Merge branch 'custom-builder'

Author: Andarist

.eslintrc

@@ -1,26 +1,19 @@
 {
-    "rules": {
-        "indent": [ 2, "tab", { "SwitchCase": 1 } ],
-        "quotes": [ 2, "single" , { "avoidEscape": true }],
-        "linebreak-style": [ 2, "unix" ],
-        "semi": [ 2, "always" ],
-        "keyword-spacing": [ 2, { "before": true, "after": true } ],
-        "space-before-blocks": [ 2, "always" ],
-        "no-mixed-spaces-and-tabs": [ 2, "smart-tabs" ],
-        "no-cond-assign": [ 0 ],
-    },
-    "env": {
-        "es6": true,
-        "browser": true,
-        "mocha": true,
-        "node": true
-    },
-    "extends": "eslint:recommended",
-    "parserOptions": {
-        "sourceType": "module",
-        "ecmaVersion": 2018,
-        "ecmaFeatures": {
-            "spread": true
-        }
-    }
+	"extends": ["eslint:recommended", "prettier", "plugin:prettier/recommended"],
+	"rules": {
+		"no-cond-assign": [0]
+	},
+	"env": {
+		"es6": true,
+		"browser": true,
+		"mocha": true,
+		"node": true
+	},
+	"parserOptions": {
+		"sourceType": "module",
+		"ecmaVersion": 2018,
+		"ecmaFeatures": {
+			"spread": true
+		}
+	}
 }

README.md

@@ -105,10 +105,10 @@ However, setting `modules: false` in your `.babelrc` may conflict if you are usi
 
 ```js
 plugins: [
-  babel({
-    babelrc: false,
-    presets: [['env', { modules: false }]],
-  }),
+	babel({
+		babelrc: false,
+		presets: [['env', { modules: false }]],
+	}),
 ];
 ```
 
@@ -137,6 +137,65 @@ npm install --save-dev rollup-plugin-babel@3 babel-preset-env babel-plugin-exter
 }
 ```
 
+## Custom plugin builder
+
+`rollup-plugin-babel` exposes a plugin-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.
+
+It's main purpose is to allow other tools for configuration of transpilation without forcing people to add extra configuration but still allow for using their own babelrc / babel config files.
+
+### Example
+
+```js
+import babel from 'rollup-plugin-babel';
+
+export default babel.custom(babelCore => {
+	function myPlugin() {
+		return {
+			visitor: {},
+		};
+	}
+
+	return {
+		// Passed the plugin options.
+		options({ opt1, opt2, ...pluginOptions }) {
+			return {
+				// Pull out any custom options that the plugin might have.
+				customOptions: { opt1, opt2 },
+
+				// Pass the options back with the two custom options removed.
+				pluginOptions,
+			};
+		},
+
+		config(cfg /* Passed Babel's 'PartialConfig' object. */, { code, customOptions }) {
+			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, { code, customOptions, config, transformOptions }) {
+			return {
+				...result,
+				code: result.code + '\n// Generated by some custom loader',
+			};
+		},
+	};
+});
+```
+
 ## License
 
 MIT