throwback

Throwback

Simple asynchronous middleware pattern.

Installation

npm install throwback --save

Usage

Compose asynchronous (promise-returning) functions.

const { compose } = require('throwback')

const fn = compose([
  async function (ctx, next) {
    console.log(1)

    try {
      await next()
    } catch (err) {
      console.log('throwback', err)
    }

    console.log(4)
  },
  async function (ctx, next) {
    console.log(2)

    return next()
  }
])

// Callback runs at the end of the stack, before
// the middleware bubbles back to the beginning.
fn({}, function (ctx) {
  console.log(3)

  ctx.status = 404
})

Tip: In development mode, debug mode will throw errors when you do something unexpected. In production, faster non-error code paths are used.

Example

Build a micro HTTP server!

const { createServer } = require('http')
const finalhandler = require('finalhandler') // Example only, not compatible with single `ctx` arg.
const { compose } = require('throwback')

const app = compose([
  function ({ req, res }, next) {
    res.end('Hello world!')
  }
])

createServer(function (req, res) {
  return app({ req, res }, finalhandler())
}).listen(3000)

Advanced

Did you know next(ctx?) accepts an optional ctx argument which will override ctx for all following middleware functions? This enables advanced functionality such as Request cloning and retries in popsicle.

async function retryRequest (req, next) {
  let retries = 5

  while (retries--) {
    try {
      const res = await next(req.clone())

      return res
    } catch (e) {
      continue
    }
  }

  throw new Error('Retry limit exceeded')
}

Use Cases

• HTTP requests (e.g. popsicle)
• HTTP servers (e.g. servie)
• Processing pipelines (e.g. scrappy)

Inspiration

Built for servie and inspired by koa-compose.

License

MIT