feat(asyncPushOnce): add asyncPushOnce function

Author: djcsdy

index.test.ts

@@ -1,6 +1,6 @@
 import type {ExecutionContext} from "ava";
 import test from "ava";
-import {asyncIterator, asyncTailOnce, asyncToArrayOnce} from "./index";
+import {asyncIterator, asyncPushOnce, asyncTailOnce, asyncToArrayOnce} from "./index";
 
 test("asyncIterator(empty)", async t => {
     t.true((await asyncIterator([]).next()).done);
@@ -93,3 +93,8 @@ test("asyncTailOnce", async t => {
     t.deepEqual(await asyncToArrayOnce(asyncTailOnce(asyncIterator([1]))), []);
     t.deepEqual(await asyncToArrayOnce(asyncTailOnce(asyncIterator([]))), []);
 });
+
+test("asyncPushOnce", async t => {
+    t.deepEqual(await asyncToArrayOnce(asyncPushOnce(asyncIterator([1, 2, 3]), 4)), [1, 2, 3, 4]);
+    t.deepEqual(await asyncToArrayOnce(asyncPushOnce(asyncIterator([]), 4)), [4]);
+});

index.ts

@@ -102,3 +102,26 @@ export function asyncTailOnce<T>(iterator: AsyncIteratorLike<T>): AsyncIterator<
     const tail = it.next().then(() => it);
     return fromPromiseOfIteratorLike(tail);
 }
+
+export function asyncPushOnce<T>(
+    iterator: AsyncIteratorLike<T>,
+    value: T | Promise<T>
+): AsyncIterator<T> {
+    const it = asyncIterator(iterator);
+    let next = async (): Promise<IteratorResult<T>> => {
+        const element = await it.next();
+        if (element.done === true) {
+            next = async () => Promise.resolve({done: true, value: undefined});
+            return {value: await value};
+        } else {
+            return element;
+        }
+    };
+    return {next: async () => next()};
+}
+
+export function asyncPushOnceFn<T>(
+    value: T | Promise<T>
+): (iterator: AsyncIteratorLike<T>) => AsyncIterator<T> {
+    return iterator => asyncPushOnce(iterator, value);
+}