chore: add implementation comments

src/internal/operators/catchError.ts

@@ -154,6 +154,9 @@ class CatchSubscriber<T, R> extends OuterSubscriber<T, T | R> {
       const innerSubscriber = new InnerSubscriber(this, undefined, undefined);
       this.add(innerSubscriber);
       const innerSubscription = subscribeToResult(this, result, undefined, undefined, innerSubscriber);
+      // The returned subscription will usually be the subscriber that was
+      // passed. However, interop subscribers will be wrapped and for
+      // unsubscriptions to chain correctly, the wrapper needs to be added, too.
       if (innerSubscription !== innerSubscriber) {
         this.add(innerSubscription);
       }

src/internal/operators/exhaustMap.ts

@@ -126,6 +126,9 @@ class ExhaustMapSubscriber<T, R> extends OuterSubscriber<T, R> {
     const destination = this.destination as Subscription;
     destination.add(innerSubscriber);
     const innerSubscription = subscribeToResult<T, R>(this, result, undefined, undefined, innerSubscriber);
+    // The returned subscription will usually be the subscriber that was
+    // passed. However, interop subscribers will be wrapped and for
+    // unsubscriptions to chain correctly, the wrapper needs to be added, too.
     if (innerSubscription !== innerSubscriber) {
       destination.add(innerSubscription);
     }

src/internal/operators/mergeMap.ts

@@ -146,6 +146,9 @@ export class MergeMapSubscriber<T, R> extends OuterSubscriber<T, R> {
     const destination = this.destination as Subscription;
     destination.add(innerSubscriber);
     const innerSubscription = subscribeToResult<T, R>(this, ish, undefined, undefined, innerSubscriber);
+    // The returned subscription will usually be the subscriber that was
+    // passed. However, interop subscribers will be wrapped and for
+    // unsubscriptions to chain correctly, the wrapper needs to be added, too.
     if (innerSubscription !== innerSubscriber) {
       destination.add(innerSubscription);
     }

src/internal/operators/mergeScan.ts

@@ -107,6 +107,9 @@ export class MergeScanSubscriber<T, R> extends OuterSubscriber<T, R> {
     const destination = this.destination as Subscription;
     destination.add(innerSubscriber);
     const innerSubscription = subscribeToResult<T, R>(this, ish, undefined, undefined, innerSubscriber);
+    // The returned subscription will usually be the subscriber that was
+    // passed. However, interop subscribers will be wrapped and for
+    // unsubscriptions to chain correctly, the wrapper needs to be added, too.
     if (innerSubscription !== innerSubscriber) {
       destination.add(innerSubscription);
     }

src/internal/operators/onErrorResumeNext.ts

@@ -163,6 +163,9 @@ class OnErrorResumeNextSubscriber<T, R> extends OuterSubscriber<T, R> {
       const destination = this.destination as Subscription;
       destination.add(innerSubscriber);
       const innerSubscription = subscribeToResult(this, next, undefined, undefined, innerSubscriber);
+      // The returned subscription will usually be the subscriber that was
+      // passed. However, interop subscribers will be wrapped and for
+      // unsubscriptions to chain correctly, the wrapper needs to be added, too.
       if (innerSubscription !== innerSubscriber) {
         destination.add(innerSubscription);
       }

src/internal/operators/skipUntil.ts

@@ -75,6 +75,9 @@ class SkipUntilSubscriber<T, R> extends OuterSubscriber<T, R> {
     this.add(innerSubscriber);
     this.innerSubscription = innerSubscriber;
     const innerSubscription = subscribeToResult(this, notifier, undefined, undefined, innerSubscriber);
+    // The returned subscription will usually be the subscriber that was
+    // passed. However, interop subscribers will be wrapped and for
+    // unsubscriptions to chain correctly, the wrapper needs to be added, too.
     if (innerSubscription !== innerSubscriber) {
       this.add(innerSubscription);
       this.innerSubscription = innerSubscription;

src/internal/operators/switchMap.ts

@@ -137,6 +137,9 @@ class SwitchMapSubscriber<T, R> extends OuterSubscriber<T, R> {
     const destination = this.destination as Subscription;
     destination.add(innerSubscriber);
     this.innerSubscription = subscribeToResult(this, result, undefined, undefined, innerSubscriber);
+    // The returned subscription will usually be the subscriber that was
+    // passed. However, interop subscribers will be wrapped and for
+    // unsubscriptions to chain correctly, the wrapper needs to be added, too.
     if (this.innerSubscription !== innerSubscriber) {
       destination.add(this.innerSubscription);
     }