docs(guides): expand ESM guide with import.meta, top-level await, and migration errors

Author: ryzrr

src/content/guides/ecma-script-modules.mdx

@@ -3,6 +3,7 @@ title: ECMAScript Modules
 sort: 19
 contributors:
   - sokra
+  - ryzrr
 related:
   - title: ECMAScript Modules in Node.js
     url: https://nodejs.org/api/esm.html
@@ -84,12 +85,143 @@ In DataURIs using the `text/javascript` or `application/javascript` mime type wi
 
 In addition to the module format, flagging modules as ESM also affect the resolving logic, interop logic and the available symbols in modules.
 
-Imports in ESM are resolved more strictly. Relative requests must include a filename and file extension (e.g. `*.js` or `*.mjs`) unless you have the behaviour disabled with [`fullySpecified=false`](/configuration/module/#resolvefullyspecified).
+## import.meta in ESM
+
+Webpack exposes several `import.meta` properties for use in ESM:
+
+| Property                     | Description                                                                                    |
+| ---------------------------- | ---------------------------------------------------------------------------------------------- |
+| `import.meta.url`            | The URL of the current module file - use it for `new Worker()` or `new URL()`                  |
+| `import.meta.webpack`        | The webpack major version number (e.g. `5`)                                                    |
+| `import.meta.webpackHot`     | Equivalent of `module.hot` - use for HMR in ESM                                                |
+| `import.meta.webpackContext` | [ESM equivalent of `require.context`](/guides/dependency-management/#importmetawebpackcontext) |
+
+**Example - using `import.meta.url` for assets:**
+
+```js
+// Resolve a sibling file relative to the current module
+const iconUrl = new URL("./icon.png", import.meta.url);
+const img = document.createElement("img");
+img.src = iconUrl.href;
+```
+
+**Example - HMR in ESM:**
+
+```js
+if (import.meta.webpackHot) {
+  import.meta.webpackHot.accept("./module.js", () => {
+    // handle update
+  });
+}
+```
+
+## Top-Level Await
+
+In ESM, you can use `await` at the top level of a module. Webpack treats the module
+as an async module automatically. Enabled by default since 5.83.0; the `experiments.topLevelAwait` option itself was removed in 5.102.0 (it just works).
+
+W> Avoid top-level await in your entry point when targeting the **browser**. It delays the entire module graph evaluation. Prefer `import()` for deferred loading. For Node.js, Electron, or Web Worker targets this restriction does not apply.
+
+```js
+// user.js (async ESM module)
+const response = await fetch("/api/user");
+export const user = await response.json();
+```
+
+```js
+// index.js - importing an async module works as expected
+import { user } from "./user.js";
+console.log(user.name);
+```
+
+## Fully Specified Imports
+
+Imports in ESM are resolved more strictly. Relative requests must include a file extension (e.g. `*.js` or `*.mjs`) following the Node.js convention when the file is flagged as ESM:
+
+```js
+// correct in ESM
+import { helper } from "./utils.js";
+
+// will fail - missing extension
+import { helper } from "./utils";
+```
 
 T> Requests to packages e.g. `import "lodash"` are still supported.
 
-Only the "default" export can be imported from non-ESM. Named exports are not available.
+To disable this check (useful when migrating a large CJS codebase), you can use [`fullySpecified=false`](/configuration/module/#resolvefullyspecified):
+
+```js
+// webpack.config.js
+export default {
+  module: {
+    rules: [
+      {
+        test: /\.m?js/,
+        resolve: {
+          fullySpecified: false,
+        },
+      },
+    ],
+  },
+};
+```
+
+## CommonJS Interop
+
+CommonJS syntax is not available in ESM: `require`, `module`, `exports`, `__filename`, `__dirname`.
+
+When importing from a CommonJS module inside ESM, only the `default` export
+is available (the entire `module.exports` object):
+
+```js
+// cjs-module.js (CommonJS)
+module.exports = { foo: 1, bar: 2 };
+
+// esm-consumer.js (ESM)
+import cjs from "./cjs-module.js";
+console.log(cjs.foo); // works - cjs is the whole exports object
+
+// named imports from CJS don't work
+import { foo } from "./cjs-module.js"; // undefined
+```
+
+This strict behavior applies when webpack treats the **imported** module as CommonJS.
+If that module itself uses ESM `export` syntax, webpack will auto-detect it as ESM
+and named imports will work normally. This commonly affects projects that mix `.js` files
+files in a project that has `"type": "module"` set - webpack may treat some files as
+ESM while third-party packages in `node_modules` remain CommonJS.
+
+T> To get named exports from CommonJS modules, consider migrating to ESM
+T> or using [`@babel/plugin-transform-modules-commonjs`](https://babeljs.io/docs/babel-plugin-transform-modules-commonjs).
+
+## Common Migration Errors
+
+**`ReferenceError: require is not defined`**
+
+When a file is treated as ESM, CommonJS globals (`require`, `module`, `exports`,
+`__filename`, `__dirname`) are unavailable.
+
+_Fix_: Replace `require()` with `import` statements. For conditional or dynamic
+loading, use `import()`.
+
+---
+
+**`Must use import to load ES Module`** (Node.js) / **`SyntaxError: Cannot use import
+statement in a module`** (browser)
+
+This happens when a file using ESM `import`/`export` syntax is not flagged as ESM -
+either `"type": "module"` is missing from `package.json`, or the file uses a `.js`
+extension instead of `.mjs`.
+
+_Fix_: Add `"type": "module"` to your `package.json`, or rename the file to `.mjs`.
+
+---
+
+**`Module not found: Error: Can't resolve './utils'` (missing extension)**
 
-CommonJs Syntax is not available: `require`, `module`, `exports`, `__filename`, `__dirname`.
+In ESM, relative imports must include the file extension. Webpack follows the Node.js
+ESM convention here.
 
-T> HMR can be used with [`import.meta.webpackHot`](/api/module-variables/#importmetawebpackhot) instead of [`module.hot`](/api/module-variables/#modulehot-webpack-specific).
+_Fix_: Change `import { helper } from './utils'` to `import { helper } from './utils.js'`,
+or set [`fullySpecified: false`](/configuration/module/#resolvefullyspecified) in your
+webpack config to disable the check while migrating.