diff --git a/src/reduce.test.js b/src/reduce.test.js
index c3331e7..aafd7f0 100644
--- a/src/reduce.test.js
+++ b/src/reduce.test.js
@@ -11,7 +11,70 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {});
+  it('should be declared', () => {
+    expect(Array.prototype.reduce2).toBeInstanceOf(Function);
+  });
+
+  it('should sum array with initial value', () => {
+    const array = [1, 2, 3, 4, 5, 6];
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = array.reduce2(callback, 0);
+
+    expect(result).toBe(21);
+    expect(callback).toHaveBeenCalledTimes(6);
+  });
+
+  it('should sum array without initial value', () => {
+    const array = [1, 2, 3, 4, 5, 6];
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = array.reduce2(callback);
+
+    expect(result).toBe(21);
+    expect(callback).toHaveBeenCalledTimes(5);
+  });
+
+  it('should return initial value for empty array', () => {
+    const array = [];
+
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = array.reduce2(callback, 0);
+
+    expect(result).toBe(0);
+    expect(callback).not.toHaveBeenCalled();
+  });
 
-  // Add tests here
+  it('should return undefined for empty array without initial value', () => {
+    const array = [];
+
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = () => array.reduce(callback);
+
+    expect(result).toThrow();
+  });
+
+  it('should work with single element and initial value', () => {
+    const array = [1];
+
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = array.reduce2(callback, 2);
+
+    expect(result).toBe(3);
+    expect(callback).toHaveBeenCalledTimes(1);
+  });
+
+  it('should return element if single element and no initial value', () => {
+    const array = [1];
+
+    const callback = jest.fn((prev, x) => prev + x);
+
+    const result = array.reduce2(callback);
+
+    expect(result).toBe(1);
+    expect(callback).not.toHaveBeenCalled();
+  });
 });