reduce? i love it

src/reduce.test.js

@@ -2,6 +2,10 @@
 
 const { reduce } = require('./reduce');
 
+const nums = [1, 2, 3, 4];
+const strings = ['1', '2', '3'];
+let callback;
+
 describe('reduce', () => {
   beforeAll(() => {
     Array.prototype.reduce2 = reduce; // eslint-disable-line
@@ -11,9 +15,62 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  beforeEach(() => {
+    callback = jest.fn((acc, curr) => acc + curr);
+  });
+
+  it('should have instance of function', () => {
+    expect(reduce).toBeInstanceOf(Function);
+  });
+
+  it('should use callback for all items in array', () => {
+    const init = 10;
+
+    nums.reduce2(callback, init);
+
+    expect(callback).toHaveBeenCalledTimes(nums.length);
+  });
+
+  it('should work without initial value', () => {
+    const result = nums.reduce2(callback);
+
+    expect(callback).toHaveBeenCalled();
+    expect(callback).toHaveBeenCalledTimes(3);
+    expect(result).toBe(10);
+  });
+
+  it('should work without initial array', () => {
+    const init = 10;
+    const result = [].reduce2(callback, init);
 
+    expect(callback).not.toHaveBeenCalled();
+    expect(callback).toHaveBeenCalledTimes(0);
+    expect(result).toBe(10);
   });
 
-  // Add tests here
+  it('should return undefined without array and initial value', () => {
+    const result = [].reduce2(callback);
+
+    expect(result).toBe(undefined);
+    expect(callback).not.toHaveBeenCalled();
+  });
+
+  it('should work with strings', () => {
+    const check = '123';
+    const result = strings.reduce2(callback);
+
+    expect(result).toBe(check);
+  });
+
+  it('should skip initial value if it is specified', () => {
+    const result = [1, 1, 2].reduce2(callback, null);
+
+    expect(result).toBe(4);
+  });
+
+  it('should invoke callback for indexes without value', () => {
+    [1, '', '', 1, 2].reduce2(callback);
+
+    expect(callback).toHaveBeenCalledTimes(4);
+  });
 });