Webpack Dev Server: Hot Module Replacement

This article goes over how to set up Hot Module Replacement (HMR) with Webpack Dev Server (WDS).

Prerequisite

We are using the package.json dependencies:

{
  "devDependencies": {
    "webpack": "^5.35.0",
    "webpack-cli": "^4.6.0",
    "webpack-dev-server": "^3.11.2"
  }
}

Quickstart

Enable HMR for WDS using the CLI by passing --hot and --inline:

npx webpack serve --hot --inline

Start the server:

$ npm start

> @ start path/to/project
> webpack serve --hot --inline

ℹ ｢wds｣: Project is running at http://localhost:8080/
ℹ ｢wds｣: webpack output is served from /
ℹ ｢wds｣: Content not from webpack is served from path/to/project
⚠ ｢wdm｣: asset main.js 160 KiB [emitted] [minimized] (name: main) 1 related asset

Open http://localhost:8080/ in the browser:

$ open http://localhost:8080/

See the console logs:

[HMR] Waiting for update signal from WDS...
[WDS] Hot Module Replacement enabled.
[WDS] Live Reloading enabled.

But that’s not enough for HMR to work. You’ll need to accept the hot update:

// src/index.js
// ...
if (module.hot) {
  module.hot.accept();
}

Edit and save the file and the page should update without a hard reload.

You should see the console logs:

[WDS] App updated. Recompiling...
[WDS] App hot update...
[HMR] Checking for updates on the server...
[HMR] Updated modules:
[HMR]  - ./src/index.js
[HMR] App is up to date.

Config

HMR can also be enabled from the config file:

// webpack.config.js
module.exports = {
  devServer: {
    hot: true,
    inline: true,
  },
};

Run webpack-dev-server without passing any options:

$ npx webpack serve

Demo

Repl.it:

Resources

Hot Module Replacement