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

src/operator/pairwise.ts

@@ -1,6 +1,5 @@
-import { Operator } from '../Operator';
 import { Observable } from '../Observable';
-import { Subscriber } from '../Subscriber';
+import { pairwise as higherOrder } from '../operators/pairwise';
 
 /**
  * Groups pairs of consecutive emissions together and emits them as an array of
@@ -38,35 +37,5 @@ import { Subscriber } from '../Subscriber';
  * @owner Observable
  */
 export function pairwise<T>(this: Observable<T>): Observable<[T, T]> {
-  return this.lift(new PairwiseOperator());
-}
-
-class PairwiseOperator<T> implements Operator<T, [T, T]> {
-  call(subscriber: Subscriber<[T, T]>, source: any): any {
-    return source.subscribe(new PairwiseSubscriber(subscriber));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class PairwiseSubscriber<T> extends Subscriber<T> {
-  private prev: T;
-  private hasPrev: boolean = false;
-
-  constructor(destination: Subscriber<[T, T]>) {
-    super(destination);
-  }
-
-  _next(value: T): void {
-    if (this.hasPrev) {
-      this.destination.next([this.prev, value]);
-    } else {
-      this.hasPrev = true;
-    }
-
-    this.prev = value;
-  }
+  return higherOrder()(this);
 }

src/operators/index.ts

@@ -46,6 +46,7 @@ export { min } from './min';
 export { multicast } from './multicast';
 export { observeOn } from './observeOn';
 export { onErrorResumeNext } from './onErrorResumeNext';
+export { pairwise } from './pairwise';
 export { publish } from './publish';
 export { race } from './race';
 export { reduce } from './reduce';

src/operators/pairwise.ts

@@ -0,0 +1,73 @@
+import { Operator } from '../Operator';
+import { Observable } from '../Observable';
+import { Subscriber } from '../Subscriber';
+import { OperatorFunction } from '../interfaces';
+
+/**
+ * Groups pairs of consecutive emissions together and emits them as an array of
+ * two values.
+ *
+ * <span class="informal">Puts the current value and previous value together as
+ * an array, and emits that.</span>
+ *
+ * <img src="./img/pairwise.png" width="100%">
+ *
+ * The Nth emission from the source Observable will cause the output Observable
+ * to emit an array [(N-1)th, Nth] of the previous and the current value, as a
+ * pair. For this reason, `pairwise` emits on the second and subsequent
+ * emissions from the source Observable, but not on the first emission, because
+ * there is no previous value in that case.
+ *
+ * @example <caption>On every click (starting from the second), emit the relative distance to the previous click</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var pairs = clicks.pairwise();
+ * var distance = pairs.map(pair => {
+ *   var x0 = pair[0].clientX;
+ *   var y0 = pair[0].clientY;
+ *   var x1 = pair[1].clientX;
+ *   var y1 = pair[1].clientY;
+ *   return Math.sqrt(Math.pow(x0 - x1, 2) + Math.pow(y0 - y1, 2));
+ * });
+ * distance.subscribe(x => console.log(x));
+ *
+ * @see {@link buffer}
+ * @see {@link bufferCount}
+ *
+ * @return {Observable<Array<T>>} An Observable of pairs (as arrays) of
+ * consecutive values from the source Observable.
+ * @method pairwise
+ * @owner Observable
+ */
+export function pairwise<T>(): OperatorFunction<T, [T, T]> {
+  return (source: Observable<T>) => source.lift(new PairwiseOperator());
+}
+
+class PairwiseOperator<T> implements Operator<T, [T, T]> {
+  call(subscriber: Subscriber<[T, T]>, source: any): any {
+    return source.subscribe(new PairwiseSubscriber(subscriber));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class PairwiseSubscriber<T> extends Subscriber<T> {
+  private prev: T;
+  private hasPrev: boolean = false;
+
+  constructor(destination: Subscriber<[T, T]>) {
+    super(destination);
+  }
+
+  _next(value: T): void {
+    if (this.hasPrev) {
+      this.destination.next([this.prev, value]);
+    } else {
+      this.hasPrev = true;
+    }
+
+    this.prev = value;
+  }
+}