test(typescript): do not permit unknown keys on octokit instance

package.json

@@ -10,7 +10,8 @@
     "lint": "prettier --check '{src,test}/**/*.{ts,md}' README.md package.json",
     "lint:fix": "prettier --write '{src,test}/**/*.{ts,md}' README.md package.json",
     "pretest": "npm run -s lint",
-    "test": "jest --coverage"
+    "test": "jest --coverage",
+    "test:typescript": "npx tsc --noEmit --declaration --noUnusedLocals test/typescript-validate.ts"
   },
   "repository": "github:octokit/core.js",
   "keywords": [

test/typescript-validate.ts

@@ -0,0 +1,39 @@
+// ************************************************************
+// THIS CODE IS NOT EXECUTED. IT IS JUST FOR TYPECHECKING
+// ************************************************************
+
+import { Octokit } from "../src";
+
+export function pluginsTest() {
+  // `octokit` instance does not permit unknown keys
+  const octokit = new Octokit();
+
+  // @ts-expect-error Property 'unknown' does not exist on type 'Octokit'.(2339)
+  octokit.unknown;
+
+  const OctokitWithDefaults = Octokit.defaults({});
+  const octokitWithDefaults = new OctokitWithDefaults();
+
+  // Error: `octokitWithDefaults` does permit unknown keys
+  // @ts-expect-error `.unknown` should not be typed as `any`
+  octokitWithDefaults.unknown;
+
+  const OctokitWithPlugin = Octokit.plugin(() => ({}));
+  const octokitWithPlugin = new OctokitWithPlugin();
+
+  // Error: `octokitWithPlugin` does permit unknown keys
+  // @ts-expect-error `.unknown` should not be typed as `any`
+  octokitWithPlugin.unknown;
+
+  const OctokitWithPluginAndDefaults = Octokit.plugin(() => ({
+    foo: 42,
+  })).defaults({
+    baz: "daz",
+  });
+
+  const octokitWithPluginAndDefaults = new OctokitWithPluginAndDefaults();
+
+  octokitWithPluginAndDefaults.foo;
+  // @ts-expect-error `.unknown` should not be typed as `any`
+  octokitWithPluginAndDefaults.unknown;
+}