[Experimental] CSS Module Support

packages/next/build/webpack/config/blocks/css/getCssModuleLocalIdent.ts

@@ -0,0 +1,46 @@
+import loaderUtils from 'loader-utils'
+import path from 'path'
+import webpack from 'webpack'
+
+export function getCssModuleLocalIdent(
+  context: webpack.loader.LoaderContext,
+  _: any,
+  exportName: string,
+  options: object
+) {
+  const relativePath = path.posix.relative(
+    context.rootContext,
+    context.resourcePath
+  )
+
+  // Generate a more meaningful name (parent folder) when the user names the
+  // file `index.module.css`.
+  const fileNameOrFolder =
+    relativePath.endsWith('index.module.css') &&
+    relativePath !== 'pages/index.module.css'
+      ? '[folder]'
+      : '[name]'
+
+  // Generate a hash to make the class name unique.
+  const hash = loaderUtils.getHashDigest(
+    Buffer.from(`filePath:${relativePath}#className:${exportName}`),
+    'md5',
+    'base64',
+    5
+  )
+
+  // Have webpack interpolate the `[folder]` or `[name]` to its real value.
+  return loaderUtils
+    .interpolateName(
+      context,
+      fileNameOrFolder + '_' + exportName + '__' + hash,
+      options
+    )
+    .replace(
+      // Webpack name interpolation returns `about.module_root__2oFM9` for
+      // `.root {}` inside a file named `about.module.css`. Let's simplify
+      // this.
+      /\.module_/,
+      '_'
+    )
+}

packages/next/build/webpack/config/blocks/css/index.ts

@@ -1,14 +1,14 @@
 import curry from 'lodash.curry'
 import MiniCssExtractPlugin from 'mini-css-extract-plugin'
 import path from 'path'
-import { Configuration } from 'webpack'
+import webpack, { Configuration } from 'webpack'
 import { loader } from '../../helpers'
 import { ConfigurationContext, ConfigurationFn, pipe } from '../../utils'
-import { getGlobalImportError } from './messages'
+import { getCssModuleLocalIdent } from './getCssModuleLocalIdent'
+import { getGlobalImportError, getModuleImportError } from './messages'
 import { getPostCssPlugins } from './plugins'
-import webpack from 'webpack'
 
-function getStyleLoader({
+function getClientStyleLoader({
   isDevelopment,
 }: {
   isDevelopment: boolean
@@ -73,14 +73,93 @@ export const css = curry(async function css(
 
   const fns: ConfigurationFn[] = []
 
+  const postCssPlugins = await getPostCssPlugins(ctx.rootDirectory)
+  // CSS Modules support must be enabled on the server and client so the class
+  // names are availble for SSR or Prerendering.
+  fns.push(
+    loader({
+      oneOf: [
+        {
+          // CSS Modules should never have side effects. This setting will
+          // allow unused CSS to be removed from the production build.
+          // We ensure this by disallowing `:global()` CSS at the top-level
+          // via the `pure` mode in `css-loader`.
+          sideEffects: false,
+          // CSS Modules are activated via this specific extension.
+          test: /\.module\.css$/,
+          // CSS Modules are only supported in the user's application. We're
+          // not yet allowing CSS imports _within_ `node_modules`.
+          issuer: {
+            include: [ctx.rootDirectory],
+            exclude: /node_modules/,
+          },
+
+          use: ([
+            // Add appropriate development more or production mode style
+            // loader
+            ctx.isClient &&
+              getClientStyleLoader({ isDevelopment: ctx.isDevelopment }),
+
+            // Resolve CSS `@import`s and `url()`s
+            {
+              loader: require.resolve('css-loader'),
+              options: {
+                importLoaders: 1,
+                sourceMap: true,
+                onlyLocals: ctx.isServer,
+                modules: {
+                  // Disallow global style exports so we can code-split CSS and
+                  // not worry about loading order.
+                  mode: 'pure',
+                  // Generate a friendly production-ready name so it's
+                  // reasonably understandable. The same name is used for
+                  // development.
+                  // TODO: Consider making production reduce this to a single
+                  // character?
+                  getLocalIdent: getCssModuleLocalIdent,
+                },
+              },
+            },
+
+            // Compile CSS
+            {
+              loader: require.resolve('postcss-loader'),
+              options: {
+                ident: 'postcss',
+                plugins: postCssPlugins,
+                sourceMap: true,
+              },
+            },
+          ] as webpack.RuleSetUseItem[]).filter(Boolean),
+        },
+      ],
+    })
+  )
+
+  // Throw an error for CSS Modules used outside their supported scope
+  fns.push(
+    loader({
+      oneOf: [
+        {
+          test: /\.module\.css$/,
+          use: {
+            loader: 'error-loader',
+            options: {
+              reason: getModuleImportError(),
+            },
+          },
+        },
+      ],
+    })
+  )
+
   if (ctx.isServer) {
     fns.push(
       loader({
         oneOf: [{ test: /\.css$/, use: require.resolve('ignore-loader') }],
       })
     )
   } else if (ctx.customAppFile) {
-    const postCssPlugins = await getPostCssPlugins(ctx.rootDirectory)
     fns.push(
       loader({
         oneOf: [
@@ -96,7 +175,7 @@ export const css = curry(async function css(
             use: [
               // Add appropriate development more or production mode style
               // loader
-              getStyleLoader({ isDevelopment: ctx.isDevelopment }),
+              getClientStyleLoader({ isDevelopment: ctx.isDevelopment }),
 
               // Resolve CSS `@import`s and `url()`s
               {

packages/next/build/webpack/config/blocks/css/messages.ts

@@ -9,3 +9,10 @@ export function getGlobalImportError(file: string | null) {
     file ? file : 'pages/_app.js'
   )}.\nRead more: https://err.sh/next.js/global-css`
 }
+
+export function getModuleImportError() {
+  // TODO: Read more link
+  return `CSS Modules ${chalk.bold(
+    'cannot'
+  )} be imported from within ${chalk.bold('node_modules')}.`
+}

packages/next/client/page-loader.js

@@ -11,12 +11,20 @@ function supportsPreload(el) {
 
 const hasPreload = supportsPreload(document.createElement('link'))
 
-function preloadScript(url) {
+function preloadLink(url, resourceType) {
   const link = document.createElement('link')
   link.rel = 'preload'
   link.crossOrigin = process.crossOrigin
   link.href = url
-  link.as = 'script'
+  link.as = resourceType
+  document.head.appendChild(link)
+}
+
+function loadStyle(url) {
+  const link = document.createElement('link')
+  link.rel = 'stylesheet'
+  link.crossOrigin = process.crossOrigin
+  link.href = url
   document.head.appendChild(link)
 }
 
@@ -105,6 +113,12 @@ export default class PageLoader {
               ) {
                 this.loadScript(d, route, false)
               }
+              if (
+                /\.css$/.test(d) &&
+                !document.querySelector(`link[rel=stylesheet][href^="${d}"]`)
+              ) {
+                loadStyle(d) // FIXME: handle failure
+              }
             })
             this.loadRoute(route)
             this.loadingRoutes[route] = true
@@ -228,7 +242,7 @@ export default class PageLoader {
     // If not fall back to loading script tags before the page is loaded
     // https://caniuse.com/#feat=link-rel-preload
     if (hasPreload) {
-      preloadScript(url)
+      preloadLink(url, url.match(/\.css$/) ? 'style' : 'script')
       return
     }
 

packages/next/package.json

@@ -87,7 +87,7 @@
     "conf": "5.0.0",
     "content-type": "1.0.4",
     "cookie": "0.4.0",
-    "css-loader": "3.2.0",
+    "css-loader": "3.3.0",
     "cssnano-simple": "1.0.0",
     "devalue": "2.0.1",
     "etag": "1.8.1",

test/integration/css/fixtures/basic-module/pages/index.js

@@ -0,0 +1,9 @@
+import { redText } from './index.module.css'
+
+export default function Home() {
+  return (
+    <div id="verify-red" className={redText}>
+      This text should be red.
+    </div>
+  )
+}

test/integration/css/fixtures/basic-module/pages/index.module.css

@@ -0,0 +1,3 @@
+.redText {
+  color: red;
+}

test/integration/css/fixtures/dev-module/pages/index.js

@@ -0,0 +1,9 @@
+import { redText } from './index.module.css'
+
+export default function Home() {
+  return (
+    <div id="verify-red" className={redText}>
+      This text should be red.
+    </div>
+  )
+}

test/integration/css/fixtures/dev-module/pages/index.module.css

@@ -0,0 +1,3 @@
+.redText {
+  color: red;
+}

test/integration/css/fixtures/hmr-module/pages/index.js

@@ -0,0 +1,15 @@
+import { redText } from './index.module.css'
+
+function Home() {
+  return (
+    <>
+      <div id="verify-red" className={redText}>
+        This text should be red.
+      </div>
+      <br />
+      <input key={'' + Math.random()} id="text-input" type="text" />
+    </>
+  )
+}
+
+export default Home

test/integration/css/fixtures/hmr-module/pages/index.module.css

@@ -0,0 +1,3 @@
+.redText {
+  color: red;
+}

test/integration/css/fixtures/invalid-module/pages/index.js

@@ -0,0 +1,7 @@
+import * as classes from 'example'
+
+function Home() {
+  return <div>This should fail at build time {JSON.stringify(classes)}.</div>
+}
+
+export default Home

test/integration/css/fixtures/multi-module/pages/blue.js

@@ -0,0 +1,20 @@
+import Link from 'next/link'
+import { blueText } from './blue.module.css'
+
+export default function Blue() {
+  return (
+    <>
+      <div id="verify-blue" className={blueText}>
+        This text should be blue.
+      </div>
+      <br />
+      <Link href="/red" prefetch>
+        <a id="link-red">Red</a>
+      </Link>
+      <br />
+      <Link href="/none" prefetch={false}>
+        <a id="link-none">None</a>
+      </Link>
+    </>
+  )
+}

test/integration/css/fixtures/multi-module/pages/blue.module.css

@@ -0,0 +1,3 @@
+.blueText {
+  color: blue;
+}

test/integration/css/fixtures/multi-module/pages/none.js

@@ -0,0 +1,19 @@
+import Link from 'next/link'
+
+export default function None() {
+  return (
+    <>
+      <div id="verify-black" style={{ color: 'black' }}>
+        This text should be black.
+      </div>
+      <br />
+      <Link href="/red" prefetch={false}>
+        <a id="link-red">Red</a>
+      </Link>
+      <br />
+      <Link href="/blue" prefetch={false}>
+        <a id="link-blue">Blue</a>
+      </Link>
+    </>
+  )
+}

test/integration/css/fixtures/multi-module/pages/red.js

@@ -0,0 +1,20 @@
+import Link from 'next/link'
+import { redText } from './red.module.css'
+
+export default function Red() {
+  return (
+    <>
+      <div id="verify-red" className={redText}>
+        This text should be red.
+      </div>
+      <br />
+      <Link href="/blue" prefetch={false}>
+        <a id="link-blue">Blue</a>
+      </Link>
+      <br />
+      <Link href="/none" prefetch={false}>
+        <a id="link-none">None</a>
+      </Link>
+    </>
+  )
+}

test/integration/css/fixtures/multi-module/pages/red.module.css

@@ -0,0 +1,3 @@
+.redText {
+  color: red;
+}