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Dev Server: Using plugins

By default, the dev server serves your code to the browser as is without modifications. There is a rich plugin API that can be used to modify code before it is served to the browser. Using a small adapter, the dev server supports most rollup plugins as well.

Rollup replace plugin

Packages published on NPM often use process.env.NODE_ENV to detect code is running in production or development mode. These variables are not defined in the browser and will crash.

In this example, we will use @rollup/plugin-replace to set up some environment variables.


Install the required dependencies:

npm install --save-dev @web/dev-server @web/dev-server-rollup @rollup/plugin-replace

Create a web-dev-server.config.mjs. The fromRollup function will convert a rollup plugin to a compatible plugin for Web Dev Server.

Note that for speed it's important to define explicitly which files to include.

import { fromRollup } from '@web/dev-server-rollup';
import rollupReplace from '@rollup/plugin-replace';

const replace = fromRollup(rollupReplace);

export default {
  plugins: [
      // setting "include" is important for performance
      include: ['src/logger.js'],
      'process.env.NODE_ENV': '"development"',

Add files

First, add a file at src/logger.js which will use the environment variable:

export function logDebug(msg) {
  if (process.env.NODE_ENV === 'development') {

And add a demo/index.html file which will use this function:

<!DOCTYPE html>
    <script type="module">
      import { logDebug } from '../src/logger.js';

      logDebug('Hello world debug');

Starting the server

Finally, we can run this example:

npx web-dev-server --open /demo/


If you are getting errors such as X does not export Y, X does not have a default export, or require is not defined you might be running into a package published on NPM as a common js or UMD module. Read our es modules guide to learn more about different module formats

You can use @rollup/plugin-commonjs to transform these modules into an es module. Note that not all commonjs modules can be transformed in this way, for example, the browser does not support node libraries like fs and path.

Note that the commonjs plugin isn't very fast, and should be limited to run only on commonjs code.

Example config:

import { fromRollup } from '@web/dev-server-rollup';
import rollupCommonjs from '@rollup/plugin-commonjs';

const commonjs = fromRollup(rollupCommonjs);

export default {
  plugins: [
      include: [
        // the commonjs plugin is slow, list the required packages explicitly:


When doing buildless development you want compilers to be as fast as possible. That's why we recommend esbuild for common transformatons like TS, JSX, and modern JS. See our guide to learn more about how to set this up.

If you need to use babel because of a specific plugin, you can use @rollup/plugin-babel. In this example we use babel with the JSX plugin for preact:

import { fromRollup } from '@web/dev-server-rollup';
import rollupBabel from '@rollup/plugin-babel';

// note that you need to use `.default` for babel
const babel = fromRollup(rollupBabel.default);

export default {
  // mimeTypes tells the dev server to treat .jsx files as .js
  mimeTypes: {
    '**/*.jsx': 'js',
  plugins: [
      include: ['**/*.jsx'],
      babelHelpers: 'bundled',
      plugins: [['@babel/plugin-transform-react-jsx', { pragma: 'h' }]],


Official dev server plugins:

Useful rollup plugins:

See the full list of rollup plugins for more.

Learn more

All the code is available on github. See the documentation of @web/dev-server.