Add declaration merging example

Author: iamWing

src/types/index.d.ts

@@ -0,0 +1,11 @@
+/* eslint-disable @typescript-eslint/no-unused-vars */
+// Must have to let TSC knows we're merging the module 'fastify'
+import * as fastify from 'fastify';
+
+// Using declaration merging, add your plugin props to the appropriate
+// fastify interfaces.
+declare module 'fastify' {
+  interface FastifyInstance {
+    exampleProperty: string; // replace this with your props
+  }
+}

tsconfig.json

@@ -11,6 +11,10 @@
     "strictNullChecks": true,
     "strictPropertyInitialization": true,
     "moduleResolution": "node",
+    "typeRoots": [
+      "./node_modules/@types",
+      "./src/types"
+    ],
     "outDir": "dist",
     "baseUrl": ".",
     "paths": {