feat: add support for async functions to preprocessor option

README.md

@@ -50,12 +50,12 @@ module.exports = {
 
 ## Options
 
-|                Name                 |        Type         |                   Default                    | Description                                      |
-| :---------------------------------: | :-----------------: | :------------------------------------------: | :----------------------------------------------- |
-|   **[`attributes`](#attributes)**   | `{Boolean\|Object}` |                    `true`                    | Enables/Disables attributes handling             |
-| **[`preprocessor`](#preprocessor)** |    `{Function}`     |                 `undefined`                  | Allows pre-processing of content before handling |
-|     **[`minimize`](#minimize)**     | `{Boolean\|Object}` | `true` in production mode, otherwise `false` | Tell `html-loader` to minimize HTML              |
-|     **[`esModule`](#esmodule)**     |     `{Boolean}`     |                   `false`                    | Use ES modules syntax                            |
+|                Name                 |            Type             |                   Default                    |                   Description                    |
+| :---------------------------------: | :-------------------------: | :------------------------------------------: | :----------------------------------------------: |
+|   **[`attributes`](#attributes)**   |     `{Boolean\|Object}`     |                    `true`                    |       Enables/Disables attributes handling       |
+| **[`preprocessor`](#preprocessor)** | `{Function\|AsyncFunction}` |                 `undefined`                  | Allows pre-processing of content before handling |
+|     **[`minimize`](#minimize)**     |     `{Boolean\|Object}`     | `true` in production mode, otherwise `false` |       Tell `html-loader` to minimize HTML        |
+|     **[`esModule`](#esmodule)**     |         `{Boolean}`         |                   `false`                    |              Use ES modules syntax               |
 
 ### `attributes`
 
@@ -336,7 +336,7 @@ module.exports = {
 
 ### `preprocessor`
 
-Type: `Function`
+Type: `Function|AsyncFunction`
 Default: `undefined`
 
 Allows pre-processing of content before handling.
@@ -352,6 +352,10 @@ Allows pre-processing of content before handling.
 <div>
 ```
 
+#### `Function`
+
+You can set the `preprocessor` option as a `Function` instance.
+
 **webpack.config.js**
 
 ```js
@@ -387,6 +391,45 @@ module.exports = {
 };
 ```
 
+#### `AsyncFunction`
+
+You can set the `preprocessor` option as an Asynchronous Function (`AsyncFunction`) instance.
+
+**webpack.config.js**
+
+```js
+const Handlebars = require('handlebars');
+
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.hbs$/i,
+        loader: 'html-loader',
+        options: {
+          preprocessor: async (content, loaderContext) => {
+            let result;
+
+            try {
+              result = await Handlebars.compile(content)({
+                firstname: 'Value',
+                lastname: 'OtherValue',
+              });
+            } catch (error) {
+              await loaderContext.emitError(error);
+
+              return content;
+            }
+
+            return result;
+          },
+        },
+      },
+    ],
+  },
+};
+```
+
 ### `minimize`
 
 Type: `Boolean|Object`

src/index.js

@@ -13,7 +13,7 @@ import {
 
 import schema from './options.json';
 
-export default function htmlLoader(content) {
+export default async function htmlLoader(content) {
   const options = getOptions(this);
 
   validateOptions(schema, options, {
@@ -23,7 +23,7 @@ export default function htmlLoader(content) {
 
   if (options.preprocessor) {
     // eslint-disable-next-line no-param-reassign
-    content = options.preprocessor(content, this);
+    content = await options.preprocessor(content, this);
   }
 
   const plugins = [];

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

@@ -1,5 +1,28 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`'process' option should work with Async "preprocessor" Function option: errors 1`] = `Array []`;
+
+exports[`'process' option should work with Async "preprocessor" Function option: module 1`] = `
+"// Imports
+var ___HTML_LOADER_GET_SOURCE_FROM_IMPORT___ = require(\\"../../src/runtime/getUrl.js\\");
+var ___HTML_LOADER_IMPORT_0___ = require(\\"./image.png\\");
+// Module
+var ___HTML_LOADER_REPLACER_0___ = ___HTML_LOADER_GET_SOURCE_FROM_IMPORT___(___HTML_LOADER_IMPORT_0___);
+var code = \\"<div>\\\\n  <p>Alexander Krasnoyarov</p>\\\\n  <img src=\\\\\\"\\" + ___HTML_LOADER_REPLACER_0___ + \\"\\\\\\" alt=\\\\\\"alt\\\\\\" />\\\\n<div>\\\\n\\";
+// Exports
+module.exports = code;"
+`;
+
+exports[`'process' option should work with Async "preprocessor" Function option: result 1`] = `
+"<div>
+  <p>Alexander Krasnoyarov</p>
+  <img src=\\"/webpack/public/path/image.png\\" alt=\\"alt\\" />
+<div>
+"
+`;
+
+exports[`'process' option should work with Async "preprocessor" Function option: warnings 1`] = `Array []`;
+
 exports[`'process' option should work with the "preprocessor" option #2: errors 1`] = `Array []`;
 
 exports[`'process' option should work with the "preprocessor" option #2: module 1`] = `
@@ -44,3 +67,25 @@ exports[`'process' option should work with the "preprocessor" option: result 1`]
 `;
 
 exports[`'process' option should work with the "preprocessor" option: warnings 1`] = `Array []`;
+
+exports[`'process' option should work with the Async "preprocessor" Function option #2: errors 1`] = `Array []`;
+
+exports[`'process' option should work with the Async "preprocessor" Function option #2: module 1`] = `
+"// Imports
+var ___HTML_LOADER_GET_SOURCE_FROM_IMPORT___ = require(\\"../../src/runtime/getUrl.js\\");
+var ___HTML_LOADER_IMPORT_0___ = require(\\"./image.png.webp\\");
+var ___HTML_LOADER_IMPORT_1___ = require(\\"./image.png\\");
+// Module
+var ___HTML_LOADER_REPLACER_0___ = ___HTML_LOADER_GET_SOURCE_FROM_IMPORT___(___HTML_LOADER_IMPORT_0___);
+var ___HTML_LOADER_REPLACER_1___ = ___HTML_LOADER_GET_SOURCE_FROM_IMPORT___(___HTML_LOADER_IMPORT_1___);
+var code = \\"<picture><source type=\\\\\\"image/webp\\\\\\" srcset=\\\\\\"\\" + ___HTML_LOADER_REPLACER_0___ + \\"\\\\\\"><img src=\\\\\\"\\" + ___HTML_LOADER_REPLACER_1___ + \\"\\\\\\"></picture>\\\\n\\";
+// Exports
+module.exports = code;"
+`;
+
+exports[`'process' option should work with the Async "preprocessor" Function option #2: result 1`] = `
+"<picture><source type=\\"image/webp\\" srcset=\\"/webpack/public/path/image.png.webp\\"><img src=\\"/webpack/public/path/image.png\\"></picture>
+"
+`;
+
+exports[`'process' option should work with the Async "preprocessor" Function option #2: warnings 1`] = `Array []`;

test/preprocessor-option.test.js

@@ -47,6 +47,40 @@ describe("'process' option", () => {
     expect(getErrors(stats)).toMatchSnapshot('errors');
   });
 
+  it('should work with Async "preprocessor" Function option', async () => {
+    const compiler = getCompiler('preprocessor.hbs', {
+      preprocessor: async (content, loaderContext) => {
+        await expect(typeof content).toBe('string');
+        await expect(loaderContext).toBeDefined();
+
+        let result;
+
+        try {
+          result = await Handlebars.compile(content)({
+            firstname: 'Alexander',
+            lastname: 'Krasnoyarov',
+          });
+        } catch (error) {
+          await loaderContext.emitError(error);
+
+          return content;
+        }
+
+        return result;
+      },
+    });
+    const stats = await compile(compiler);
+
+    expect(getModuleSource('./preprocessor.hbs', stats)).toMatchSnapshot(
+      'module'
+    );
+    expect(
+      execute(readAsset('main.bundle.js', compiler, stats))
+    ).toMatchSnapshot('result');
+    expect(getWarnings(stats)).toMatchSnapshot('warnings');
+    expect(getErrors(stats)).toMatchSnapshot('errors');
+  });
+
   it('should work with the "preprocessor" option #2', async () => {
     const plugin = posthtmlWebp();
     const compiler = getCompiler('posthtml.html', {
@@ -71,6 +105,37 @@ describe("'process' option", () => {
     });
     const stats = await compile(compiler);
 
+    expect(getModuleSource('./posthtml.html', stats)).toMatchSnapshot('module');
+    expect(
+      execute(readAsset('main.bundle.js', compiler, stats))
+    ).toMatchSnapshot('result');
+    expect(getWarnings(stats)).toMatchSnapshot('warnings');
+    expect(getErrors(stats)).toMatchSnapshot('errors');
+  });
+  it('should work with the Async "preprocessor" Function option #2', async () => {
+    const plugin = posthtmlWebp();
+    const compiler = getCompiler('posthtml.html', {
+      preprocessor: async (content, loaderContext) => {
+        await expect(typeof content).toBe('string');
+        await expect(loaderContext).toBeDefined();
+
+        let result;
+
+        try {
+          result = await posthtml()
+            .use(plugin)
+            .process(content, { sync: true });
+        } catch (error) {
+          loaderContext.emitError(error);
+
+          return content;
+        }
+
+        return result.html;
+      },
+    });
+    const stats = await compile(compiler);
+
     expect(getModuleSource('./posthtml.html', stats)).toMatchSnapshot('module');
     expect(
       execute(readAsset('main.bundle.js', compiler, stats))