Document the importInterop option

docs/plugin-transform-modules-commonjs.md

@@ -66,6 +66,90 @@ require("@babel/core").transformSync("code", {
 
 ## Options
 
+### `importInterop`
+
+`"babel" | "node" | "none"`, or `(specifier: string) => "babel" | "node" | "none"`. Defaults to `"babel"`.
+
+CommonJS modules and ECMAScript modules are not fully compatible. However, compilers, bundlers and JavaScript
+runtimes developed different strategies to make them work together as well as possible.
+
+This option specify which interop strategy Babel should use. When it's a function, Babel calls this function
+passing the import specifier (for example, `@babel/core` in `import { transform } from "@babel/core"`), and the
+function should return the interop to use for that specific import.
+
+#### `"babel"`
+
+When using exports with babel a non-enumerable `__esModule` property is exported. This property is then used to determine if the import _is_ the default export or if it _contains_ the default export.
+
+```javascript
+import foo from "foo";
+import { bar } from "bar";
+foo;
+bar;
+
+// Is compiled to ...
+
+"use strict";
+
+function _interopRequireDefault(obj) {
+  return obj && obj.__esModule ? obj : { default: obj };
+}
+
+var _foo = _interopRequireDefault(require("foo"));
+var _bar = require("bar");
+
+_foo.default;
+_bar.bar;
+```
+
+When this import interop is used, if both the imported and the importer module are compiled with Babel they behave as if none of them was copiled.
+
+This is the default behavior.
+
+#### `"node"`
+
+When importing CommonJS files (either directly written in CommonJS, or generated with a compiler) Node.js always binds the `default` export to the value of `module.exports`.
+
+```javascript
+import foo from "foo";
+import { bar } from "bar";
+foo;
+bar;
+
+// Is compiled to ...
+
+"use strict";
+
+var _foo = require("foo");
+var _bar = require("bar");
+
+_foo;
+_bar.bar;
+```
+
+This is not exactly the same as what Node.js does since Babel allows accessing any property of `module.exports` as a named export, while Node.js only allows importing _statically analyzable_ properties of `module.exports`. However, any import working in Node.js will also work when compiled with Babel using `importInterop: "node"`.
+
+#### `"none"`
+
+If you know that the imported file has been transformed with a compiler that stores the `default` export on `exports.default` (such as Babel), you can safely omit the `_interopRequireDefault` helper.
+
+```javascript
+import foo from "foo";
+import { bar } from "bar";
+foo;
+bar;
+
+// Is compiled to ...
+
+"use strict";
+
+var _foo = require("foo");
+var _bar = require("bar");
+
+_foo.default;
+_bar.bar;
+```
+
 ### `loose`
 
 `boolean`, defaults to `false`.
@@ -108,28 +192,6 @@ Object.defineProperty(exports, "__esModule", {
 In order to prevent the `__esModule` property from being exported, you can set
 the `strict` option to `true`.
 
-### `noInterop`
-
-`boolean`, defaults to `false`
-
-By default, when using exports with babel a non-enumerable `__esModule` property
-is exported. This property is then used to determine if the import _is_ the default
-export or if it _contains_ the default export.
-
-```javascript
-"use strict";
-
-var _foo = _interopRequireDefault(require("foo"));
-
-function _interopRequireDefault(obj) {
-  return obj && obj.__esModule ? obj : { default: obj };
-}
-```
-
-In cases where the auto-unwrapping of `default` is not needed, you can set the
-`noInterop` option to `true` to avoid the usage of the `interopRequireDefault`
-helper (shown in inline form above).
-
 ### `lazy`
 
 `boolean`, `Array<string>`, or `(string) => boolean`, defaults to `false`
@@ -165,3 +227,11 @@ The two cases where imports can never be lazy are:
   way to know what names need to be exported.
 
 > You can read more about configuring plugin options [here](https://babeljs.io/docs/en/plugins#plugin-options)
+
+### `noInterop`
+
+`boolean`, defaults to `false`
+
+> ⚠️ **Deptecated**: Use the `importInterop` option instead.
+
+When set to `true`, this option has the same behavior as setting `importInterop: "none"`.