feat(count): add higher-order lettable version of count

src/operator/count.ts

@@ -1,7 +1,5 @@
 import { Observable } from '../Observable';
-import { Operator } from '../Operator';
-import { Observer } from '../Observer';
-import { Subscriber } from '../Subscriber';
+import { count as higherOrder } from '../operators';
 
 /**
  * Counts the number of emissions on the source and emits that number when the
@@ -52,59 +50,5 @@ import { Subscriber } from '../Subscriber';
  * @owner Observable
  */
 export function count<T>(this: Observable<T>, predicate?: (value: T, index: number, source: Observable<T>) => boolean): Observable<number> {
-  return this.lift(new CountOperator(predicate, this));
-}
-
-class CountOperator<T> implements Operator<T, number> {
-  constructor(private predicate?: (value: T, index: number, source: Observable<T>) => boolean,
-              private source?: Observable<T>) {
-  }
-
-  call(subscriber: Subscriber<number>, source: any): any {
-    return source.subscribe(new CountSubscriber(subscriber, this.predicate, this.source));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class CountSubscriber<T> extends Subscriber<T> {
-  private count: number = 0;
-  private index: number = 0;
-
-  constructor(destination: Observer<number>,
-              private predicate?: (value: T, index: number, source: Observable<T>) => boolean,
-              private source?: Observable<T>) {
-    super(destination);
-  }
-
-  protected _next(value: T): void {
-    if (this.predicate) {
-      this._tryPredicate(value);
-    } else {
-      this.count++;
-    }
-  }
-
-  private _tryPredicate(value: T) {
-    let result: any;
-
-    try {
-      result = this.predicate(value, this.index++, this.source);
-    } catch (err) {
-      this.destination.error(err);
-      return;
-    }
-
-    if (result) {
-      this.count++;
-    }
-  }
-
-  protected _complete(): void {
-    this.destination.next(this.count);
-    this.destination.complete();
-  }
+  return higherOrder(predicate)(this);
 }

src/operators/count.ts

@@ -0,0 +1,111 @@
+import { Observable } from '../Observable';
+import { Operator } from '../Operator';
+import { Observer } from '../Observer';
+import { Subscriber } from '../Subscriber';
+import { OperatorFunction } from '../interfaces';
+
+/**
+ * Counts the number of emissions on the source and emits that number when the
+ * source completes.
+ *
+ * <span class="informal">Tells how many values were emitted, when the source
+ * completes.</span>
+ *
+ * <img src="./img/count.png" width="100%">
+ *
+ * `count` transforms an Observable that emits values into an Observable that
+ * emits a single value that represents the number of values emitted by the
+ * source Observable. If the source Observable terminates with an error, `count`
+ * will pass this error notification along without emitting a value first. If
+ * the source Observable does not terminate at all, `count` will neither emit
+ * a value nor terminate. This operator takes an optional `predicate` function
+ * as argument, in which case the output emission will represent the number of
+ * source values that matched `true` with the `predicate`.
+ *
+ * @example <caption>Counts how many seconds have passed before the first click happened</caption>
+ * var seconds = Rx.Observable.interval(1000);
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var secondsBeforeClick = seconds.takeUntil(clicks);
+ * var result = secondsBeforeClick.count();
+ * result.subscribe(x => console.log(x));
+ *
+ * @example <caption>Counts how many odd numbers are there between 1 and 7</caption>
+ * var numbers = Rx.Observable.range(1, 7);
+ * var result = numbers.count(i => i % 2 === 1);
+ * result.subscribe(x => console.log(x));
+ *
+ * // Results in:
+ * // 4
+ *
+ * @see {@link max}
+ * @see {@link min}
+ * @see {@link reduce}
+ *
+ * @param {function(value: T, i: number, source: Observable<T>): boolean} [predicate] A
+ * boolean function to select what values are to be counted. It is provided with
+ * arguments of:
+ * - `value`: the value from the source Observable.
+ * - `index`: the (zero-based) "index" of the value from the source Observable.
+ * - `source`: the source Observable instance itself.
+ * @return {Observable} An Observable of one number that represents the count as
+ * described above.
+ * @method count
+ * @owner Observable
+ */
+export function count<T>(predicate?: (value: T, index: number, source: Observable<T>) => boolean): OperatorFunction<T, number> {
+  return (source: Observable<T>) => source.lift(new CountOperator(predicate, source));
+}
+
+class CountOperator<T> implements Operator<T, number> {
+  constructor(private predicate?: (value: T, index: number, source: Observable<T>) => boolean,
+              private source?: Observable<T>) {
+  }
+
+  call(subscriber: Subscriber<number>, source: any): any {
+    return source.subscribe(new CountSubscriber(subscriber, this.predicate, this.source));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class CountSubscriber<T> extends Subscriber<T> {
+  private count: number = 0;
+  private index: number = 0;
+
+  constructor(destination: Observer<number>,
+              private predicate?: (value: T, index: number, source: Observable<T>) => boolean,
+              private source?: Observable<T>) {
+    super(destination);
+  }
+
+  protected _next(value: T): void {
+    if (this.predicate) {
+      this._tryPredicate(value);
+    } else {
+      this.count++;
+    }
+  }
+
+  private _tryPredicate(value: T) {
+    let result: any;
+
+    try {
+      result = this.predicate(value, this.index++, this.source);
+    } catch (err) {
+      this.destination.error(err);
+      return;
+    }
+
+    if (result) {
+      this.count++;
+    }
+  }
+
+  protected _complete(): void {
+    this.destination.next(this.count);
+    this.destination.complete();
+  }
+}

src/operators/index.ts

@@ -10,6 +10,7 @@ export { concat } from './concat';
 export { concatAll } from './concatAll';
 export { concatMap } from './concatMap';
 export { concatMapTo } from './concatMapTo';
+export { count } from './count';
 export { defaultIfEmpty } from './defaultIfEmpty';
 export { dematerialize } from './dematerialize';
 export { filter } from './filter';