feat: add index param to callback of .map and .forEach

fix felixfbecker#30

src/iterate.ts

@@ -30,7 +30,7 @@ export class IteratorWithOperators<T> implements IterableIterator<T> {
     /**
      * Returns a new Iterator by running each element thru iteratee
      */
-    map<R>(iteratee: (value: T) => R): IteratorWithOperators<R> {
+    map<R>(iteratee: (value: T, index: number) => R): IteratorWithOperators<R> {
         return new IteratorWithOperators(new MapIterator(this.source, iteratee))
     }
 
@@ -192,14 +192,15 @@ export class IteratorWithOperators<T> implements IterableIterator<T> {
     /**
      * Iterates and invokes `iteratee` for every element emitted by the Iterator
      */
-    forEach(iteratee: (value: T) => any): void {
+    forEach(iteratee: (value: T, index: number) => any): void {
         let result: IteratorResult<T>
+        let index = 0;
         while (true) {
             result = this.source.next()
             if (result.done) {
                 break
             }
-            iteratee(result.value)
+            iteratee(result.value, index++)
         }
     }
 

src/map.ts

@@ -2,10 +2,14 @@
  * An iterator that emits results by running each element through a provided predicate
  */
 export class MapIterator<T, R> implements Iterator<R> {
-    constructor(private source: Iterator<T>, private iteratee: (value: T) => R) {}
+    private index: number
+
+    constructor(private source: Iterator<T>, private iteratee: (value: T, index: number) => R) {
+        this.index = 0
+    }
 
     next(): IteratorResult<R> {
         const { value, done } = this.source.next()
-        return { value: !done && this.iteratee(value), done } as IteratorResult<R>
+        return { value: !done && this.iteratee(value, this.index++), done } as IteratorResult<R>
     }
 }