feat(only): add only function

Author: djcsdy

index.test.ts

@@ -1,5 +1,5 @@
 import test from "ava";
-import {asyncIterable, initial, last, push, tail, toArray, unshift} from "./index";
+import {asyncIterable, initial, last, only, push, tail, toArray, unshift} from "./index";
 
 test("tail", async t => {
     t.deepEqual(await toArray(tail(asyncIterable([1, 2, 3, 4]))), [2, 3, 4]);
@@ -27,3 +27,9 @@ test("last", async t => {
     t.is(await last(asyncIterable([])), null);
     t.is(await last(asyncIterable([1, 2, 3])), 3);
 });
+
+test("only", async t => {
+    t.is(await only(asyncIterable([])), null);
+    t.is(await only(asyncIterable([4])), 4);
+    t.is(await only(asyncIterable([3, 4, 5])), null);
+});

index.ts

@@ -110,3 +110,12 @@ export async function last<T>(iterable: AsyncIterableLike<T>): Promise<T | null>
 }
 
 export const asyncLast = last;
+
+export async function only<T>(iterable: AsyncIterableLike<T>): Promise<T | null> {
+    const iterator = asyncIterator(iterable);
+    const first = await iterator.next();
+
+    return !(first.done ?? false) && ((await iterator.next()).done ?? false) ? first.value : null;
+}
+
+export const asyncOnly = only;