feat(v2): allow extend PostCSS config

packages/docusaurus-types/src/index.d.ts

@@ -199,6 +199,9 @@ export type AllContent = Record<
   >
 >;
 
+// TODO improve type (not exposed by postcss-loader)
+export type PostCssOptions = Record<string, any> & {plugins: any[]};
+
 export interface Plugin<T, U = unknown> {
   name: string;
   loadContent?(): Promise<T>;
@@ -220,6 +223,7 @@ export interface Plugin<T, U = unknown> {
     isServer: boolean,
     utils: ConfigureWebpackUtils,
   ): Configuration & {mergeStrategy?: ConfigureWebpackFnMergeStrategy};
+  configurePostCss?(options: PostCssOptions): PostCssOptions;
   getThemePath?(): string;
   getTypeScriptThemePath?(): string;
   getPathsToWatch?(): string[];
@@ -253,6 +257,7 @@ export interface Plugin<T, U = unknown> {
 
 export type ConfigureWebpackFn = Plugin<unknown>['configureWebpack'];
 export type ConfigureWebpackFnMergeStrategy = Record<string, MergeStrategy>;
+export type ConfigurePostCssFn = Plugin<unknown>['configurePostCss'];
 
 export type PluginOptions = {id?: string} & Record<string, unknown>;
 

packages/docusaurus/src/commands/build.ts

@@ -20,7 +20,11 @@ import {handleBrokenLinks} from '../server/brokenLinks';
 import {BuildCLIOptions, Props} from '@docusaurus/types';
 import createClientConfig from '../webpack/client';
 import createServerConfig from '../webpack/server';
-import {compile, applyConfigureWebpack} from '../webpack/utils';
+import {
+  compile,
+  applyConfigureWebpack,
+  applyConfigurePostCss,
+} from '../webpack/utils';
 import CleanWebpackPlugin from '../webpack/plugins/CleanWebpackPlugin';
 import {loadI18n} from '../server/i18n';
 import {mapAsyncSequencial} from '@docusaurus/utils';
@@ -166,24 +170,27 @@ async function buildLocale({
     });
   }
 
-  // Plugin Lifecycle - configureWebpack.
+  // Plugin Lifecycle - configureWebpack and configurePostCss.
   plugins.forEach((plugin) => {
-    const {configureWebpack} = plugin;
-    if (!configureWebpack) {
-      return;
+    const {configureWebpack, configurePostCss} = plugin;
+
+    if (configurePostCss) {
+      clientConfig = applyConfigurePostCss(configurePostCss, clientConfig);
     }
 
-    clientConfig = applyConfigureWebpack(
-      configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
-      clientConfig,
-      false,
-    );
+    if (configureWebpack) {
+      clientConfig = applyConfigureWebpack(
+        configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
+        clientConfig,
+        false,
+      );
 
-    serverConfig = applyConfigureWebpack(
-      configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
-      serverConfig,
-      true,
-    );
+      serverConfig = applyConfigureWebpack(
+        configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
+        serverConfig,
+        true,
+      );
+    }
   });
 
   // Make sure generated client-manifest is cleaned first so we don't reuse

packages/docusaurus/src/commands/start.ts

@@ -24,7 +24,11 @@ import {load} from '../server';
 import {StartCLIOptions} from '@docusaurus/types';
 import {CONFIG_FILE_NAME, STATIC_DIR_NAME} from '../constants';
 import createClientConfig from '../webpack/client';
-import {applyConfigureWebpack, getHttpsConfig} from '../webpack/utils';
+import {
+  applyConfigureWebpack,
+  applyConfigurePostCss,
+  getHttpsConfig,
+} from '../webpack/utils';
 import {getCLIOptionHost, getCLIOptionPort} from './commandUtils';
 import {getTranslationsLocaleDirPath} from '../server/translations/translations';
 
@@ -134,18 +138,21 @@ export default async function start(
     ],
   });
 
-  // Plugin Lifecycle - configureWebpack.
+  // Plugin Lifecycle - configureWebpack and configurePostCss.
   plugins.forEach((plugin) => {
-    const {configureWebpack} = plugin;
-    if (!configureWebpack) {
-      return;
+    const {configureWebpack, configurePostCss} = plugin;
+
+    if (configurePostCss) {
+      config = applyConfigurePostCss(configurePostCss, config);
     }
 
-    config = applyConfigureWebpack(
-      configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
-      config,
-      false,
-    );
+    if (configureWebpack) {
+      config = applyConfigureWebpack(
+        configureWebpack.bind(plugin), // The plugin lifecycle may reference `this`.
+        config,
+        false,
+      );
+    }
   });
 
   // https://webpack.js.org/configuration/dev-server

packages/docusaurus/src/webpack/__tests__/utils.test.ts

@@ -12,7 +12,11 @@ import {
 } from 'webpack';
 import path from 'path';
 
-import {applyConfigureWebpack, getFileLoaderUtils} from '../utils';
+import {
+  applyConfigureWebpack,
+  applyConfigurePostCss,
+  getFileLoaderUtils,
+} from '../utils';
 import {
   ConfigureWebpackFn,
   ConfigureWebpackFnMergeStrategy,
@@ -148,3 +152,123 @@ describe('getFileLoaderUtils()', () => {
     );
   });
 });
+
+describe('extending PostCSS', () => {
+  test('user plugin should be appended in PostCSS loader', () => {
+    let webpackConfig: Configuration = {
+      output: {
+        path: __dirname,
+        filename: 'bundle.js',
+      },
+      module: {
+        rules: [
+          {
+            test: 'any',
+            use: [
+              {
+                loader: 'some-loader-1',
+                options: {},
+              },
+              {
+                loader: 'some-loader-2',
+                options: {},
+              },
+              {
+                loader: 'postcss-loader-1',
+                options: {
+                  postcssOptions: {
+                    plugins: [['default-postcss-loader-1-plugin']],
+                  },
+                },
+              },
+              {
+                loader: 'some-loader-3',
+                options: {},
+              },
+            ],
+          },
+          {
+            test: '2nd-test',
+            use: [
+              {
+                loader: 'postcss-loader-2',
+                options: {
+                  postcssOptions: {
+                    plugins: [['default-postcss-loader-2-plugin']],
+                  },
+                },
+              },
+            ],
+          },
+        ],
+      },
+    };
+
+    function createFakePlugin(name: string) {
+      return [name, {}];
+    }
+
+    // Run multiple times: ensure last run does not override previous runs
+    webpackConfig = applyConfigurePostCss((postCssOptions) => {
+      return {
+        ...postCssOptions,
+        plugins: [
+          ...postCssOptions.plugins,
+          createFakePlugin('postcss-plugin-1'),
+        ],
+      };
+    }, webpackConfig);
+
+    webpackConfig = applyConfigurePostCss((postCssOptions) => {
+      return {
+        ...postCssOptions,
+        plugins: [
+          createFakePlugin('postcss-plugin-2'),
+          ...postCssOptions.plugins,
+        ],
+      };
+    }, webpackConfig);
+
+    webpackConfig = applyConfigurePostCss((postCssOptions) => {
+      return {
+        ...postCssOptions,
+        plugins: [
+          ...postCssOptions.plugins,
+          createFakePlugin('postcss-plugin-3'),
+        ],
+      };
+    }, webpackConfig);
+
+    // @ts-expect-error: relax type
+    const postCssLoader1 = webpackConfig.module?.rules[0].use[2];
+    expect(postCssLoader1.loader).toEqual('postcss-loader-1');
+
+    const pluginNames1 = postCssLoader1.options.postcssOptions.plugins.map(
+      // @ts-expect-error: relax type
+      (p: unknown) => p[0],
+    );
+    expect(pluginNames1).toHaveLength(4);
+    expect(pluginNames1).toEqual([
+      'postcss-plugin-2',
+      'default-postcss-loader-1-plugin',
+      'postcss-plugin-1',
+      'postcss-plugin-3',
+    ]);
+
+    // @ts-expect-error: relax type
+    const postCssLoader2 = webpackConfig.module?.rules[1].use[0];
+    expect(postCssLoader2.loader).toEqual('postcss-loader-2');
+
+    const pluginNames2 = postCssLoader2.options.postcssOptions.plugins.map(
+      // @ts-expect-error: relax type
+      (p: unknown) => p[0],
+    );
+    expect(pluginNames2).toHaveLength(4);
+    expect(pluginNames2).toEqual([
+      'postcss-plugin-2',
+      'default-postcss-loader-2-plugin',
+      'postcss-plugin-1',
+      'postcss-plugin-3',
+    ]);
+  });
+});

packages/docusaurus/src/webpack/utils.ts

@@ -11,6 +11,7 @@ import merge from 'webpack-merge';
 import webpack, {
   Configuration,
   Loader,
+  NewLoader,
   Plugin,
   RuleSetRule,
   Stats,
@@ -23,7 +24,7 @@ import path from 'path';
 import crypto from 'crypto';
 import chalk from 'chalk';
 import {TransformOptions} from '@babel/core';
-import {ConfigureWebpackFn} from '@docusaurus/types';
+import {ConfigureWebpackFn, ConfigurePostCssFn} from '@docusaurus/types';
 import CssNanoPreset from '@docusaurus/cssnano-preset';
 import {version as cacheLoaderVersion} from 'cache-loader/package.json';
 import {BABEL_CONFIG_FILE_NAME, STATIC_ASSETS_DIR_NAME} from '../constants';
@@ -175,6 +176,31 @@ export function applyConfigureWebpack(
   return config;
 }
 
+export function applyConfigurePostCss(
+  configurePostCss: NonNullable<ConfigurePostCssFn>,
+  config: Configuration,
+): Configuration {
+  type LocalPostCSSLoader = Loader & {options: {postcssOptions: any}};
+
+  function isPostCssLoader(loader: Loader): loader is LocalPostCSSLoader {
+    // TODO not ideal heuristic but good enough for our usecase?
+    return !!(loader as any)?.options?.postcssOptions;
+  }
+
+  // Does not handle all edge cases, but good enough for now
+  config.module?.rules.map((rule) => {
+    for (const loader of rule.use as NewLoader[]) {
+      if (isPostCssLoader(loader)) {
+        loader.options.postcssOptions = configurePostCss(
+          loader.options.postcssOptions,
+        );
+      }
+    }
+  });
+
+  return config;
+}
+
 // See https://webpack.js.org/configuration/stats/#statswarningsfilter
 // @slorber: note sure why we have to re-implement this logic
 // just know that legacy had this only partially implemented, so completed it

website/docs/lifecycle-apis.md

@@ -346,6 +346,45 @@ module.exports = function (context, options) {
 
 Read the [webpack-merge strategy doc](https://github.com/survivejs/webpack-merge#merging-with-strategies) for more details.
 
+## `configurePostCss(options)`
+
+Modifies [`postcssOptions` of `postcss-loader`](https://webpack.js.org/loaders/postcss-loader/#postcssoptions) during the generation of the client bundle.
+
+Should return the mutated `postcssOptions`.
+
+By default, `postcssOptions` looks like this:
+
+```js
+const postcssOptions = {
+  ident: 'postcss',
+  plugins: [
+    require('postcss-preset-env')({
+      autoprefixer: {
+        flexbox: 'no-2009',
+      },
+      stage: 4,
+    }),
+  ],
+};
+```
+
+Example:
+
+```js title="docusaurus-plugin/src/index.js"
+module.exports = function (context, options) {
+  return {
+    name: 'docusaurus-plugin',
+    // highlight-start
+    configurePostCss(postcssOptions) {
+      // Appends new PostCSS plugin.
+      postcssOptions.plugins.push(require('postcss-import'));
+      return postcssOptions;
+    },
+    // highlight-end
+  };
+};
+```
+
 ## `postBuild(props)`
 
 Called when a (production) build finishes.