Note: This page covers Parcel 1, the documentation for Parcel 2 is being worked on here:

🔥 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.

As of version 1.12.0, the default implementation has changed to fully refresh the page when files change. You can opt-in to enable true HMR by adding the following in your app. This will only apply in development; HMR is automatically disabled when bundling in production mode.

if ( {

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

if ( { {
    // module is about to be replaced
  }) {
    // module or one of its dependencies was just updated

Automagically installed dependencies

Whenever Parcel comes across a dependency that fits the node_module pattern and can't find it, Parcel tries to install this dependency using yarn or npm depending on finding a yarn.lock file or not. This prevents the developer from having to exit parcel or having multiple terminal windows open.

This only occurs in development (using serve or watch), however in production (using build) autoinstall is disabled to prevent unwanted side-effects on deployment.

You can disable this feature using --no-autoinstall.

Safe Write

Some text editors and IDE's have a feature called safe write that basically prevents data loss, by taking a copy of the file and renaming it when saved.

When using Hot Module Reload (HMR) this feature blocks the automatic detection of file updates, to disable safe write use the options provided below:

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