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JavaScript function to use generators for request handlers and middleware. Calls next only if there was an error. Useful for Express routes.
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NPM version

CoNext.js is a small function for JavaScript to wrap a generator function with Co for use as request handlers and middlewares. Calls the last argument (usually called next) if there was an error, but not if the generator finishes successfully. Useful with Express routes.


npm install co-next

CoNext.js follows semantic versioning, so feel free to depend on its major version with something like >= 1.0.0 < 2 (a.k.a ^1.0.0).


var next = require("co-next")
var app = require("express")()

app.get("/", next(function*(req, res, next) {
  if (req.account == null) throw new Error("Unauthorized")
  res.send("Hello, world!")

In the above example, the thrown error will be passed to next, which in Express's case calls the error handling middleware later on. If the generator succeeds, the next callback will not be called.

If you need to, like in middleware handlers, you can always call next yourself:

app.use(next(function*(req, res, next) {
  var account = yield
  if (account == null) return void next(new Error("Unauthorized"))
  req.account = account


CoNext.js is released under a Lesser GNU Affero General Public License, which in summary means:

  • You can use this program for no cost.
  • You can use this program for both personal and commercial reasons.
  • You do not have to share your own program's code which uses this program.
  • You have to share modifications (e.g. bug-fixes) you've made to this program.

For more convoluted language, see the LICENSE file.


Andri Möll typed this and the code.
Monday Calendar supported the engineering work.

If you find CoNext.js needs improving, please don't hesitate to type to me now at or create an issue online.

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