feat(asyncLastOnce): add asyncLastOnce function

Author: djcsdy

index.test.ts

@@ -3,6 +3,7 @@ import test from "ava";
 import {
     asyncInitialOnce,
     asyncIterator,
+    asyncLastOnce,
     asyncPushOnce,
     asyncTailOnce,
     asyncToArrayOnce,
@@ -119,3 +120,8 @@ test("asyncInitialOnce", async t => {
     t.deepEqual(await asyncToArrayOnce(asyncInitialOnce(asyncIterator([1]))), []);
     t.deepEqual(await asyncToArrayOnce(asyncInitialOnce(asyncIterator([]))), []);
 });
+
+test("asyncLastOnce", async t => {
+    t.is(await asyncLastOnce(asyncIterator([])), null);
+    t.is(await asyncLastOnce(asyncIterator([1, 2, 3])), 3);
+});

index.ts

@@ -158,3 +158,17 @@ export function asyncInitialOnce<T>(iterator: AsyncIteratorLike<T>): AsyncIterat
         }
     };
 }
+
+export async function asyncLastOnce<T>(iterator: AsyncIteratorLike<T>): Promise<T | null> {
+    const it = asyncIterator(iterator);
+    let last = await it.next();
+    if (last.done === true) {
+        return null;
+    }
+    let element = await it.next();
+    while (element.done !== true) {
+        last = element;
+        element = await it.next();
+    }
+    return last.value;
+}