the promise function returns the value of next.

The problem is that the when next() is passed “next” as in next("next") it receives it as an error.

Ideally I can run code like this:

For express-promise-router.

var promiseRouter = require("express-promise-router")()
promiseRouter.get("/hello-two", function(req, res){
  return somePromise()
})

For express’ router

var expressRouter = require("express").Router()
expressRouter.get("/hello-one", function(req, res, next){
  return somePromise().then(next)
})

Ideally Promise.resolve(null) was supported to return next as well as next, that way I can pass the value null into next and get the next route.

Promise.resolve(null).then(next) // no error
Promise.resolve("next").then(next) // error

Currently I have to write this & don’t worry about errors because they’re caught and returned.

var expressRouter = require("express").Router()
expressRouter.get("/hello-one", function(req, res, next){
  return somePromise().then(function(val){
    if(val == "next") return next()
    return next(val)
  })
})

Thoughts?

[TheThing]

Oh wow, I just took a better look at the code and now I realize what your problem is. The problem that I see though with making Promise.resolve(null) call next is routes will probably most likely return null under normal circumstances.

Also I think you should be be careful with calling next(val) from a resolved promise. Especially considering routes might accidentally return values from a successful promise (especially with arrow functions)

[TheThing]

I think your solution would be to change:

return somePromise().then(next)

to

return somePromise().then(() => next())

That is what I do if I have to call stuff like next or done from promises.

Also, next is primarily being used to be called either empty (no value = everything was success) or with an error (parameter = something went wrong) so passing it directly into then like .then(next) would be serious anti pattern as anything that gets resolved, any value or anything will suddenly be interpreted as an error. The more correct pattern would be:

return somePromise().then(() => next()).catch(next)

Effectively what you’re with this code:

return somePromise().then(function(val){
    if(val == "next") return next()
    return next(val)
  })

Is turning any and all values (excluding the literal value "next" that is being resolved (remember, resolved = success) as a failure which is pretty serious antipattern as I mentioned.

Edit: See new reply below.