.map(value, counter), as per tc39/proposal-iterator-helpers#211

src/operations/async/map.ts

@@ -1,13 +1,15 @@
 import { Arguments, AsyncOperation, Name } from "../operation";
 
 export interface MapFn<T, O> {
-  (value: T): O | Promise<O>;
+  (value: T, counter: number): O | Promise<O>;
 }
 
 export function map<T, O>(callbackFn: MapFn<T, O>) {
   const fn: AsyncOperation<T, AsyncIterable<O>> = async function *map(iterable) {
+    let counter: number = -1;
     for await (const value of async(iterable)) {
-      yield callbackFn(value);
+      counter += 1;
+      yield callbackFn(value, counter);
     }
     async function *async(value: AsyncIterable<T>) {
       yield * value;

src/operations/sync/map.ts

@@ -2,15 +2,17 @@ import { Arguments, AsyncFn, GetAsync, Name, SyncOperation } from "../operation"
 import { isAsyncIterable, isIterable } from "../../async-like";
 import * as Async from "../async";
 
-export type MapFn<T, O> = (value: T) => O;
+export type MapFn<T, O> = (value: T, counter: number) => O;
 
 export function map<T, O>(callbackFn: MapFn<T, O>) {
   const fn: SyncOperation<T, Iterable<O>> = function *map(iterable) {
     if (isAsyncIterable(iterable) && !isIterable(iterable)) throw new Async.ExpectedAsyncOperationError(
       fn[GetAsync]()
     );
+    let counter = -1;
     for (const value of iterable) {
-      yield callbackFn(value);
+      counter += 1;
+      yield callbackFn(value, counter);
     }
   };
   fn[Name] = "map";