Development tool that enables hot-swapping Express server code without a restart

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2251.7.04 months ago5 years agoMinified + gzip package size for @artsy/express-reloadable in KB


When developing a Node app it's common to rely on tools like node-dev or nodemon to make the development process more rapid by automatically restarting the server instance on file-change. What express-reloadable does is listen for source-code changes within a subset of your app and, scanning Node's internal module cache, clears the require call if found. This tricks Node into thinking the module has not yet been loaded, effectively hot-swapping out your code without a full restart. Additionally, when the watchModules option is passed, express-reloadable will listen for changes to NPM module code and reload on change. Useful when working with yarn link across packages / repos. Crazy-fast development speed!
Disclaimer: While this works for most of our use-cases, this is an example of "require hacking" and hasn't been tested in all environments. Your mileage may vary.

How it works:
  • express-reloadable is called with a path to an app, which it then mounts
  • When source-code within that folder / app changes an internal lookup is made to Node, scanning its require cache for the changed file
  • If found, it is cleared internally via delete require.cache[id]
  • When a new request is made express-reloadable executes a callback that re-requires the code and changes are instantly available.

yarn add @artsy/express-reloadable

The below example assumes that the folders /api and /client exist, and that each contain an index file that exports a mountable express.js route.
import express from 'express'
import { createReloadable, isDevelopment } from '@artsy/express-reloadable'

const app = express()

if (isDevelopment) {

  // Pass in `app` and current `require` context
  const mountAndReload = createReloadable(app, require)

  // Pass in the path to an express sub-app and everything is taken care of
  mountAndReload(path.resolve(__dirname, './client'))

  // Full example:
  app.use('/api', mountAndReload(path.resolve(__dirname, './api')), {

    // If you need to mount an app at a particular root (`/api`), pass in
    // `mountPoint` as an option.
    mountPoint: '/api',

    // Or if you're using `yarn link` (or npm) to symlink external dependencies
    // during dev, pass in an array of modules to watch. Changes made internally
    // will be instantly available in the app. Additionally, using something like 
    // `glob`, other modules outside of express route path can be passed.
    watchModules: [

  // If prod, mount apps like normal
} else {
  app.use('/api', require('./api')

app.listen(3000, () => {
  console.log(`Listening on port 3000`)

Help! I've mounted my app using reloadable but I'm not seeing any changes?

For the utility to work you need to a) ensure that NODE_ENV=development (for safety) and b) the path to your app is absolute:
// Incorrect

// Correct 
app.use(reloadAndMount(path.resolve(__dirname, 'path/to/app')))

This package was heavily inspired by @glenjamin's ultimate-hot-loading-example.