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

Author: jasonaden

src/operator/skipLast.ts

@@ -1,8 +1,5 @@
-import { Operator } from '../Operator';
-import { Subscriber } from '../Subscriber';
-import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
 import { Observable } from '../Observable';
-import { TeardownLogic } from '../Subscription';
+import { skipLast as higherOrder } from '../operators/skipLast';
 
 /**
  * Skip the last `count` values emitted by the source Observable.
@@ -37,54 +34,5 @@ import { TeardownLogic } from '../Subscription';
  * @owner Observable
  */
 export function skipLast<T>(this: Observable<T>, count: number): Observable<T> {
-  return this.lift(new SkipLastOperator(count));
+  return higherOrder(count)(this);
 }
-
-class SkipLastOperator<T> implements Operator<T, T> {
-  constructor(private _skipCount: number) {
-    if (this._skipCount < 0) {
-      throw new ArgumentOutOfRangeError;
-    }
-  }
-
-  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    if (this._skipCount === 0) {
-      // If we don't want to skip any values then just subscribe
-      // to Subscriber without any further logic.
-      return source.subscribe(new Subscriber(subscriber));
-    } else {
-      return source.subscribe(new SkipLastSubscriber(subscriber, this._skipCount));
-    }
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class SkipLastSubscriber<T> extends Subscriber<T> {
-  private _ring: T[];
-  private _count: number = 0;
-
-  constructor(destination: Subscriber<T>, private _skipCount: number) {
-    super(destination);
-    this._ring = new Array<T>(_skipCount);
-  }
-
-  protected _next(value: T): void {
-    const skipCount = this._skipCount;
-    const count = this._count++;
-
-    if (count < skipCount) {
-      this._ring[count] = value;
-    } else {
-      const currentIndex = count % skipCount;
-      const ring = this._ring;
-      const oldValue = ring[currentIndex];
-
-      ring[currentIndex] = value;
-      this.destination.next(oldValue);
-    }
-  }
-}

src/operators/index.ts

@@ -61,6 +61,7 @@ export { reduce } from './reduce';
 export { refCount } from './refCount';
 export { scan } from './scan';
 export { skip } from './skip';
+export { skipLast } from './skipLast';
 export { subscribeOn } from './subscribeOn';
 export { switchAll } from './switchAll';
 export { switchMap } from './switchMap';

src/operators/skipLast.ts

@@ -0,0 +1,91 @@
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
+import { Observable } from '../Observable';
+import { TeardownLogic } from '../Subscription';
+import { MonoTypeOperatorFunction } from '../interfaces';
+
+/**
+ * Skip the last `count` values emitted by the source Observable.
+ *
+ * <img src="./img/skipLast.png" width="100%">
+ *
+ * `skipLast` returns an Observable that accumulates a queue with a length
+ * enough to store the first `count` values. As more values are received,
+ * values are taken from the front of the queue and produced on the result
+ * sequence. This causes values to be delayed.
+ *
+ * @example <caption>Skip the last 2 values of an Observable with many values</caption>
+ * var many = Rx.Observable.range(1, 5);
+ * var skipLastTwo = many.skipLast(2);
+ * skipLastTwo.subscribe(x => console.log(x));
+ *
+ * // Results in:
+ * // 1 2 3
+ *
+ * @see {@link skip}
+ * @see {@link skipUntil}
+ * @see {@link skipWhile}
+ * @see {@link take}
+ *
+ * @throws {ArgumentOutOfRangeError} When using `skipLast(i)`, it throws
+ * ArgumentOutOrRangeError if `i < 0`.
+ *
+ * @param {number} count Number of elements to skip from the end of the source Observable.
+ * @returns {Observable<T>} An Observable that skips the last count values
+ * emitted by the source Observable.
+ * @method skipLast
+ * @owner Observable
+ */
+export function skipLast<T>(count: number): MonoTypeOperatorFunction<T> {
+  return (source: Observable<T>) => source.lift(new SkipLastOperator(count));
+}
+
+class SkipLastOperator<T> implements Operator<T, T> {
+  constructor(private _skipCount: number) {
+    if (this._skipCount < 0) {
+      throw new ArgumentOutOfRangeError;
+    }
+  }
+
+  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+    if (this._skipCount === 0) {
+      // If we don't want to skip any values then just subscribe
+      // to Subscriber without any further logic.
+      return source.subscribe(new Subscriber(subscriber));
+    } else {
+      return source.subscribe(new SkipLastSubscriber(subscriber, this._skipCount));
+    }
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class SkipLastSubscriber<T> extends Subscriber<T> {
+  private _ring: T[];
+  private _count: number = 0;
+
+  constructor(destination: Subscriber<T>, private _skipCount: number) {
+    super(destination);
+    this._ring = new Array<T>(_skipCount);
+  }
+
+  protected _next(value: T): void {
+    const skipCount = this._skipCount;
+    const count = this._count++;
+
+    if (count < skipCount) {
+      this._ring[count] = value;
+    } else {
+      const currentIndex = count % skipCount;
+      const ring = this._ring;
+      const oldValue = ring[currentIndex];
+
+      ring[currentIndex] = value;
+      this.destination.next(oldValue);
+    }
+  }
+}