Express Co

Small, flexible utility for wrapping express middleware in co so that you can use async/await-like syntax via generators yielding promises, and not deal with calling the next function or handling errors directly (unless you want to!).

Usage

expressCo.wrap(..)

Wrap is the main function of the library. It accepts a generator and an optional options object. It will wrap the generator using co.wrap(..) allowing you to use all the same yield-ing semantics as co. It will extract the next function and call it automatically unless you return false (the check is strict), throw an error (in which case is calls next(error)), or pass in options.terminal = true in the options object.

const express = require('express');
const router = express.Router();
const expressCo = require('scl-express-co');

// the simplest usage: run an async function. if promise is rejected, next(error) is called
// otherwise next() is called once promise is reolved
router.use(expressCo.wrap(function * (req, res) {
  req.user = yield parseToken(req); // some function returning a promise
}));

// do something more complex: for instance check user permissions
// here we incorporate custom error handling for one specific case and fall back
// on next(error) for the rest
router.use(expressCo.wrap(function * (req, res) {
  try {
    // some function that returns a promise which resolves if user can view books
    // and rejects if user cannot view books.
    yield canUserViewBooks(req.user);
  } catch (error) {
    // catch errors as if this was a standard/non-async call
    if (error.name === 'user-cannot-view-books') {
      res.status(403).json('error': 'You cannot view books.');
      return false; // dont call next, we already handled the response. end the request here
    }
    // thrown errors will cause next(error) to be called,
    // passing control over to the express error handling-mechanism
    // if you didnt care about outputting the message above, you could just skip
    // the try-catch block altogether and the rejected promise would be passed to next(error)
    throw error; 
  }
}));

expressCo.wrapParam(..)

This is a simple wrapper to the expressCo.wrap(..) function that sets options.nextIndex according to the position of the next function in the param handler (its actually the same index right now, but if it changes in the future, you will be happy about the extra layer of abstraction).

router.param('book', expressCo.wrapParam(function * (req, res, bookId) {
  // next is automatically called, or next(error) is called if promise is rejected.
  req.book = yield req.db.getBookWithId(userId); // some function returning a promise
}));

expressCo.wrapTerminal(..)

In some cases you know this is a "terminal" handler. expressCo.wrapTerminal is the same as wrap except it sets options.terminal = true, so that next() will not be called (unless an error is thrown in which case it will call next(error))

router.get('/:book', expressCo.wrapTerminal(function * (req, res) {
  const book = req.book;
  const isFavorite = yield req.user.isBookInFavorites(book);
  
  //  next will not be called
  res.status(200).json({book, isFavorite});
}));