fix: return an empty string if iterable that passed to the 'join' fun…

…ction is empty-iterable

src/join.ts

@@ -9,11 +9,13 @@ type ReturnJoinType<T extends Iterable<unknown> | AsyncIterable<unknown>> =
     : never;
 
 function sync<A>(sep: string, iterable: Iterable<A>) {
-  return reduce((a: string, b) => `${a}${sep}${b}`, iterable);
+  return reduce((a: string, b) => `${a}${sep}${b}`, iterable) ?? "";
 }
 
 function async<A>(sep: string, iterable: AsyncIterable<A>) {
-  return reduce((a: string, b) => `${a}${sep}${b}`, iterable);
+  return reduce((a: string, b) => `${a}${sep}${b}`, iterable).then(
+    (a) => a ?? "",
+  );
 }
 
 /**

test/join.spec.ts

@@ -1,7 +1,14 @@
+import { asyncEmpty, empty } from "../src/_internal/utils";
 import { filter, join, map, pipe, toAsync } from "../src/index";
 
 describe("join", function () {
   describe("sync", function () {
+    it("should return an empty string if there are no iterable items", function () {
+      expect(join(", ", "")).toEqual("");
+      expect(join(", ", [])).toEqual("");
+      expect(join(", ", empty())).toEqual("");
+    });
+
     it("should be joined with separator", function () {
       expect(join(", ", "hello")).toEqual("h, e, l, l, o");
     });
@@ -26,6 +33,10 @@ describe("join", function () {
   });
 
   describe("async", function () {
+    it("should return an empty string if there are no iterable items", async function () {
+      expect(await join(", ", asyncEmpty())).toEqual("");
+    });
+
     it("should be discarded elements by length", async function () {
       const res = await join("-", toAsync([1, 2, 3, 4, 5]));
       expect(res).toEqual("1-2-3-4-5");