Merge aae7ceb into c4a84cc

src/internal/operators/isEmpty.ts

@@ -3,6 +3,66 @@ import { Subscriber } from '../Subscriber';
 import { Observable } from '../Observable';
 import { OperatorFunction } from '../types';
 
+/**
+ * Emits false if the input observable emits any values, or emits true if the
+ * input observable completes without emitting any values.
+ *
+ * <span class="informal">Tells whether any values are emitted by an observable</span>
+ *
+ * ![](isEmpty.png)
+ *
+ * `isEmpty` transforms an Observable that emits values into an Observable that
+ * emits a single boolean value representing whether or not any values were
+ * emitted by the source Observable. As soon as the source Observable emits a
+ * value, `isEmpty` will emit a `false` and complete.  If the source Observable
+ * completes having not emitted anything, `isEmpty` will emit a `true` and
+ * complete.
+ *
+ * A similar effect could be achieved with {@link count}, but `isEmpty` can emit
+ * a `false` value sooner.
+ *
+ * ## Examples
+ *
+ * Emit `false` for a non-empty Observable
+ * ```javascript
+ * import { Subject } from 'rxjs';
+ * import { isEmpty } from 'rxjs/operators';
+ *
+ * const source = new Subject<string>();
+ * const result = source.pipe(isEmpty());
+ * source.subscribe(x => console.log(x));
+ * result.subscribe(x => console.log(x));
+ * source.next('a');
+ * source.next('b');
+ * source.next('c');
+ * source.complete();
+ *
+ * // Results in:
+ * // a
+ * // false
+ * // b
+ * // c
+ * ```
+ *
+ * Emit `true` for an empty Observable
+ * ```javascript
+ * import { EMPTY } from 'rxjs';
+ * import { isEmpty } from 'rxjs/operators';
+ *
+ * const result = EMPTY.pipe(isEmpty());
+ * result.subscribe(x => console.log(x));
+ * // Results in:
+ * // true
+ * ```
+ *
+ * @see {@link count}
+ * @see {@link EMPTY}
+ *
+ * @return {OperatorFunction<T, boolean>} An Observable of a boolean value indicating whether observable was empty or not
+ * @method isEmpty
+ * @owner Observable
+ */
+
 export function isEmpty<T>(): OperatorFunction<T, boolean> {
   return (source: Observable<T>) => source.lift(new IsEmptyOperator());
 }