🔥 Hot Module Replacement

Hot Module Replacement (HMR) improves the development experience by automatically updating modules in the browser at runtime without needing a whole page refresh. This means that application state can be retained as you change small things. Parcel's HMR implementation supports both JavaScript and CSS assets out of the box. HMR is automatically disabled when bundling in production mode.

As you save files, Parcel rebuilds what changed and sends an update to any running clients containing the new code. The new code then replaces the old version, and is re-evaluated along with all parents. You can hook into this process using the module.hot API, which can notify your code when a module is about to be disposed, or when a new version comes in. Projects like react-hot-loader can help with this process, and work out of the box with Parcel.

There are two methods to know about: module.hot.accept and module.hot.dispose. You call module.hot.accept with a callback function which is executed when that module or any of its dependencies are updated. module.hot.dispose accepts a callback which is called when that module is about to be replaced.

if (module.hot) {
  module.hot.dispose(function () {
    // module is about to be replaced

  module.hot.accept(function () {
    // module or one of its dependencies was just updated