qa array reduce method solution

src/reduce.test.js

@@ -11,9 +11,62 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  let callback;
 
+  beforeEach(() => {
+    callback = jest.fn().mockImplementation((a, b) => a + b);
   });
 
-  // Add tests here
+  it('should be declared', () => {
+    expect(reduce).toBeInstanceOf(Function);
+  });
+
+  it('should not mutate array', () => {
+    const array = [1, 2, 3, 4];
+
+    const copy = [...array];
+
+    array.reduce2(callback, 0);
+
+    expect(array).toEqual(copy);
+  });
+
+  it('should run callback array`s length times if initialValue is', () => {
+    const array = [1, 2, 3, 4];
+
+    const result = array.reduce2(callback, 0);
+
+    expect(callback).toHaveBeenCalledTimes(array.length); ;
+    expect(result).toBe(10);
+  });
+
+  it('should run callback array`s length - 1 times if no initialValue', () => {
+    const array = [1, 2, 3, 4];
+
+    const result = array.reduce2(callback);
+
+    expect(callback).toHaveBeenCalledTimes(array.length - 1); ;
+    expect(result).toBe(10);
+  });
+
+  it('should run callback with correct arguments', () => {
+    const array = [1, 2, 3, 4];
+
+    const result = array.reduce2(callback, 0);
+
+    expect(callback).toHaveBeenNthCalledWith(1, 0, 1, 0, array);
+    expect(callback).toHaveBeenNthCalledWith(2, 1, 2, 1, array);
+    expect(callback).toHaveBeenNthCalledWith(3, 3, 3, 2, array);
+    expect(callback).toHaveBeenNthCalledWith(4, 6, 4, 3, array);
+
+    expect(result).toBe(10);
+  });
+
+  it('should return initial value if array is empty', () => {
+    const array = [];
+    const initialValue = 0;
+    const result = array.reduce2(callback, initialValue);
+
+    expect(result).toBe(initialValue); ;
+  });
 });