feat: expose webpack target via babel caller (#826)

Co-authored-by: Brian Ng <bng412@gmail.com>

README.md

@@ -193,6 +193,37 @@ In the case one of your dependencies is installing `babel` and you cannot uninst
   }
 ```
 
+## Customize config based on webpack target
+
+Webpack supports bundling multiple [targets](https://webpack.js.org/concepts/targets/). For cases where you may want different Babel configurations for each target (like `web` _and_ `node`), this loader provides a `target` property via Babel's [caller](https://babeljs.io/docs/en/config-files#apicallercb) API.
+
+For example, to change the environment targets passed to `@babel/preset-env` based on the webpack target:
+
+```javascript
+// babel.config.js
+
+module.exports = api => {
+  return {
+    plugins: [
+      "@babel/plugin-proposal-nullish-coalescing-operator",
+      "@babel/plugin-proposal-optional-chaining"
+    ],
+    presets: [
+      [
+        "@babel/preset-env",
+        {
+          useBuiltIns: "entry",
+          // caller.target will be the same as the target option from webpack
+          targets: api.caller(caller => caller && caller.target === "node")
+            ? { node: "current" }
+            : { chrome: "58", ie: "11" }
+        }
+      ]
+    ]
+  }
+}
+```
+
 ## Customized Loader
 
 `babel-loader` exposes a loader-builder utility that allows users to add custom handling

src/index.js

@@ -158,7 +158,9 @@ async function loader(source, inputSourceMap, overrides) {
     );
   }
 
-  const config = babel.loadPartialConfig(injectCaller(programmaticOptions));
+  const config = babel.loadPartialConfig(
+    injectCaller(programmaticOptions, this.target),
+  );
   if (config) {
     let options = config.options;
     if (overrides && overrides.config) {

src/injectCaller.js

@@ -1,13 +1,17 @@
 const babel = require("@babel/core");
 
-module.exports = function injectCaller(opts) {
+module.exports = function injectCaller(opts, target) {
   if (!supportsCallerOption()) return opts;
 
   return Object.assign({}, opts, {
     caller: Object.assign(
       {
         name: "babel-loader",
 
+        // Provide plugins with insight into webpack target.
+        // https://github.com/babel/babel-loader/issues/787
+        target,
+
         // Webpack >= 2 supports ESM and dynamic import.
         supportsStaticESM: true,
         supportsDynamicImport: true,