feat(findIndex): add higher-order lettable findIndex

src/operator/findIndex.ts

@@ -1,6 +1,5 @@
 import { Observable } from '../Observable';
-import { FindValueOperator } from '../operators/find';
-
+import { findIndex as higherOrder } from '../operators';
 /**
  * Emits only the index of the first value emitted by the source Observable that
  * meets some condition.
@@ -37,5 +36,5 @@ import { FindValueOperator } from '../operators/find';
  */
 export function findIndex<T>(this: Observable<T>, predicate: (value: T, index: number, source: Observable<T>) => boolean,
                              thisArg?: any): Observable<number> {
-  return <any>this.lift<any>(new FindValueOperator(predicate, this, true, thisArg));
+  return higherOrder(predicate, thisArg)(this);
 }

src/operators/findIndex.ts

@@ -0,0 +1,41 @@
+import { Observable } from '../Observable';
+import { FindValueOperator } from '../operators/find';
+import { OperatorFunction } from '../interfaces';
+/**
+ * Emits only the index of the first value emitted by the source Observable that
+ * meets some condition.
+ *
+ * <span class="informal">It's like {@link find}, but emits the index of the
+ * found value, not the value itself.</span>
+ *
+ * <img src="./img/findIndex.png" width="100%">
+ *
+ * `findIndex` searches for the first item in the source Observable that matches
+ * the specified condition embodied by the `predicate`, and returns the
+ * (zero-based) index of the first occurrence in the source. Unlike
+ * {@link first}, the `predicate` is required in `findIndex`, and does not emit
+ * an error if a valid value is not found.
+ *
+ * @example <caption>Emit the index of first click that happens on a DIV element</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var result = clicks.findIndex(ev => ev.target.tagName === 'DIV');
+ * result.subscribe(x => console.log(x));
+ *
+ * @see {@link filter}
+ * @see {@link find}
+ * @see {@link first}
+ * @see {@link take}
+ *
+ * @param {function(value: T, index: number, source: Observable<T>): boolean} predicate
+ * A function called with each item to test for condition matching.
+ * @param {any} [thisArg] An optional argument to determine the value of `this`
+ * in the `predicate` function.
+ * @return {Observable} An Observable of the index of the first item that
+ * matches the condition.
+ * @method find
+ * @owner Observable
+ */
+export function findIndex<T>(predicate: (value: T, index: number, source: Observable<T>) => boolean,
+                             thisArg?: any): OperatorFunction<T, number> {
+  return (source: Observable<T>) => source.lift(new FindValueOperator(predicate, source, true, thisArg)) as Observable<any>;
+}

src/operators/index.ts

@@ -27,6 +27,7 @@ export { expand } from './expand';
 export { filter } from './filter';
 export { finalize } from './finalize';
 export { find } from './find';
+export { findIndex } from './findIndex';
 export { ignoreElements } from './ignoreElements';
 export { map } from './map';
 export { materialize } from './materialize';