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

src/operator/takeLast.ts

@@ -1,9 +1,6 @@
-import { Operator } from '../Operator';
-import { Subscriber } from '../Subscriber';
-import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
-import { EmptyObservable } from '../observable/EmptyObservable';
+
 import { Observable } from '../Observable';
-import { TeardownLogic } from '../Subscription';
+import { takeLast as higherOrderTakeLast } from '../operators';
 
 /**
  * Emits only the last `count` values emitted by the source Observable.
@@ -42,65 +39,5 @@ import { TeardownLogic } from '../Subscription';
  * @owner Observable
  */
 export function takeLast<T>(this: Observable<T>, count: number): Observable<T> {
-  if (count === 0) {
-    return new EmptyObservable<T>();
-  } else {
-    return this.lift(new TakeLastOperator(count));
-  }
-}
-
-class TakeLastOperator<T> implements Operator<T, T> {
-  constructor(private total: number) {
-    if (this.total < 0) {
-      throw new ArgumentOutOfRangeError;
-    }
-  }
-
-  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    return source.subscribe(new TakeLastSubscriber(subscriber, this.total));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class TakeLastSubscriber<T> extends Subscriber<T> {
-  private ring: Array<T> = new Array();
-  private count: number = 0;
-
-  constructor(destination: Subscriber<T>, private total: number) {
-    super(destination);
-  }
-
-  protected _next(value: T): void {
-    const ring = this.ring;
-    const total = this.total;
-    const count = this.count++;
-
-    if (ring.length < total) {
-      ring.push(value);
-    } else {
-      const index = count % total;
-      ring[index] = value;
-    }
-  }
-
-  protected _complete(): void {
-    const destination = this.destination;
-    let count = this.count;
-
-    if (count > 0) {
-      const total = this.count >= this.total ? this.total : this.count;
-      const ring  = this.ring;
-
-      for (let i = 0; i < total; i++) {
-        const idx = (count++) % total;
-        destination.next(ring[idx]);
-      }
-    }
-
-    destination.complete();
-  }
+  return higherOrderTakeLast(count)(this);
 }

src/operators/index.ts

@@ -4,3 +4,4 @@ export { map } from './map';
 export { mergeMap } from './mergeMap';
 export { scan } from './scan';
 export { switchMap } from './switchMap';
+export { takeLast } from './takeLast';

src/operators/takeLast.ts

@@ -0,0 +1,109 @@
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
+import { EmptyObservable } from '../observable/EmptyObservable';
+import { Observable } from '../Observable';
+import { TeardownLogic } from '../Subscription';
+import { OperatorFunction } from '../interfaces';
+
+/**
+ * Emits only the last `count` values emitted by the source Observable.
+ *
+ * <span class="informal">Remembers the latest `count` values, then emits those
+ * only when the source completes.</span>
+ *
+ * <img src="./img/takeLast.png" width="100%">
+ *
+ * `takeLast` returns an Observable that emits at most the last `count` values
+ * emitted by the source Observable. If the source emits fewer than `count`
+ * values then all of its values are emitted. This operator must wait until the
+ * `complete` notification emission from the source in order to emit the `next`
+ * values on the output Observable, because otherwise it is impossible to know
+ * whether or not more values will be emitted on the source. For this reason,
+ * all values are emitted synchronously, followed by the complete notification.
+ *
+ * @example <caption>Take the last 3 values of an Observable with many values</caption>
+ * var many = Rx.Observable.range(1, 100);
+ * var lastThree = many.takeLast(3);
+ * lastThree.subscribe(x => console.log(x));
+ *
+ * @see {@link take}
+ * @see {@link takeUntil}
+ * @see {@link takeWhile}
+ * @see {@link skip}
+ *
+ * @throws {ArgumentOutOfRangeError} When using `takeLast(i)`, it delivers an
+ * ArgumentOutOrRangeError to the Observer's `error` callback if `i < 0`.
+ *
+ * @param {number} count The maximum number of values to emit from the end of
+ * the sequence of values emitted by the source Observable.
+ * @return {Observable<T>} An Observable that emits at most the last count
+ * values emitted by the source Observable.
+ * @method takeLast
+ * @owner Observable
+ */
+export function takeLast<T>(count: number): OperatorFunction<T, T> {
+  return function takeLastOperatorFunction(source: Observable<T>): Observable<T> {
+    if (count === 0) {
+      return new EmptyObservable<T>();
+    } else {
+      return source.lift(new TakeLastOperator(count));
+    }
+  };
+}
+
+class TakeLastOperator<T> implements Operator<T, T> {
+  constructor(private total: number) {
+    if (this.total < 0) {
+      throw new ArgumentOutOfRangeError;
+    }
+  }
+
+  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+    return source.subscribe(new TakeLastSubscriber(subscriber, this.total));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class TakeLastSubscriber<T> extends Subscriber<T> {
+  private ring: Array<T> = new Array();
+  private count: number = 0;
+
+  constructor(destination: Subscriber<T>, private total: number) {
+    super(destination);
+  }
+
+  protected _next(value: T): void {
+    const ring = this.ring;
+    const total = this.total;
+    const count = this.count++;
+
+    if (ring.length < total) {
+      ring.push(value);
+    } else {
+      const index = count % total;
+      ring[index] = value;
+    }
+  }
+
+  protected _complete(): void {
+    const destination = this.destination;
+    let count = this.count;
+
+    if (count > 0) {
+      const total = this.count >= this.total ? this.total : this.count;
+      const ring  = this.ring;
+
+      for (let i = 0; i < total; i++) {
+        const idx = (count++) % total;
+        destination.next(ring[idx]);
+      }
+    }
+
+    destination.complete();
+  }
+}