Using webpack-dev-server and HMR

5.3 version
Symfony 5.3 is backed by JoliCode.

Using webpack-dev-server and HMR

While developing, instead of using yarn encore dev --watch, you can use the webpack-dev-server:

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$ yarn encore dev-server

This builds and serves the front-end assets from a new server. This server runs at localhost:8080 by default, meaning your build assets are available at localhost:8080/build. This server does not actually write the files to disk; instead it serves them from memory, allowing for hot module reloading.

As a consequence, the link and script tags need to point to the new server. If you’re using the encore_entry_script_tags() and encore_entry_link_tags() Twig shortcuts (or are processing your assets through entrypoints.json in some other way), you’re done: the paths in your templates will automatically point to the dev server.

dev-server Options

Caution

Encore uses webpack-dev-server version 4, which at the time of Encore’s 1.0 release was still in beta and was not documented. See the 4.0 CHANGELOG for changes.

The dev-server command supports all the options defined by webpack-dev-server. You can set these options via command line options:

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$ yarn encore dev-server --port 9000

You can also set these options using the Encore.configureDevServerOptions() method in your webpack.config.js file:

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// webpack.config.js
// ...

Encore
    // ...

    .configureDevServerOptions(options => {
        options.https = {
            key: '/path/to/server.key',
            cert: '/path/to/server.crt',
        }
    })
;

New in version 0.28.4: The Encore.configureDevServerOptions() method was introduced in Encore 0.28.4.

Enabling HTTPS using the Symfony Web Server

If you’re using the Symfony web server locally with HTTPS, you’ll need to also tell the dev-server to use HTTPS. To do this, you can reuse the Symfony web server SSL certificate:

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  // webpack.config.js
  // ...
+ const path = require('path');

  Encore
      // ...

+     .configureDevServerOptions(options => {
+         options.https = {
+             pfx: path.join(process.env.HOME, '.symfony/certs/default.p12'),
+         }
+     })

Caution

Make sure to not pass the --https flag at the command line when running encore dev-server. This flag was required before 1.0, but now will cause your config to be overridden.

CORS Issues

If you experience issues related to CORS (Cross Origin Resource Sharing), set the following option:

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// webpack.config.js
// ...

Encore
    // ...

    .configureDevServerOptions(options => {
        options.allowedHosts = 'all';
        // in older Webpack Dev Server versions, use this option instead:
        // options.firewall = false;
    })

Beware that this is not a recommended security practice in general, but here it’s required to solve the CORS issue.

Hot Module Replacement HMR

Hot module replacement is a superpower of the dev-server where styles and (in some cases) JavaScript can automatically update without needing to reload your page. HMR works automatically with CSS (as long as you’re using the dev-server and Encore 1.0 or higher) but only works with some JavaScript (like Vue.js).

New in version 1.0.0: Before Encore 1.0, you needed to pass a --hot flag at the command line to enable HMR. You also needed to disable CSS extraction to enable HMR for CSS. That is no longer needed.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.