feat(asyncFold1Once): add asyncFold1Once function

Author: djcsdy

index.test.ts

@@ -10,6 +10,7 @@ import {
     asyncExcludeNullOnce,
     asyncExcludeOnce,
     asyncFilterOnce,
+    asyncFold1Once,
     asyncFoldOnce,
     asyncInitialOnce,
     asyncIterator,
@@ -410,3 +411,7 @@ test("asyncRemoveFirstOnce", async t => {
 test("asyncFoldOnce", async t => {
     t.is(await asyncFoldOnce(asyncIterator([1, 2, 3]), (a, e, i) => a + e * i, 0), 8);
 });
+
+test("asyncFold1Once", async t => {
+    t.is(await asyncFold1Once(asyncIterator([1, 2, 3]), (a, e, i) => a + e * i), 9);
+});

index.ts

@@ -648,3 +648,27 @@ export function asyncFoldOnceFn<T, U>(
 ): (iterator: AsyncIteratorLike<T>) => Promise<U> {
     return async iterator => asyncFoldOnce(iterator, f, initial);
 }
+
+export async function asyncFold1Once<T>(
+    iterator: AsyncIteratorLike<T>,
+    f: (accumulator: T, element: T, index: number) => T | Promise<T>
+): Promise<T> {
+    const it = asyncIterator(iterator);
+    const element = await it.next();
+
+    if (element.done === true) {
+        throw new TypeError("asyncFold1Once: empty AsyncIterator");
+    }
+
+    return asyncFoldOnce(
+        it,
+        async (accumulator, element, index) => f(accumulator, element, index + 1),
+        element.value
+    );
+}
+
+export function asyncFold1OnceFn<T>(
+    f: (accumulator: T, element: T, index: number) => T | Promise<T>
+): (iterator: AsyncIteratorLike<T>) => Promise<T> {
+    return async iterator => asyncFold1Once(iterator, f);
+}