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An express middleware to bundle modules with rollup on demand (but cached)


Express middleware for rollup

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npm install --save express-middleware-rollup

Basic Usage

Assuming a directory setup like the following of your project:

├── client
│   └── js
│       ├── main.bundle
│       └── test.js
├── server.js
└── static
    └── index.html

In your server.js write the following:

const express = require('express');
const rollup  = require('express-middleware-rollup');
const path    = require('path');
const app = express();
  src: 'client/js',
  dest: 'static',
  root: __dirname,
  prefix: '/js'
app.use(express.static(path.join(__dirname, 'static')));

Now, if you request localhost:3000/main.js, the middleware will automatically bundle client/js/main.bundle using rollup into a file that is ready to be served by express.static middleware. You can see this in action by looking into the basic example

Alternative Usage Scenarios

There are basically two different approaches developers follow which lead them to use this middleware:

  1. Differentiate between source code (es6) and production code (es5). Use the middleware to compile the former to the latter.
  2. Write es6 for the frontend today, including modules. Use the middleware as a polyfill until browsers support es6 modules natively.

Both approaches are supported by this middleware. But since they are so different from each other, we were not able to come up with a unique set of options to configure both in a sensible way. This is why we have the mode option: Depending on which mode you set (compile or polyfill), you can customize the middleware's behavior with a different set of options. The default mode is compile.


Options which are available in both modes are:

  • src: (String, required). Directory where to look for bundle entries
  • root: (String, default: process.cwd()). Directory which other paths (like src) are relative to
  • rebuild: (String, default: 'deps-change'). Strategy used to determine whether to re-run rollup if a compiled/cached bundle exists. Can be 'deps-change', 'never' or 'always'
  • rollupOpts: (Object, default: {}). Options that will be passed to rollup.rollup. entry is set by the plugin, though.
  • bundleOpts: (Object, default: { format: 'iife' }). Options passed to bundle.generate
  • prefix: (String, default: null)
  • maxAge: (Integer, default: 0).
  • type: (String, default: javascript). MIME type of served bundles. Can be anything, e.g. application/javascript
  • debug: (Bool, default: false)

Options compile Mode

  • dest: (String, default: value of src)
  • bundleExtension: (String, default: '.bundle')
  • serve: (Bool|String, default: false). Can be true, false or 'on-compile'. 'on-compile' has the benefit that the bundle which is already in memory will be written directly into the response

Options polyfill Mode

  • cache: (String, default: 'cache'). Directory where to store bundles as cache.


Different module file extensions than .js

Let's say you have files with .jsx or .es6 as file extension in your project but you still want to import them without any extension specified in your code. Then you were probably hoping for an option similar to browserify's --extension option. Unfortunately, the rollup team does not seem to favor a solution like that. Therefore, I am afraid yo're stuck specifying the extension of the files you import in your code.


Essentially, the reasons for why you would want to use this middleware are the same as for middlewares like browserify-middleware or node-sass-middleware: You like it simple and don't want to set up a build pipeline with gulp/grunt/broccoli/webpack/file watchers etc. Also, you don't really need hot-reloading on save, since you are able to press f5 on your own. And maybe you also have the problem that you don't want to choose between having compiled files in your repo and forcing the server guys to build the client code each time they pull. With this package, you can simply have your server handle the build process, just when it's needed and only if it's needed.


This middleware is heavily influenced by node-sass-middleware


Copyright (c) 2016+ Lukas Böhm. See LICENSE for details.