[express middleware boilerplate]

express-middleware-boilerplate.js

// the function that will transform the content or whatever
function myFunction (data, opts) {
  // do your stuff here
  return `${data}`
}

// the middleware that gets passed to app.use()
function myMiddleware (opts) {
  // options passed to the middleware
  // eg.: app.use(myMiddleware({opt1: "opt1", opt2: "opt2"})
  const default_opts = {
    opt1: "opt1",
    opt2: "opt2",
    override: false
  }
  opts = opts || {}
  opts = Object.assign(opts, default_opts)

  // the response handler
  function doThing (req, res, next) {
    const theThing = function (data, opts, cb) {
      if (typeof cb === 'undefined') {
        // if no callback do this
        return function (err, html) {
          if (err) return next(err)
          html = myFunction(data, opts)
          res.send(html)
        }
      } else {
        // if callback do this      
        return function (err, html) {
          if (html) html = myFunction(data, opts)
          cb(err, html)
        }
      }
    }

    // returns your middleware in stream
    res.myMiddleware = function (view, renderOpts, cb) {
      this.render(view, renderOpts, theThing(data, opts, cb))
    }

    // returns your middleware only if explicitly called
    if (opts.override) {
      res.oldRender = res.render
      res.render = function (view, renderOpts, cb) {
        this.oldRender(view, renderOpts, theThing(data, opts, cb))
      }
    }
    return next()
  }

  return doThing
}

module.exports = myMiddleware