feat(average): add average function

Author: djcsdy

index.test.ts

@@ -1,6 +1,7 @@
 import test from "ava";
 import {
     asyncIterable,
+    average,
     contains,
     drop,
     dropWhile,
@@ -263,3 +264,9 @@ test("product", async t => {
     t.is(await product(asyncIterable([1, 2, 3, 2])), 12);
     t.is(await product(asyncIterable([])), 1);
 });
+
+test("average", async t => {
+    t.is(await average(asyncIterable([1, 2, 3])), 2);
+    t.is(await average(asyncIterable([1, 2, 3, 2])), 2);
+    t.is(await average(asyncIterable([])), null);
+});

index.ts

@@ -836,3 +836,14 @@ export async function product(iterable: AsyncIterableLike<number>): Promise<numb
 }
 
 export const asyncProduct = product;
+
+export async function average(iterable: AsyncIterableLike<number>): Promise<number | null> {
+    const [sum, count] = await fold(
+        iterable,
+        ([sum], element, index) => [sum + element, index + 1],
+        [0, 0]
+    );
+    return count === 0 ? null : sum / count;
+}
+
+export const asyncAverage = average;