HMR is an opt-in feature that only affects modules containing HMR code. One example would be patching styling through the [`style-loader`](https://github.com/webpack-contrib/style-loader). In order for patching to work, the `style-loader` implements the HMR interface; when it receives an update through HMR, it replaces the old styles with the new ones.

HMR is an opt-in feature that only affects modules containing HMR code. One example would be patching styling through the [`style-loader`](https://github.com/webpack/style-loader). In order for patching to work, the `style-loader` implements the HMR interface; when it receives an update through HMR, it replaces the old styles with the new ones.

Similarly, when implementing the HMR interface in a module, you can describe what should happen when the module is updated. However, in most cases, it's not mandatory to write HMR code in every module. If a module has no HMR handlers, the update bubbles up. This means that a single handler can update a complete module tree. If a single module from the tree is updated, the entire set of dependencies is reloaded.

Take advantage of the [`loader-utils`](https://github.com/webpack/loader-utils) package which provides a variety of useful tools. Along with `loader-utils`, the [`schema-utils`](https://github.com/webpack-contrib/schema-utils) package should be used for consistent JSON Schema based validation of loader options. Here's a brief example that utilizes both:

Take advantage of the [`loader-utils`](https://github.com/webpack/loader-utils) package which provides a variety of useful tools. Along with `loader-utils`, the [`schema-utils`](https://github.com/webpack/schema-utils) package should be used for consistent JSON Schema based validation of loader options. Here's a brief example that utilizes both:

**loader.js**

[ExtractTextPlugin](https://github.com/webpack-contrib/extract-text-webpack-plugin) requires version 2 to work with webpack 2.

[ExtractTextPlugin](https://github.com/webpack/extract-text-webpack-plugin) requires version 2 to work with webpack 2.