feat: add getJSON option to output CSS modules mapping

This changeset adds a `getJSON` option to output CSS modules mappings to JSON.  This value can be a boolean or a function, and it employs similar logic to [postcss-modules#getJSON](https://github.com/madyankin/postcss-modules?tab=readme-ov-file#saving-exported-classes) as a function.  This is particularly useful for SSR/SSG/templating languages when CSS modules mappings need to be present at build time.

Addresses [webpack-contrib#988](webpack-contrib#988).

README.md

@@ -327,6 +327,9 @@ type modules =
         | "dashesOnly"
         | ((name: string) => string);
       exportOnlyLocals: boolean;
+      getJSON:
+        | string
+        | ((resourcePath: string, json: object, outputPath: string) => any);
     };
 ```
 
@@ -592,6 +595,7 @@ module.exports = {
             namedExport: true,
             exportLocalsConvention: "camelCase",
             exportOnlyLocals: false,
+            getJSON: false,
           },
         },
       },
@@ -1372,6 +1376,75 @@ module.exports = {
 };
 ```
 
+##### `getJSON`
+
+Type:
+
+```ts
+type getJSON =
+  | boolean
+  | ((resourcePath: string, json: object, outputPath: string) => any);
+```
+
+Default: `undefined`
+
+Enables the outputting of the CSS modules mapping JSON. This can be omitted or set to a falsy value to disable any output.
+
+###### `boolean`
+
+Possible values:
+
+- `true` - writes a JSON file next located in the same directory as the loaded resource file. For example, given a resource file located at /foo/bar/baz.css, this would write the CSS modules mapping JSON to /foo/bar/baz.css.json
+- `false` - disables CSS modules mapping JSON output
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        loader: "css-loader",
+        options: {
+          modules: {
+            getJSON: true,
+          },
+        },
+      },
+    ],
+  },
+};
+```
+
+###### `function`
+
+Enables custom handling of the CSS modules mapping JSON output. The return value of the function is not consumed.
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        loader: "css-loader",
+        options: {
+          modules: {
+            getJSON: (resourcePath, json, outputPath) => {
+              // `resourcePath` is the original resource file path, e.g., /foo/bar/baz.css
+              // `json` is the CSS modules map
+              // `outputPath` is the expected output file path, e.g., /foo/bar/baz.css.json
+            },
+          },
+        },
+      },
+    ],
+  },
+};
+```
+
 ### `importLoaders`
 
 Type:

src/index.js

@@ -27,6 +27,7 @@ import {
   stringifyRequest,
   warningFactory,
   syntaxErrorFactory,
+  writeModulesMap,
 } from "./utils";
 
 export default async function loader(content, map, meta) {
@@ -274,5 +275,16 @@ export default async function loader(content, map, meta) {
     isTemplateLiteralSupported
   );
 
+  try {
+    const { getJSON } = options.modules;
+    if (getJSON) {
+      await writeModulesMap(getJSON, resourcePath, exports);
+    }
+  } catch (error) {
+    callback(error);
+
+    return;
+  }
+
   callback(null, `${importCode}${moduleCode}${exportCode}`);
 }

src/options.json

@@ -169,6 +169,18 @@
               "description": "Export only locals.",
               "link": "https://github.com/webpack-contrib/css-loader#exportonlylocals",
               "type": "boolean"
+            },
+            "getJSON": {
+              "description": "Output CSS modules mapping to a JSON file or through a callback.",
+              "link": "https://github.com/webpack-contrib/css-loader#getJSON",
+              "anyOf": [
+                {
+                  "type": "boolean"
+                },
+                {
+                  "instanceof": "Function"
+                }
+              ]
             }
           }
         }

src/utils.js

@@ -4,6 +4,7 @@
 */
 import { fileURLToPath } from "url";
 import path from "path";
+import fsp from "fs/promises";
 
 import modulesValues from "postcss-modules-values";
 import localByDefault from "postcss-modules-local-by-default";
@@ -1412,6 +1413,26 @@ function syntaxErrorFactory(error) {
   return obj;
 }
 
+async function writeModulesMap(getJSON, resourcePath, exports) {
+  const json = exports.reduce((acc, { name, value }) => {
+    return { ...acc, [name]: value };
+  }, {});
+
+  const outputPath = path.resolve(
+    path.dirname(resourcePath),
+    `${path.basename(resourcePath)}.json`
+  );
+
+  if (getJSON === true) {
+    // If true, output a JSON CSS modules mapping file in the same directory as the resource
+    await fsp.writeFile(outputPath, JSON.stringify(json));
+  } else if (typeof getJSON === "function") {
+    // If function, call function with call getJSON with similar args as postcss-modules#getJSON
+    // https://github.com/madyankin/postcss-modules/tree/master?tab=readme-ov-file#saving-exported-classes
+    getJSON(resourcePath, json, outputPath);
+  }
+}
+
 export {
   normalizeOptions,
   shouldUseModulesPlugins,
@@ -1439,4 +1460,5 @@ export {
   defaultGetLocalIdent,
   warningFactory,
   syntaxErrorFactory,
+  writeModulesMap,
 };

test/__snapshots__/modules-option.test.js.snap

@@ -1122,6 +1122,75 @@ exports[`"modules" option should emit warning when localIdentName is emoji: erro
 
 exports[`"modules" option should emit warning when localIdentName is emoji: warnings 1`] = `Array []`;
 
+exports[`"modules" option should invoke the custom getJSON function with getJSON as a function: errors 1`] = `Array []`;
+
+exports[`"modules" option should invoke the custom getJSON function with getJSON as a function: mapping 1`] = `
+Object {
+  "a": "RT7ktT7mB7tfBR25sJDZ",
+  "b": "IZmhTnK9CIeu6ww6Zjbv",
+  "c": "PV11nPFlF7mzEgCXkQw4",
+}
+`;
+
+exports[`"modules" option should invoke the custom getJSON function with getJSON as a function: module 1`] = `
+"// Imports
+import ___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___ from \\"../../../../src/runtime/noSourceMaps.js\\";
+import ___CSS_LOADER_API_IMPORT___ from \\"../../../../src/runtime/api.js\\";
+var ___CSS_LOADER_EXPORT___ = ___CSS_LOADER_API_IMPORT___(___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___);
+// Module
+___CSS_LOADER_EXPORT___.push([module.id, \`.RT7ktT7mB7tfBR25sJDZ {
+  background-color: aliceblue;
+}
+
+.IZmhTnK9CIeu6ww6Zjbv {
+  background-color: burlywood;
+}
+
+.PV11nPFlF7mzEgCXkQw4 {
+  background-color: chartreuse;
+}
+
+.d {
+  background-color: darkgoldenrod
+}
+\`, \\"\\"]);
+// Exports
+___CSS_LOADER_EXPORT___.locals = {
+	\\"a\\": \`RT7ktT7mB7tfBR25sJDZ\`,
+	\\"b\\": \`IZmhTnK9CIeu6ww6Zjbv\`,
+	\\"c\\": \`PV11nPFlF7mzEgCXkQw4\`
+};
+export default ___CSS_LOADER_EXPORT___;
+"
+`;
+
+exports[`"modules" option should invoke the custom getJSON function with getJSON as a function: result 1`] = `
+Array [
+  Array [
+    "./modules/getJSON/source.css",
+    ".RT7ktT7mB7tfBR25sJDZ {
+  background-color: aliceblue;
+}
+
+.IZmhTnK9CIeu6ww6Zjbv {
+  background-color: burlywood;
+}
+
+.PV11nPFlF7mzEgCXkQw4 {
+  background-color: chartreuse;
+}
+
+.d {
+  background-color: darkgoldenrod
+}
+",
+    "",
+  ],
+]
+`;
+
+exports[`"modules" option should invoke the custom getJSON function with getJSON as a function: warnings 1`] = `Array []`;
+
 exports[`"modules" option should keep order: errors 1`] = `Array []`;
 
 exports[`"modules" option should keep order: module 1`] = `
@@ -1194,6 +1263,69 @@ Array [
 
 exports[`"modules" option should keep order: warnings 1`] = `Array []`;
 
+exports[`"modules" option should output a co-located CSS modules map file with getJSON as true: errors 1`] = `Array []`;
+
+exports[`"modules" option should output a co-located CSS modules map file with getJSON as true: mapping 1`] = `"{\\"a\\":\\"RT7ktT7mB7tfBR25sJDZ\\",\\"b\\":\\"IZmhTnK9CIeu6ww6Zjbv\\",\\"c\\":\\"PV11nPFlF7mzEgCXkQw4\\"}"`;
+
+exports[`"modules" option should output a co-located CSS modules map file with getJSON as true: module 1`] = `
+"// Imports
+import ___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___ from \\"../../../../src/runtime/noSourceMaps.js\\";
+import ___CSS_LOADER_API_IMPORT___ from \\"../../../../src/runtime/api.js\\";
+var ___CSS_LOADER_EXPORT___ = ___CSS_LOADER_API_IMPORT___(___CSS_LOADER_API_NO_SOURCEMAP_IMPORT___);
+// Module
+___CSS_LOADER_EXPORT___.push([module.id, \`.RT7ktT7mB7tfBR25sJDZ {
+  background-color: aliceblue;
+}
+
+.IZmhTnK9CIeu6ww6Zjbv {
+  background-color: burlywood;
+}
+
+.PV11nPFlF7mzEgCXkQw4 {
+  background-color: chartreuse;
+}
+
+.d {
+  background-color: darkgoldenrod
+}
+\`, \\"\\"]);
+// Exports
+___CSS_LOADER_EXPORT___.locals = {
+	\\"a\\": \`RT7ktT7mB7tfBR25sJDZ\`,
+	\\"b\\": \`IZmhTnK9CIeu6ww6Zjbv\`,
+	\\"c\\": \`PV11nPFlF7mzEgCXkQw4\`
+};
+export default ___CSS_LOADER_EXPORT___;
+"
+`;
+
+exports[`"modules" option should output a co-located CSS modules map file with getJSON as true: result 1`] = `
+Array [
+  Array [
+    "./modules/getJSON/source.css",
+    ".RT7ktT7mB7tfBR25sJDZ {
+  background-color: aliceblue;
+}
+
+.IZmhTnK9CIeu6ww6Zjbv {
+  background-color: burlywood;
+}
+
+.PV11nPFlF7mzEgCXkQw4 {
+  background-color: chartreuse;
+}
+
+.d {
+  background-color: darkgoldenrod
+}
+",
+    "",
+  ],
+]
+`;
+
+exports[`"modules" option should output a co-located CSS modules map file with getJSON as true: warnings 1`] = `Array []`;
+
 exports[`"modules" option should resolve absolute path in composes: errors 1`] = `Array []`;
 
 exports[`"modules" option should resolve absolute path in composes: module 1`] = `

test/fixtures/modules/getJSON/source.css

@@ -0,0 +1,15 @@
+.a {
+  background-color: aliceblue;
+}
+
+.b {
+  background-color: burlywood;
+}
+
+.c {
+  background-color: chartreuse;
+}
+
+:global(.d) {
+  background-color: darkgoldenrod
+}

test/fixtures/modules/getJSON/source.js

@@ -0,0 +1,5 @@
+import css from './source.css';
+
+__export__ = css;
+
+export default css;