DOM: Implement the first() Observable operator

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

@@ -0,0 +1,114 @@
+promise_test(async () => {
+  const results = [];
+
+  const source = new Observable(subscriber => {
+    subscriber.addTeardown(() => results.push('teardown'));
+    subscriber.next(1);
+    results.push(subscriber.active ? 'active' : 'inactive');
+    results.push(subscriber.signal.aborted ? 'aborted' : 'not aborted')
+
+    // Ignored.
+    subscriber.next(2);
+    subscriber.complete();
+  });
+
+  const value = await source.first();
+
+  assert_array_equals(results, ['teardown', 'inactive', 'aborted']);
+  assert_equals(value, 1,
+      "Promise resolves with the first value from the source Observable");
+}, "first(): Promise resolves with the first value from the source Observable");
+
+promise_test(async () => {
+  const error = new Error("error from source");
+  const source = new Observable(subscriber => {
+    subscriber.error(error);
+  });
+
+  let rejection;
+  try {
+    await source.first();
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_equals(rejection, error, "Promise rejects with source Observable error");
+}, "first(): Promise rejects with the error emitted from the source Observable");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {
+    subscriber.complete();
+  });
+
+  let rejection;
+  try {
+    await source.first();
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_true(rejection instanceof RangeError,
+      "Upon complete(), first() Promise rejects with RangeError");
+  assert_equals(rejection.message, "No values in Observable");
+}, "first(): Promise rejects with RangeError when source Observable " +
+   "completes without emitting any values");
+
+promise_test(async () => {
+  const source = new Observable(subscriber => {});
+
+  const controller = new AbortController();
+  const promise = source.first({ signal: controller.signal });
+
+  controller.abort();
+
+  let rejection;
+  try {
+    await promise;
+  } catch (e) {
+    rejection = e;
+  }
+
+  assert_true(rejection instanceof DOMException,
+      "Promise rejects with a DOMException for abortion");
+  assert_equals(rejection.name, "AbortError",
+      "Rejected with 'AbortError' DOMException");
+  assert_equals(rejection.message, "signal is aborted without reason");
+}, "first(): Aborting a signal rejects the Promise with an AbortError DOMException");
+
+promise_test(async () => {
+  const results = [];
+
+  const source = new Observable(subscriber => {
+    results.push("source subscribe");
+    subscriber.addTeardown(() => results.push("source teardown"));
+    subscriber.signal.addEventListener("abort", () => results.push("source abort"));
+    results.push("before source next 1");
+    subscriber.next(1);
+    results.push("after source next 1");
+  });
+
+  results.push("calling first");
+  const promise = source.first();
+
+  assert_array_equals(results, [
+    "calling first",
+    "source subscribe",
+    "before source next 1",
+    "source teardown",
+    "source abort",
+    "after source next 1"
+  ], "Array values after first() is called");
+
+  const firstValue = await promise;
+  results.push(`first resolved with: ${firstValue}`);
+
+  assert_array_equals(results, [
+    "calling first",
+    "source subscribe",
+    "before source next 1",
+    "source teardown",
+    "source abort",
+    "after source next 1",
+    "first resolved with: 1",
+  ], "Array values after Promise is awaited");
+}, "first(): Lifecycle");