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

Author: jasonaden

src/operator/pluck.ts

@@ -1,5 +1,5 @@
 import { Observable } from '../Observable';
-import { map } from './map';
+import { pluck as higherOrder } from '../operators/pluck';
 
 /**
  * Maps each source value (an object) to its specified nested property.
@@ -28,26 +28,5 @@ import { map } from './map';
  * @owner Observable
  */
 export function pluck<T, R>(this: Observable<T>, ...properties: string[]): Observable<R> {
-  const length = properties.length;
-  if (length === 0) {
-    throw new Error('list of properties cannot be empty.');
-  }
-  return map.call(this, plucker(properties, length));
-}
-
-function plucker(props: string[], length: number): (x: string) => any {
-  const mapper = (x: string) => {
-    let currentProp = x;
-    for (let i = 0; i < length; i++) {
-      const p = currentProp[props[i]];
-      if (typeof p !== 'undefined') {
-        currentProp = p;
-      } else {
-        return undefined;
-      }
-    }
-    return currentProp;
-  };
-
-  return mapper;
+  return higherOrder(...properties)(this);
 }

src/operators/index.ts

@@ -48,6 +48,7 @@ export { observeOn } from './observeOn';
 export { onErrorResumeNext } from './onErrorResumeNext';
 export { pairwise } from './pairwise';
 export { partition } from './partition';
+export { pluck } from './pluck';
 export { publish } from './publish';
 export { race } from './race';
 export { reduce } from './reduce';

src/operators/pluck.ts

@@ -0,0 +1,54 @@
+import { Observable } from '../Observable';
+import { map } from './map';
+import { OperatorFunction } from '../interfaces';
+
+/**
+ * Maps each source value (an object) to its specified nested property.
+ *
+ * <span class="informal">Like {@link map}, but meant only for picking one of
+ * the nested properties of every emitted object.</span>
+ *
+ * <img src="./img/pluck.png" width="100%">
+ *
+ * Given a list of strings describing a path to an object property, retrieves
+ * the value of a specified nested property from all values in the source
+ * Observable. If a property can't be resolved, it will return `undefined` for
+ * that value.
+ *
+ * @example <caption>Map every click to the tagName of the clicked target element</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var tagNames = clicks.pluck('target', 'tagName');
+ * tagNames.subscribe(x => console.log(x));
+ *
+ * @see {@link map}
+ *
+ * @param {...string} properties The nested properties to pluck from each source
+ * value (an object).
+ * @return {Observable} A new Observable of property values from the source values.
+ * @method pluck
+ * @owner Observable
+ */
+export function pluck<T, R>(...properties: string[]): OperatorFunction<T, R> {
+  const length = properties.length;
+  if (length === 0) {
+    throw new Error('list of properties cannot be empty.');
+  }
+  return (source: Observable<T>) => map(plucker(properties, length))(source as any);
+}
+
+function plucker(props: string[], length: number): (x: string) => any {
+  const mapper = (x: string) => {
+    let currentProp = x;
+    for (let i = 0; i < length; i++) {
+      const p = currentProp[props[i]];
+      if (typeof p !== 'undefined') {
+        currentProp = p;
+      } else {
+        return undefined;
+      }
+    }
+    return currentProp;
+  };
+
+  return mapper;
+}