Add some type tests

addon/src/services/page-title.ts

@@ -44,7 +44,7 @@ export default class PageTitleService extends Service {
 
   // in fastboot context "document" is instance of
   // ember-fastboot/simple-dom document
-  @service('-document') declare document: FastBootDocument;
+  @service('-document') private declare document: FastBootDocument;
 
   tokens: PageTitleToken[] = [];
 

test-types/index.ts

@@ -1,9 +1,51 @@
 import 'ember-source/types';
 import '@glint/environment-ember-loose';
 
-import Helper from '@ember/component/helper';
 
 import { expectTypeOf } from 'expect-type';
+import type Owner from '@ember/owner';
+
+/************************
+ * The Helper
+ ************************/
+import Helper from '@ember/component/helper';
 import { pageTitle } from 'ember-page-title';
 
-expectTypeOf(new pageTitle()).toMatchTypeOf<Helper<any>>();
+let instance = new pageTitle({} as Owner);
+
+expectTypeOf(instance).toMatchTypeOf<Helper<any>>();
+expectTypeOf<ReturnType<typeof instance['compute']>>().toBeString(/* an empty string, but still a string */);
+
+/************************
+ * The Service
+ ************************/
+import PageTitleService from 'ember-page-title/services/page-title';
+import type Service from '@ember/service';
+
+let service = new PageTitleService({} as Owner);
+
+expectTypeOf(service).toMatchTypeOf<Service>();
+expectTypeOf(service.tokens).toBeArray();
+expectTypeOf(service.tokens[0]).toMatchTypeOf<object | undefined>();
+expectTypeOf<NonNullable<typeof service.tokens[0]>>()
+  .toMatchTypeOf<{
+    id: string, title?: string,
+    separator?: string;
+    prepend?: boolean;
+    replace?: boolean;
+    front?: unknown;
+  }>();
+
+expectTypeOf<PageTitleService['tokens']>().toBeArray();
+expectTypeOf<PageTitleService['sortedTokens']>().toBeArray();
+expectTypeOf<ReturnType<PageTitleService['toString']>>().toBeString();
+expectTypeOf<PageTitleService['titleDidUpdate']>().toMatchTypeOf<(title: string) => void>();
+expectTypeOf<PageTitleService['remove']>().toMatchTypeOf<(id: string) => void>();
+
+expectTypeOf<PageTitleService['push']>()
+  .toMatchTypeOf<(token: { id: string }) => void>();
+expectTypeOf<PageTitleService['push']>()
+  .toMatchTypeOf<(token: { id: string, title: string }) => void>();
+
+// @ts-expect-error
+expectTypeOf<PageTitleService['push']>().toMatchTypeOf<(token: undefined) => void>();