Async Middleware

A handy tool to write async/promise style middleware for express, connect-like.

Why is this tool needed?

Let's check this code

app.use(async (req, res, next) => {
  req.user = await User.findById(req.params.id).exec()
  next()
})

The next() will be executed after User.findById(...).exec() is fulfilled because express allows middlewares returning a promise.

However, express does not support if the promise returned by the middleware is rejected. The following middlewares will never be called, and the response will never be returned to the client.

The solution is simple by wrapping the middleware with

const {asyncMiddleware} = require('middleware-async')
app.use(asyncMiddleware(async (req, res, next) => {
  req.user = await User.findById(req.params.id).exec()
  next()  
}))

Note that once the next function is called, following errors will not be thrown, and vice versa. Sample usage:

const {asyncMiddleware} = require('middleware-async')
app.use(asyncMiddleware(async (req, res, next) => {
  next()  
  throw new Error('my error')
}))

the error new Error('my error') will not be thrown because the next function is called.

Or

const {asyncMiddleware} = require('middleware-async')
app.use(asyncMiddleware((req, res, next) => {
  return Promise((resolve, reject)=> {
    reject()
    setTimeout(() => next(new Error('next error')), 0)
  })
}))

the new Error('next error') error will not be thrown because the promise is already rejected

How to install

Install it via npm or yarn

npm install --save middleware-async
#or
yarn add middleware-async

API

• asyncMiddleware(middleware): returns a middleware that covers the error thrown (throw err) or rejected (next(err)) by middlewares. The next parameter of the returned middleware is called at most once.

Sample usage:

app.use(asyncMiddleware(async (req, res, next) => {/*middleware code*/}))

• combineMiddlewares(middleware, list of middlewares, or list of middlewares with any depth): combine one or many middlewares into one middleware. Very useful for testing.

You can use this function with any combination of middleware or array of middlewares, for example combineMiddlewares(mdw) or combineMiddlewares([mdw1, mdw2], [[mdw3], [mdw4, [mdw5, mdw6]], mdw7], mdw8). The function will take care of expanding parameters.

Note that this function does not wrap the middleware with asyncMiddleware. If the middleware returns a promise, you need to manually wrap the middleware; otherwise, the error will never be caught.

• middlewareToPromise: convert express-style middleware to a function that returns a promise.

await middlewareToPromise(mdw)(req, res) is rejected if the middleware mdw throws an error (in express/connect-like style via calling next(err)), otherwise the promise is resolved normally.

• combineToAsync: a combination of middlewareToPromise and combineMiddlewares.

Sample usage: await combineToAsync(mdw1, mdw2)(req, res)

Besides, I highly recommend using flip-promise package to assert a rejected promise.

Sample usages

const {asyncMiddleware, combineMiddlewares, combineToAsync, middlewareToPromise} = require('middleware-async')

describe('combineMiddlewares', () => {
  test('should go through all middlewares', async () => {
    const req = {val: 0}
    await combineToAsync([
      async (req, res, next) => {
        await Promise.resolve()
        req.val += 1
        next()
      },
      (req, res, next) => {
        req.val++
        next()
      },
    ])(req)
    expect(req.val).toBe(2)
  })
})

Check test directory for more sample usages.