test: fix types

Author: posva

__tests__/src/index.ts

@@ -51,11 +51,12 @@ export function spyOnSnapshotCallback(ref: FirestoreReference) {
 // This makes sure some tests fail by delaying callbacks
 export function delayUpdate(ref: firestore.DocumentReference, time = 0) {
   const onSnapshot = ref.onSnapshot.bind(ref)
-  // @ts-ignore
   ref.onSnapshot = (fn) =>
-    // @ts-ignore
     onSnapshot(async (...args) => {
       await delay(time)
+      if (typeof fn !== 'function') {
+        throw new Error('onSnapshot can only be called on function')
+      }
       fn(...args)
     })
 }