Add solution

src/reduce.test.js

@@ -11,9 +11,70 @@ 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 and return initial value
+      if array is empty`, () => {
+    const cb = jest.fn();
+    const result = [].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', () => {
+    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);
+  });
 });