Check if postcss-load-config will resolve config if not use default options

- Update post-processing-css.md
- Add test
- Add postcss-load-config as react-scripts dependency

Author: piecyk

docusaurus/docs/post-processing-css.md

@@ -42,3 +42,19 @@ If you need to disable autoprefixing for some reason, [follow this section](http
 If you'd like to opt-in to CSS Grid prefixing, [first familiarize yourself about its limitations](https://github.com/postcss/autoprefixer#does-autoprefixer-polyfill-grid-layout-for-ie).
 
 To enable CSS Grid prefixing, add `/* autoprefixer grid: autoplace */` to the top of your CSS file.
+
+### Customizing PostCSS Config
+
+However, if you want to add custom plugins, add a `postcss.config.js` file to the root of your project. It will replace the internal CRA postcss plugins.
+
+You can read more about common PostCSS Config [here](https://github.com/michael-ciniawsky/postcss-load-config).
+
+```js
+module.exports = {
+  parser: 'sugarss',
+  map: false,
+  plugins: {
+    'postcss-plugin': {},
+  },
+};
+```

packages/react-scripts/config/webpack.config.js

@@ -36,7 +36,15 @@ const ReactRefreshWebpackPlugin = require('@pmmmwh/react-refresh-webpack-plugin'
 // @remove-on-eject-begin
 const getCacheIdentifier = require('react-dev-utils/getCacheIdentifier');
 // @remove-on-eject-end
-const postcssNormalize = require('postcss-normalize');
+const postcssLoadConfig = require('postcss-load-config');
+
+const hasPostcssConfig = (() => {
+  try {
+    return !!postcssLoadConfig.sync();
+  } catch (_error) {
+    return false;
+  }
+})();
 
 const appPackageJson = require(paths.appPackageJson);
 
@@ -110,19 +118,23 @@ module.exports = function (webpackEnv) {
           // Necessary for external CSS imports to work
           // https://github.com/facebook/create-react-app/issues/2677
           ident: 'postcss',
-          plugins: () => [
-            require('postcss-flexbugs-fixes'),
-            require('postcss-preset-env')({
-              autoprefixer: {
-                flexbox: 'no-2009',
-              },
-              stage: 3,
-            }),
-            // Adds PostCSS Normalize as the reset css with default options,
-            // so that it honors browserslist config in package.json
-            // which in turn let's users customize the target behavior as per their needs.
-            postcssNormalize(),
-          ],
+          ...(hasPostcssConfig
+            ? {}
+            : {
+                plugins: () => [
+                  require('postcss-flexbugs-fixes'),
+                  require('postcss-preset-env')({
+                    autoprefixer: {
+                      flexbox: 'no-2009',
+                    },
+                    stage: 3,
+                  }),
+                  // Adds PostCSS Normalize as the reset css with default options,
+                  // so that it honors browserslist config in package.json
+                  // which in turn let's users customize the target behavior as per their needs.
+                  require('postcss-normalize')(),
+                ],
+              }),
           sourceMap: isEnvProduction ? shouldUseSourceMap : isEnvDevelopment,
         },
       },

packages/react-scripts/package.json

@@ -64,6 +64,7 @@
     "pnp-webpack-plugin": "1.6.4",
     "postcss-flexbugs-fixes": "4.2.1",
     "postcss-loader": "3.0.0",
+    "postcss-load-config": "2.1.0",
     "postcss-normalize": "8.0.1",
     "postcss-preset-env": "6.7.0",
     "postcss-safe-parser": "4.0.2",

test/fixtures/postcss-config/.disable-pnp

@@ -0,0 +1,6 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`builds with custom postcss config 1`] = `
+"body{background:#056ef0}
+/*# sourceMappingURL=main.9b96d08a.chunk.css.map */"
+`;

test/fixtures/postcss-config/__snapshots__/index.test.js.snap

@@ -0,0 +1,17 @@
+const testSetup = require('../__shared__/test-setup');
+
+const fs = require('fs-extra');
+const globby = require('globby');
+const path = require('path');
+
+test('builds with custom postcss config', async () => {
+  await testSetup.scripts.build();
+
+  const buildDir = path.join(testSetup.testDirectory, 'build');
+  const cssFile = path.join(
+    buildDir,
+    globby.sync('**/*.css', { cwd: buildDir }).pop()
+  );
+
+  expect(fs.readFileSync(cssFile, 'utf8')).toMatchSnapshot();
+});

test/fixtures/postcss-config/index.test.js

@@ -0,0 +1,7 @@
+{
+  "dependencies": {
+    "react": "latest",
+    "react-dom": "latest",
+    "postcss-simple-vars": "latest"
+  }
+}

test/fixtures/postcss-config/package.json

@@ -0,0 +1,3 @@
+module.exports = {
+  plugins: [require('postcss-simple-vars')],
+};

test/fixtures/postcss-config/postcss.config.js

@@ -0,0 +1,5 @@
+$blue: #056ef0;
+
+body {
+  background: $blue;
+}

test/fixtures/postcss-config/src/index.css

@@ -0,0 +1,5 @@
+import React from 'react';
+import ReactDOM from 'react-dom';
+import './index.css';
+
+ReactDOM.render(<div />, document.getElementById('root'));