feat(index): add index function

Author: djcsdy

index.test.ts

@@ -12,6 +12,7 @@ import {
     filter,
     fold,
     fold1,
+    index,
     indexOf,
     initial,
     last,
@@ -197,6 +198,11 @@ test("fold1", async t => {
     t.is(await fold1(asyncIterable([1, 2, 3]), (a, e, i) => a + e * i), 9);
 });
 
+test("index", async t => {
+    t.is(await index(asyncIterable([1, 2, 3, 4, 3, 2, 1]), 2), 3);
+    t.is(await index(asyncIterable([1, 2, 3, 4, 3, 2, 1]), 7), null);
+});
+
 test("contains", async t => {
     t.true(await contains(asyncIterable([1, 2, 3]), 1));
     t.false(await contains(asyncIterable([1, 2, 3]), 0));

index.ts

@@ -578,6 +578,28 @@ export function fold1Fn<T>(
 
 export const asyncFold1Fn = fold1Fn;
 
+export async function index<T>(iterable: AsyncIterableLike<T>, index: number): Promise<T | null> {
+    if (index < 0 || !isFinite(index) || Math.floor(index) !== index) {
+        throw new RangeError("illegal index");
+    }
+
+    let i = 0;
+    for await (const element of await iterable) {
+        if (i++ === index) {
+            return element;
+        }
+    }
+    return null;
+}
+
+export const asyncIndex = index;
+
+export function indexFn<T>(index: number): (iterable: AsyncIterableLike<T>) => Promise<T | null> {
+    return async iterable => asyncIndex(iterable, index);
+}
+
+export const asyncIndexFn = indexFn;
+
 export async function contains<T>(iterable: AsyncIterableLike<T>, value: T): Promise<boolean> {
     for await (const element of await iterable) {
         if (element === value) {