chore: init delegates as explicitly undefined

src/internal/scheduler/dateTimestampProvider.ts

@@ -1,9 +1,14 @@
 import { TimestampProvider } from "../types";
 
 interface DateTimestampProvider extends TimestampProvider {
-  delegate?: TimestampProvider;
+  delegate: TimestampProvider | undefined;
 }
 
 export const dateTimestampProvider: DateTimestampProvider = {
-  now() { return (this.delegate || Date).now(); }
+  now() {
+    // Use the variable rather than `this` so that the function can be called
+    // without being bound to the provider.
+    return (dateTimestampProvider.delegate || Date).now();
+  },
+  delegate: undefined
 };

src/internal/scheduler/performanceTimestampProvider.ts

@@ -1,9 +1,14 @@
 import { TimestampProvider } from "../types";
 
 interface PerformanceTimestampProvider extends TimestampProvider {
-  delegate?: TimestampProvider;
+  delegate: TimestampProvider | undefined;
 }
 
 export const performanceTimestampProvider: PerformanceTimestampProvider = {
-  now() { return (this.delegate || performance).now(); }
+  now() {
+    // Use the variable rather than `this` so that the function can be called
+    // without being bound to the provider.
+    return (performanceTimestampProvider.delegate || performance).now();
+  },
+  delegate: undefined
 };

src/internal/scheduler/requestAnimationFrameProvider.ts

@@ -2,18 +2,21 @@ import { Subscription } from "../Subscription";
 
 type RequestAnimationFrameProvider = {
   schedule(callback: FrameRequestCallback): Subscription;
-  delegate?: {
+  delegate: {
     requestAnimationFrame: typeof requestAnimationFrame;
     cancelAnimationFrame: typeof cancelAnimationFrame;
-  };
+  } | undefined;
 };
 
 export const requestAnimationFrameProvider: RequestAnimationFrameProvider = {
   schedule(callback) {
-    const { delegate } = this;
+    // Use the variable rather than `this` so that the function can be called
+    // without being bound to the provider.
+    const { delegate } = requestAnimationFrameProvider;
     const request = delegate?.requestAnimationFrame || requestAnimationFrame;
     const cancel = delegate?.cancelAnimationFrame || cancelAnimationFrame;
     const handle = request(callback);
     return new Subscription(() => cancel(handle));
-  }
+  },
+  delegate: undefined
 };