From ebedae17bb514779691f9c1cf84118c6eddd4d96 Mon Sep 17 00:00:00 2001
From: Logan Smyth <loganfsmyth@gmail.com>
Date: Thu, 17 May 2018 10:50:08 -0700
Subject: [PATCH] Expose API to allow implementing custom loaders around
 babel-loader. (#619)

---
 README.md    | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/index.js | 49 ++++++++++++++++++++++++++-------
 2 files changed, 115 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index e1a7a5eb..09a1b466 100644
--- a/README.md
+++ b/README.md
@@ -205,4 +205,80 @@ In the case one of your dependencies is installing `babel` and you cannot uninst
   }
 ```
 
+## Customized Loader
+
+`babel-loader` exposes a loader-builder utility that allows users to add custom handling
+of Babel's configuration for each file that it processes.
+
+`.custom` accepts a callback that will be called with the loader's instance of
+`babel` so that tooling can ensure that it using exactly the same `@babel/core`
+instance as the loader itself.
+
+### Example
+
+```js
+module.exports = require("babel-loader").custom(babel => {
+  function myPlugin() {
+    return {
+      visitor: {},
+    };
+  }
+
+  return {
+    // Passed the loader options.
+    customOptions({ opt1, opt2, ...loader }) {
+      return {
+        // Pull out any custom options that the loader might have.
+        custom: { opt1, opt2 },
+
+        // Pass the options back with the two custom options removed.
+        loader,
+      };
+    },
+
+    // Passed Babel's 'PartialConfig' object. 
+    config(cfg) {
+      if (cfg.hasFilesystemConfig()) {
+        // Use the normal config
+        return cfg.options;
+      }
+
+      return {
+        ...cfg.options,
+        plugins: [
+          ...(cfg.options.plugins || []),
+
+          // Include a custom plugin in the options.
+          myPlugin,
+        ],
+      };
+    },
+
+    result(result) {
+      return {
+        ...result,
+        code: result.code + "\n// Generated by some custom loader",
+      };
+    },
+  };
+});
+```
+
+### `customOptions(options: Object): { custom: Object, loader: Object }`
+
+Given the loader's options, split custom options out of `babel-loader`'s
+options.
+
+
+### `config(cfg: PartialConfig): Object`
+
+Given Babel's `PartialConfig` object, return the `options` object that should
+be passed to `babel.transform`.
+
+
+### `result(result: Result): Result`
+
+Given Babel's result object, allow loaders to make additional tweaks to it.
+
+
 ## [License](http://couto.mit-license.org/)
diff --git a/src/index.js b/src/index.js
index b743905b..353fd4c8 100644
--- a/src/index.js
+++ b/src/index.js
@@ -14,19 +14,33 @@ function subscribe(subscriber, metadata, context) {
   }
 }
 
-module.exports = function(source, inputSourceMap) {
-  // Make the loader async
-  const callback = this.async();
+module.exports = makeLoader();
+module.exports.custom = makeLoader;
 
-  loader
-    .call(this, source, inputSourceMap)
-    .then(args => callback(null, ...args), err => callback(err));
-};
+function makeLoader(callback) {
+  const overrides = callback ? callback(babel) : undefined;
 
-async function loader(source, inputSourceMap) {
+  return function(source, inputSourceMap) {
+    // Make the loader async
+    const callback = this.async();
+
+    loader
+      .call(this, source, inputSourceMap, overrides)
+      .then(args => callback(null, ...args), err => callback(err));
+  };
+}
+
+async function loader(source, inputSourceMap, overrides) {
   const filename = this.resourcePath;
 
-  const loaderOptions = loaderUtils.getOptions(this) || {};
+  let loaderOptions = loaderUtils.getOptions(this) || {};
+
+  let customOptions;
+  if (overrides && overrides.customOptions) {
+    const result = await overrides.customOptions.call(this, loaderOptions);
+    customOptions = result.custom;
+    loaderOptions = result.loader;
+  }
 
   // Deprecation handling
   if ("forceEnv" in loaderOptions) {
@@ -73,7 +87,13 @@ async function loader(source, inputSourceMap) {
 
   const config = babel.loadPartialConfig(programmaticOptions);
   if (config) {
-    const options = config.options;
+    let options = config.options;
+    if (overrides && overrides.config) {
+      options = await overrides.config.call(this, config, {
+        source,
+        customOptions,
+      });
+    }
 
     const {
       cacheDirectory = null,
@@ -105,6 +125,15 @@ async function loader(source, inputSourceMap) {
     }
 
     if (result) {
+      if (overrides && overrides.result) {
+        result = await overrides.result.call(this, result, {
+          source,
+          customOptions,
+          config,
+          options,
+        });
+      }
+
       const { code, map, metadata } = result;
 
       metadataSubscribers.forEach(subscriber => {