docs: use declare module

README.md

@@ -6,44 +6,46 @@ A feature flag plugin for Vite.
 
 ## How to use
 
-1.  Install the plugin.
-
-    ```bash
-    pnpm add -D @virtual-live-lab/vite-plugin-feature-flags
-    ```
-
-2.  Add this plugin to `vite.config.ts`.
-
-    ```ts
-    import { defineConfig } from "vite";
-    import { featuresPlugin } from "vite-plugin-feature-flags";
-
-    export default defineConfig({
-      plugins: [
-        featuresPlugin({
-          // your feature flags
-          hoge: true,
-          fuga: false,
-        }),
-      ],
-    });
-    ```
-
-3.  Add type declaration for feature flags.
-
-    ```ts
-    /// <reference types="@virtual-live-lab/vite-plugin-feature-flags/client" />
-
-    interface ImportMetaFeatures {
-      hoge: boolean;
-      fuga: boolean;
-    }
-    ```
-
-4.  Use feature flags in your code.
-
-    ```ts
-    if (import.meta.features.hoge) {
-      console.log("hoge is enabled");
-    }
-    ```
+1. Install the plugin.
+
+   ```bash
+   pnpm add -D @virtual-live-lab/vite-plugin-feature-flags
+   ```
+
+2. Add this plugin to `vite.config.ts`.
+
+   ```ts
+   import { defineConfig } from "vite";
+   import { featuresPlugin } from "vite-plugin-feature-flags";
+
+   export default defineConfig({
+     plugins: [
+       featuresPlugin({
+         // your feature flags
+         hoge: true,
+         fuga: false,
+       }),
+     ],
+   });
+   ```
+
+3. Add type declaration for feature flags.
+
+   ```ts
+   declare module "@virtual-live-lab/vite-plugin-feature-flags/client" {
+     interface ImportMetaFeatures {
+       hoge: boolean;
+       fuga: boolean;
+     }
+   }
+
+   export {}; // Declaration merging is only available in modules.
+   ```
+
+4. Use feature flags in your code.
+
+   ```ts
+   if (import.meta.features.hoge) {
+     console.log("hoge is enabled");
+   }
+   ```