Testing

src/reduce.js

@@ -7,6 +7,10 @@
  * @returns {*}
  */
 function reduce(callback, startValue) {
+  if (this.length === 0 && arguments.length < 2) {
+    throw new TypeError('Reduce of empty array with no initial value');
+  }
+
   let prev = startValue;
   let startIndex = 0;
 

src/reduce.test.js

@@ -2,16 +2,65 @@
 
 const { reduce } = require('./reduce');
 
-describe('reduce', () => {
-  beforeAll(() => {
-    Array.prototype.reduce2 = reduce; // eslint-disable-line
+describe('Custom reduce method', () => {
+  let numbers;
+
+  beforeEach(() => {
+    numbers = [1, 2, 3, 4];
+  });
+
+  it('should sum all numbers with initial value', () => {
+    const result = reduce.call(numbers, (acc, num) => acc + num, 0);
+
+    expect(result).toBe(10);
+  });
+
+  it('should sum all numbers without initial value', () => {
+    const result = reduce.call(numbers, (acc, num) => acc + num);
+
+    expect(result).toBe(10);
+  });
+
+  it('should multiply all numbers', () => {
+    const result = reduce.call(numbers, (acc, num) => acc * num, 1);
+
+    expect(result).toBe(24);
   });
 
-  afterAll(() => {
-    delete Array.prototype.reduce2;
+  it('should concatenate strings', () => {
+    const words = ['Hello', ' ', 'World'];
+
+    const result = reduce.call(words, (acc, word) => acc + word, '');
+
+    expect(result).toBe('Hello World');
   });
 
-  it('should ', () => {});
+  it('should return initial value for empty array', () => {
+    const result = reduce.call([], (acc, num) => acc + num, 100);
 
-  // Add tests here
+    expect(result).toBe(100);
+  });
+
+  it('should throw TypeError for empty array without initial value', () => {
+    expect(() => {
+      reduce.call([], (acc, num) => acc + num);
+    }).toThrow(TypeError);
+  });
+
+  it('should call callback correct number of times', () => {
+    const callback = jest.fn((acc, num) => acc + num);
+
+    reduce.call(numbers, callback, 0);
+
+    expect(callback).toHaveBeenCalledTimes(4);
+  });
+
+  it('should pass correct arguments to callback', () => {
+    const callback = jest.fn((acc, num) => acc + num);
+
+    reduce.call(numbers, callback, 0);
+
+    expect(callback).toHaveBeenNthCalledWith(1, 0, 1, 0, numbers);
+    expect(callback).toHaveBeenNthCalledWith(2, 1, 2, 1, numbers);
+  });
 });