Tweak the customize implementation to be a bit more strict.

README.md

@@ -90,7 +90,7 @@ This loader also supports the following loader-specific option:
 
 * `cacheCompression`: Default `true`. When set, each Babel transform output will be compressed with Gzip. If you want to opt-out of cache compression, set it to `false` -- your project may benefit from this if it transpiles thousands of files.
 
-* `customize`: Default `null`. See [Customized Loader](#customized-loader).
+* `customize`: Default `null`. The path of a module that exports a `custom` callback [like the one that you'd pass to `.custom()`](#customized-loader). Since you already have to make a new file to use this, it is recommended that you instead use `.custom` to create a wrapper loader. Only use this is you _must_ continue using `babel-loader` directly, but still want to customize.
 
 **Note**: The `sourceMap` option is ignored. Instead, source maps are automatically enabled when webpack is configured to use them (via the [`devtool`](https://webpack.js.org/configuration/devtool/#devtool) config option).
 
@@ -210,12 +210,14 @@ of Babel's configuration for each file that it processes.
 `babel` so that tooling can ensure that it using exactly the same `@babel/core`
 instance as the loader itself.
 
-This same callback can be provided under the `customize` key in the loader options.
-You may also pass `customize` a file name which exports this callback.
+In cases where you want to customize without actually having a file to call `.custom`, you
+may also pass the `customize` option with a string pointing at a file that exports
+your `custom` callback function.
 
 ### Example
 
 ```js
+// Export from "./my-custom-loader.js" or whatever you want.
 module.exports = require("babel-loader").custom(babel => {
   function myPlugin() {
     return {
@@ -263,6 +265,20 @@ module.exports = require("babel-loader").custom(babel => {
 });
 ```
 
+```js
+// And in your Webpack config
+module.exports = {
+  // ..
+  module: {
+    rules: [{
+      // ...
+      loader: path.join(__dirname, 'my-custom-loader.js'),
+      // ...
+    }]
+  }
+};
+```
+
 ### `customOptions(options: Object): { custom: Object, loader: Object }`
 
 Given the loader's options, split custom options out of `babel-loader`'s

src/index.js

@@ -23,6 +23,7 @@ const pkg = require("../package.json");
 const cache = require("./cache");
 const transform = require("./transform");
 
+const path = require("path");
 const loaderUtils = require("loader-utils");
 
 function subscribe(subscriber, metadata, context) {
@@ -52,15 +53,32 @@ async function loader(source, inputSourceMap, overrides) {
 
   let loaderOptions = loaderUtils.getOptions(this) || {};
 
-  overrides = overrides || loaderOptions.customize;
-  // customize may have been passed as a file, so we should load it
-  if (typeof overrides === "string") {
-    overrides = require(overrides);
-  }
-  // customize may have been passed as a function and not an object (to access
-  // the `babel` variable), so let's build the overrides
-  if (typeof overrides === "function") {
-    overrides = overrides(babel);
+  if (loaderOptions.customize != null) {
+    if (typeof loaderOptions.customize !== "string") {
+      throw new Error(
+        "Customized loaders must be implemented as standalone modules.",
+      );
+    }
+    if (!path.isAbsolute(loaderOptions.customize)) {
+      throw new Error(
+        "Customized loaders must be passed as absolute paths, since " +
+          "babel-loader has no way to know what they would be relative to.",
+      );
+    }
+    if (overrides) {
+      throw new Error(
+        "babel-loader's 'customize' option is not available when already " +
+          "using a customized babel-loader wrapper.",
+      );
+    }
+
+    let override = require(loaderOptions.customize);
+    if (override.__esModule) override = override.default;
+
+    if (typeof override === "function") {
+      throw new Error("Custom overrides must be functions.");
+    }
+    overrides = override(babel);
   }
 
   let customOptions;