From f5b9840c4bf5a55436750e373d72af8c91f494bf Mon Sep 17 00:00:00 2001
From: Dmytro Nikulin <dminikulin2003@gmail.com>
Date: Wed, 4 Mar 2026 16:40:22 +0200
Subject: [PATCH 1/2] Add solution

---
 src/reduce.test.js | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/src/reduce.test.js b/src/reduce.test.js
index 47a892f..4cdec53 100644
--- a/src/reduce.test.js
+++ b/src/reduce.test.js
@@ -11,9 +11,48 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  it('should call a callback for every item in an array', () => {
+    const cb = jest.fn();
 
+    [1, 2, 3, 4].reduce2(cb, 0);
+    expect(cb).toHaveBeenCalledTimes(4);
   });
 
   // Add tests here
+
+  it(`should call a callback for every item in an array
+    (minus one when no initial value is provided)`, () => {
+    const cb = jest.fn();
+
+    [1, 2, 3, 4].reduce2(cb);
+    expect(cb).toHaveBeenCalledTimes(3);
+  });
+
+  it('should not call a callback if array is empty', () => {
+    const cb = jest.fn();
+
+    [].reduce2(cb, 0);
+    expect(cb).not.toHaveBeenCalled();
+  });
+
+  it('should pass an accumulator and current value to a callback', () => {
+    const cb = jest.fn((acc, curr) => acc + curr);
+
+    [1, 2, 3, 4].reduce2(cb, 0);
+
+    expect(cb).toHaveBeenNthCalledWith(
+      4, 6, 4, expect.anything(), expect.anything()
+    );
+  });
+
+  it('should return a value', () => {
+    expect([].reduce2(() => {}, 0)).toBeDefined();
+  });
+
+  it('should correctly return a value calculated from an array', () => {
+    const cb = jest.fn((acc, curr) => acc + curr);
+    const result = [1, 2, 3, 4].reduce2(cb, 0);
+
+    expect(result).toBe(10);
+  });
 });

From ba5cff32f6b6ed366b6f3ef3dedb478566e87b0c Mon Sep 17 00:00:00 2001
From: Dmytro Nikulin <dminikulin2003@gmail.com>
Date: Wed, 4 Mar 2026 16:48:33 +0200
Subject: [PATCH 2/2] Edge cases added

---
 src/reduce.test.js | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/src/reduce.test.js b/src/reduce.test.js
index 4cdec53..82054f0 100644
--- a/src/reduce.test.js
+++ b/src/reduce.test.js
@@ -28,11 +28,29 @@ describe('reduce', () => {
     expect(cb).toHaveBeenCalledTimes(3);
   });
 
-  it('should not call a callback if array is empty', () => {
+  it(`should not call a callback and return initial value
+      if array is empty`, () => {
     const cb = jest.fn();
+    const result = [].reduce2(cb, 0);
 
-    [].reduce2(cb, 0);
     expect(cb).not.toHaveBeenCalled();
+    expect(result).toBe(0);
+  });
+
+  it(`should throw a TypeError if array is empty
+      while no initial value is given`, () => {
+    const cb = jest.fn();
+
+    expect(() => [].reduce2(cb)).toThrow(TypeError);
+  });
+
+  it(`should return an item from array with a single item
+      when no initial value is given`, () => {
+    const cb = jest.fn();
+    const result = [1].reduce2(cb);
+
+    expect(cb).not.toHaveBeenCalled();
+    expect(result).toBe(1);
   });
 
   it('should pass an accumulator and current value to a callback', () => {
@@ -41,7 +59,11 @@ describe('reduce', () => {
     [1, 2, 3, 4].reduce2(cb, 0);
 
     expect(cb).toHaveBeenNthCalledWith(
-      4, 6, 4, expect.anything(), expect.anything()
+      4,
+      6,
+      4,
+      expect.anything(),
+      expect.anything(),
     );
   });