feat(index): Add indexElements.

source/operators/index.ts

@@ -8,6 +8,7 @@ export * from "./debounceTimeSubsequent";
 export * from "./defaultObservableIfEmpty";
 export * from "./endWith";
 export * from "./guard";
+export * from "./indexElements";
 export * from "./inexorably";
 export * from "./initial";
 export * from "./instanceOf";

source/operators/indexElements-spec.ts

@@ -0,0 +1,33 @@
+/**
+ * @license Use of this source code is governed by an MIT-style license that
+ * can be found in the LICENSE file at https://github.com/cartant/rxjs-etc
+ */
+/*tslint:disable:no-unused-expression*/
+
+import { marbles } from "rxjs-marbles";
+import { indexElements } from "./indexElements";
+
+describe("indexElements", () => {
+
+    it("should index without a project function", marbles(m => {
+
+        const source =   m.cold("ab-cd-");
+        const sourceSubs =      "^-----";
+        const expected = m.cold("ab-cd-", { a: 0, b: 1, c: 2, d: 3 });
+
+        const destination = source.pipe(indexElements());
+        m.expect(destination).toBeObservable(expected);
+        m.expect(source).toHaveSubscriptions(sourceSubs);
+    }));
+
+    it("should index with a project function", marbles(m => {
+
+        const source =   m.cold("ab-cd-");
+        const sourceSubs =      "^-----";
+        const expected = m.cold("ab-cd-", { a: "a:0", b: "b:1", c: "c:2", d: "d:3" });
+
+        const destination = source.pipe(indexElements((value, index) => `${value}:${index}`));
+        m.expect(destination).toBeObservable(expected);
+        m.expect(source).toHaveSubscriptions(sourceSubs);
+    }));
+});

source/operators/indexElements.ts

@@ -0,0 +1,13 @@
+/**
+ * @license Use of this source code is governed by an MIT-style license that
+ * can be found in the LICENSE file at https://github.com/cartant/rxjs-etc
+ */
+
+import { OperatorFunction } from "rxjs";
+import { map } from "rxjs/operators";
+
+export function indexElements<T>(): OperatorFunction<T, number>;
+export function indexElements<T, R>(project: (value: T, index: number) => R): OperatorFunction<T, R>;
+export function indexElements<T>(project: (value: T, index: number) => any = (value, index) => index): OperatorFunction<T, any> {
+    return map<T, any>(project);
+}