Solution

src/reduce.test.js

@@ -11,9 +11,41 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  it('should sum numbers with initial value', () => {
+    const result = [1, 2, 3, 4].reduce2(
+      (acc, val) => acc + val,
+      0
+    );
 
+    expect(result).toBe(10);
   });
 
-  // Add tests here
+  it('should multiply numbers with initial value', () => {
+    const result = [2, 3, 4].reduce2(
+      (acc, val) => acc * val,
+      1
+    );
+
+    expect(result).toBe(24);
+  });
+
+  it('should reduce correctly without an initial value', () => {
+    const result = [1, 2, 3].reduce2(
+      (acc, val) => acc + val
+    );
+
+    expect(result).toBe(6);
+  });
+
+  it('should handle single-element array without initial value', () => {
+    const result = [5].reduce2((acc, val) => acc + val);
+
+    expect(result).toBe(5);
+  });
+
+  it('should handle empty array with initial value', () => {
+    const result = [].reduce2((acc, val) => acc + val, 100);
+
+    expect(result).toBe(100);
+  });
 });