ESM hot-reloading for Node.js

hot ./server.js

Install hot-esm by running

yarn add hot-esm

hot-esm provides a loader that clears the module cache for files that are edited and the files that import them. This allows you to re-import your application and get updated code.

node --experimental-loader hot-esm ./server.js

# Or use this shorthand:
hot ./server.js

import * as http from 'http'

const server = http.createServer(async (request, response) => {
  const app = await import('./app.js')
  app.default(request, response)
})
server.listen(8080)

You'll have to find an appropriate place in your application to place an import() expression. This expression needs to run often enough to not miss updates.

State that is local to a file will be lost when that file is re-imported. To share state between the old and new copies of a file, put that state in global.

Also, any ongoing side-effects will need to be cleaned up when a new version of a file is imported. Otherwise, for example, multiple of the same event listener will be running. Again, this can be managed using global state.

Note that this only works with ECMAScript Modules and not with CommonJS modules.

Also, edits to files in node_modules, even if they are ECMAScript Modules are ignored by default as attaching filesystem watchers to so many directories is too expensive.

But, specific packages inside of node_modules can be watched for updates by setting the environment variable:

HOT_INCLUDE_PACKAGES=package1,package2

hot-esm provides debug logging detailing which files are watched, when they get invalidated, and when they get re-imported. Those logs can be enabled by setting the environment variable:

DEBUG=hot-esm