DOM: Implement the switchMap() Observable operator

See WICG/observable#130 for the spec PR, and
https://chromium-review.googlesource.com/c/chromium/src/+/5381640 for
documentation about the `flatMap()` operator, which is structured in
almost the same way as `switchMap()`.

R=masonf@chromium.org

Bug: 40282760
Change-Id: Id2b0de2d60dd985be843f154bebd66f8948f36f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5410391
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280836}

Author: domfarolino

dom/observable/tentative/observable-switchMap.any.js

@@ -0,0 +1,252 @@
+test(() => {
+  const source = createTestSubject();
+  const inner1 = createTestSubject();
+  const inner2 = createTestSubject();
+
+  const result = source.switchMap((value, index) => {
+    if (value === 1) {
+      return inner1;
+    }
+    if (value === 2) {
+      return inner2;
+    }
+    throw new Error("invalid ");
+  });
+
+  const results = [];
+
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push("complete"),
+  });
+
+  assert_equals(source.subscriberCount(), 1,
+      "source observable is subscribed to");
+
+  source.next(1);
+  assert_equals(inner1.subscriberCount(), 1,
+      "inner1 observable is subscribed to");
+
+  inner1.next("1a");
+  assert_array_equals(results, ["1a"]);
+
+  inner1.next("1b");
+  assert_array_equals(results, ["1a", "1b"]);
+
+  source.next(2);
+  assert_equals(inner1.subscriberCount(), 0,
+      "inner1 observable is unsubscribed from");
+  assert_equals(inner2.subscriberCount(), 1,
+      "inner2 observable is subscribed to");
+
+  inner2.next("2a");
+  assert_array_equals(results, ["1a", "1b", "2a"]);
+
+  inner2.next("2b");
+  assert_array_equals(results, ["1a", "1b", "2a", "2b"]);
+
+  inner2.complete();
+  assert_array_equals(results, ["1a", "1b", "2a", "2b"]);
+
+  source.complete();
+  assert_array_equals(results, ["1a", "1b", "2a", "2b", "complete"]);
+}, "switchMap(): result subscribes to one inner observable at a time, " +
+   "unsubscribing from the previous active one when a new one replaces it");
+
+test(() => {
+  const source = createTestSubject();
+  const inner = createTestSubject();
+
+  const result = source.switchMap(() => inner);
+
+  const results = [];
+
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push("complete"),
+  });
+
+  assert_equals(source.subscriberCount(), 1,
+      "source observable is subscribed to");
+  assert_equals(inner.subscriberCount(), 0,
+      "inner observable is not subscribed to");
+
+  source.next(1);
+  assert_equals(inner.subscriberCount(), 1,
+      "inner observable is subscribed to");
+
+  inner.next("a");
+  assert_array_equals(results, ["a"]);
+
+  inner.next("b");
+  assert_array_equals(results, ["a", "b"]);
+
+  source.complete();
+  assert_array_equals(results, ["a", "b"],
+      "Result observable does not complete when source observable completes, " +
+      "because inner is still active");
+
+  inner.next("c");
+  assert_array_equals(results, ["a", "b", "c"]);
+
+  inner.complete();
+  assert_array_equals(results, ["a", "b", "c", "complete"],
+      "Result observable completes when inner observable completes, because " +
+      "source is already complete");
+}, "switchMap(): result does not complete when the source observable " +
+   "completes, if the inner observable is still active");
+
+test(() => {
+  const source = createTestSubject();
+
+  const e = new Error('thrown from mapper');
+  const result = source.switchMap(() => {
+    throw e;
+  });
+
+  const results = [];
+
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push("complete"),
+  });
+
+  assert_equals(source.subscriberCount(), 1,
+      "source observable is subscribed to");
+
+  source.next(1);
+  assert_array_equals(results, [e]);
+  assert_equals(source.subscriberCount(), 0,
+      "source observable is unsubscribed from");
+}, "switchMap(): result emits an error if Mapper callback throws an error");
+
+test(() => {
+  const source = createTestSubject();
+  const inner = createTestSubject();
+
+  const result = source.switchMap(() => inner);
+
+  const results = [];
+
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push("complete"),
+  });
+
+  source.next(1);
+  inner.next("a");
+  assert_array_equals(results, ["a"]);
+
+  const e = new Error('error from source');
+  source.error(e);
+  assert_array_equals(results, ["a", e],
+      "switchMap result emits an error if the source emits an error");
+  assert_equals(inner.subscriberCount(), 0,
+      "inner observable is unsubscribed from");
+  assert_equals(source.subscriberCount(), 0,
+      "source observable is unsubscribed from");
+}, "switchMap(): result emits an error if the source observable emits an " +
+   "error");
+
+test(() => {
+  const source = createTestSubject();
+  const inner = createTestSubject();
+
+  const result = source.switchMap(() => inner);
+
+  const results = [];
+
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.push(e),
+    complete: () => results.push("complete"),
+  });
+
+  source.next(1);
+  inner.next("a");
+  assert_array_equals(results, ["a"]);
+
+  const e = new Error("error from inner");
+  inner.error(e);
+  assert_array_equals(results, ["a", e],
+      "result emits an error if the inner observable emits an error");
+  assert_equals(inner.subscriberCount(), 0,
+      "inner observable is unsubscribed from");
+  assert_equals(source.subscriberCount(), 0,
+      "source observable is unsubscribed from");
+}, "switchMap(): result emits an error if the inner observable emits an error");
+
+test(() => {
+  const results = [];
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.addTeardown(() => {
+      results.push('source teardown');
+    });
+    subscriber.signal.onabort = e => {
+      results.push('source onabort');
+    };
+  });
+
+  const inner = new Observable(subscriber => {
+    subscriber.addTeardown(() => {
+      results.push('inner teardown');
+    });
+    subscriber.signal.onabort = () => {
+      results.push('inner onabort');
+    };
+  });
+
+  const result = source.switchMap(() => inner);
+
+  const ac = new AbortController();
+  result.subscribe({
+    next: v => results.push(v),
+    error: e => results.error(e),
+    complete: () => results.complete("complete"),
+  }, {signal: ac.signal});
+
+  ac.abort();
+  assert_array_equals(results, [
+    "source teardown",
+    "source onabort",
+    "inner teardown",
+    "inner onabort",
+  ], "Unsubscription order is correct");
+}, "switchMap(): should unsubscribe in the correct order when user aborts " +
+   "the subscription");
+
+// A helper function to create an Observable that can be externally controlled
+// and examined for testing purposes.
+function createTestSubject() {
+  const subscribers = new Set();
+  const subject = new Observable(subscriber => {
+    subscribers.add(subscriber);
+    subscriber.addTeardown(() => subscribers.delete(subscriber));
+  });
+
+  subject.next = value => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.next(value);
+    }
+  };
+  subject.error = error => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.error(error);
+    }
+  };
+  subject.complete = () => {
+    for (const subscriber of Array.from(subscribers)) {
+      subscriber.complete();
+    }
+  };
+  subject.subscriberCount = () => {
+    return subscribers.size;
+  };
+
+  return subject;
+}