initial commit: scaffolding

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diff --git a/node_modules/rxjs/src/internal/operators/defaultIfEmpty.ts b/node_modules/rxjs/src/internal/operators/defaultIfEmpty.ts
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+import { OperatorFunction } from '../types';
+import { operate } from '../util/lift';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+
+/**
+ * Emits a given value if the source Observable completes without emitting any
+ * `next` value, otherwise mirrors the source Observable.
+ *
+ * <span class="informal">If the source Observable turns out to be empty, then
+ * this operator will emit a default value.</span>
+ *
+ * ![](defaultIfEmpty.png)
+ *
+ * `defaultIfEmpty` emits the values emitted by the source Observable or a
+ * specified default value if the source Observable is empty (completes without
+ * having emitted any `next` value).
+ *
+ * ## Example
+ *
+ * If no clicks happen in 5 seconds, then emit 'no clicks'
+ *
+ * ```ts
+ * import { fromEvent, takeUntil, interval, defaultIfEmpty } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const clicksBeforeFive = clicks.pipe(takeUntil(interval(5000)));
+ * const result = clicksBeforeFive.pipe(defaultIfEmpty('no clicks'));
+ * result.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link empty}
+ * @see {@link last}
+ *
+ * @param defaultValue The default value used if the source
+ * Observable is empty.
+ * @return A function that returns an Observable that emits either the
+ * specified `defaultValue` if the source Observable emits no items, or the
+ * values emitted by the source Observable.
+ */
+export function defaultIfEmpty<T, R>(defaultValue: R): OperatorFunction<T, T | R> {
+  return operate((source, subscriber) => {
+    let hasValue = false;
+    source.subscribe(
+      createOperatorSubscriber(
+        subscriber,
+        (value) => {
+          hasValue = true;
+          subscriber.next(value);
+        },
+        () => {
+          if (!hasValue) {
+            subscriber.next(defaultValue!);
+          }
+          subscriber.complete();
+        }
+      )
+    );
+  });
+}