diff --git a/goldens/public-api/core/core.d.ts b/goldens/public-api/core/core.d.ts
index 52ac3b1f5a..99595a7ec4 100644
--- a/goldens/public-api/core/core.d.ts
+++ b/goldens/public-api/core/core.d.ts
@@ -25,12 +25,12 @@ export declare const APP_BOOTSTRAP_LISTENER: InjectionToken<((compRef: Component
 
 export declare const APP_ID: InjectionToken<string>;
 
-export declare const APP_INITIALIZER: InjectionToken<(() => void)[]>;
+export declare const APP_INITIALIZER: InjectionToken<readonly (() => Observable<unknown> | Promise<unknown> | void)[]>;
 
 export declare class ApplicationInitStatus {
     readonly done = false;
     readonly donePromise: Promise<any>;
-    constructor(appInits: (() => any)[]);
+    constructor(appInits: ReadonlyArray<() => Observable<unknown> | Promise<unknown> | void>);
 }
 
 export declare class ApplicationModule {
diff --git a/packages/core/src/application_init.ts b/packages/core/src/application_init.ts
index 76707e8aa0..b89ed0908a 100644
--- a/packages/core/src/application_init.ts
+++ b/packages/core/src/application_init.ts
@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {Observable} from 'rxjs';
 import {Inject, Injectable, InjectionToken, Optional} from './di';
 import {isObservable, isPromise} from './util/lang';
 import {noop} from './util/noop';
@@ -28,7 +29,9 @@ import {noop} from './util/noop';
  *
  * @publicApi
  */
-export const APP_INITIALIZER = new InjectionToken<Array<() => void>>('Application Initializer');
+export const APP_INITIALIZER =
+    new InjectionToken<ReadonlyArray<() => Observable<unknown>| Promise<unknown>| void>>(
+        'Application Initializer');
 
 /**
  * A class that reflects the state of running {@link APP_INITIALIZER} functions.
@@ -43,7 +46,8 @@ export class ApplicationInitStatus {
   public readonly donePromise: Promise<any>;
   public readonly done = false;
 
-  constructor(@Inject(APP_INITIALIZER) @Optional() private appInits: (() => any)[]) {
+  constructor(@Inject(APP_INITIALIZER) @Optional() private readonly appInits:
+                  ReadonlyArray<() => Observable<unknown>| Promise<unknown>| void>) {
     this.donePromise = new Promise((res, rej) => {
       this.resolve = res;
       this.reject = rej;
diff --git a/packages/router/src/router_module.ts b/packages/router/src/router_module.ts
index 26e2a4ce6d..810fd68a11 100644
--- a/packages/router/src/router_module.ts
+++ b/packages/router/src/router_module.ts
@@ -584,7 +584,7 @@ export function getBootstrapListener(r: RouterInitializer) {
 export const ROUTER_INITIALIZER =
     new InjectionToken<(compRef: ComponentRef<any>) => void>('Router Initializer');
 
-export function provideRouterInitializer() {
+export function provideRouterInitializer(): ReadonlyArray<Provider> {
   return [
     RouterInitializer,
     {