feat(empty): add empty function

Author: djcsdy

index.test.ts

@@ -1,5 +1,5 @@
 import test from "ava";
-import {asyncIterable, initial, last, only, push, tail, toArray, unshift} from "./index";
+import {asyncIterable, empty, 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]);
@@ -33,3 +33,9 @@ test("only", async t => {
     t.is(await only(asyncIterable([4])), 4);
     t.is(await only(asyncIterable([3, 4, 5])), null);
 });
+
+test("empty", async t => {
+    t.is(await empty(asyncIterable([])), true);
+    t.is(await empty(asyncIterable([1])), false);
+    t.is(await empty(asyncIterable([1, 2, 3])), false);
+});

index.ts

@@ -119,3 +119,9 @@ export async function only<T>(iterable: AsyncIterableLike<T>): Promise<T | null>
 }
 
 export const asyncOnly = only;
+
+export async function empty(iterable: AsyncIterableLike<unknown>): Promise<boolean> {
+    return (await asyncIterator(iterable).next()).done === true;
+}
+
+export const asyncEmpty = empty;