🔥👀 hot-watch

Watch files and hot reload them

Installation

yarn add -D hot-watch

Usage

const http = require('http');
const watch = require('hot-watch');

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  // Module to be hot-reloaded
  // Be sure to dynamic require them, or else have to re-require them in `onAfterInvalidate`
  res.end(require('./app'));
});

server.listen(3000, () => {
  // Returns a unwatch function
  const unwatch = watch();

  server.on('close', () => {
    // Remember to call un-watch to free the process
    unwatch();
  });
});

Express middleware

There is also an useful middleware to help get started in express application.

const express = require('express');
const hotWatchMiddleware = require('hot-watch/middleware');

const app = express();

// It's also very common to only use it during development
if (process.env.NODE_ENV !== 'production') {
  // Initialize it and insert it before request handler or router
  app.use(
    hotWatchMiddleware({
      // It accepts the same options as watch
    })
  );
}

app.get('/', (req, res) => {
  res.end(require('./a'));
});

app.listen(3000);

Once setup, the express server will wait for the files changed and perform necessary cleanups/restart before running the next request handlers or sending back the data.

API

// watch accepts an optional option object and here are their default values
const unwatch = watch({
  // The directory the watcher is going to watch on,
  // can be either a string or an array of strings
  cwd: process.cwd(),
  // Files should not be watched, expected an array of strings
  ignore: [],
  // The function which will be fired with changed file path whenever the watcher detect a change and before hot-reloading,
  // Can return void or a Promise
  onBeforeInvalidate: path => {},
  // The function which will be fired with changed file path after old modules are invalidated,
  // you can re-require modules here to update to the latest modules if necessary
  // Can return void or a Promise
  onAfterInvalidate: path => {},
});

unwatch(); // Stop and close the watcher

How

The basic idea is to clean up require.cache key whenever the files changed. We have to walk up the require tree to delete the caches for all it's parents. The native module API only stores the first cached parent for each module. In order to get all the parents and their parents and so on, we monkey-patch require to store all their parents in an internal parentsMap.

License

MIT