diff --git a/src/reduce.js b/src/reduce.js
index 38be21c..fdd0761 100644
--- a/src/reduce.js
+++ b/src/reduce.js
@@ -7,6 +7,14 @@
  * @returns {*}
  */
 function reduce(callback, startValue) {
+  if (typeof callback !== 'function') {
+    throw new TypeError(callback + ' is not a function');
+  }
+
+  if (this.length === 0 && arguments.length < 2) {
+    throw new TypeError('Reduce of empty array with no initial value');
+  }
+
   let prev = startValue;
   let startIndex = 0;
 
diff --git a/src/reduce.test.js b/src/reduce.test.js
index 47a892f..16012bc 100644
--- a/src/reduce.test.js
+++ b/src/reduce.test.js
@@ -1,8 +1,11 @@
 'use strict';
 
-const { reduce } = require('./reduce');
+describe('Reduce function', () => {
+  const { reduce } = require('./reduce');
+  let mockReducer;
+  const testArr = [10, 20, 30];
+  const initialValue = 5;
 
-describe('reduce', () => {
   beforeAll(() => {
     Array.prototype.reduce2 = reduce; // eslint-disable-line
   });
@@ -11,9 +14,123 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  beforeEach(() => {
+    mockReducer = jest.fn((acc, value) => acc + value);
+  });
+
+  it('should be declared', () => {
+    expect(reduce).toBeInstanceOf(Function);
+  });
+
+  describe('if initialValue is provided', () => {
+    it('should execute correct sum', () => {
+      const actual = testArr.reduce2(mockReducer, initialValue);
+      const expected = 5 + 10 + 20 + 30;
+
+      expect(mockReducer).toHaveBeenCalledTimes(3);
+      expect(mockReducer).toHaveBeenCalledWith(5, 10, 0, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(15, 20, 1, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(35, 30, 2, [10, 20, 30]);
+      expect(actual).toBe(expected);
+    });
+
+    it('should handle single-element array', () => {
+      const actual = [10].reduce2(mockReducer, initialValue);
+      const expected = 5 + 10;
+
+      expect(mockReducer).toHaveBeenCalledTimes(1);
+      expect(mockReducer).toHaveBeenCalledWith(5, 10, 0, [10]);
+      expect(actual).toBe(expected);
+    });
+
+    it('should execute correct multiply', () => {
+      mockReducer = jest.fn((acc, value) => acc * value);
+
+      const actual = testArr.reduce2(mockReducer, initialValue);
+      const expected = 5 * 10 * 20 * 30;
+
+      expect(mockReducer).toHaveBeenCalledTimes(3);
+      expect(mockReducer).toHaveBeenCalledWith(5, 10, 0, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(50, 20, 1, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(1000, 30, 2, [10, 20, 30]);
+      expect(actual).toBe(expected);
+    });
+
+    it('should execute correct concatenate', () => {
+      const actual = ['Mate ', 'Academy ', '!'].reduce2(mockReducer, '');
+      const expected = 'Mate Academy !';
+
+      expect(mockReducer).toHaveBeenCalledTimes(3);
+
+      expect(mockReducer).toHaveBeenCalledWith('',
+        'Mate ', 0, ['Mate ', 'Academy ', '!']);
+
+      expect(mockReducer).toHaveBeenCalledWith('Mate ',
+        'Academy ', 1, ['Mate ', 'Academy ', '!']);
+
+      expect(mockReducer).toHaveBeenCalledWith('Mate Academy ',
+        '!', 2, ['Mate ', 'Academy ', '!']);
 
+      expect(actual).toBe(expected);
+    });
+
+    it('should handle empty array', () => {
+      const actual = [].reduce2(mockReducer, initialValue);
+      const expected = initialValue;
+
+      expect(mockReducer).toHaveBeenCalledTimes(0);
+      expect(actual).toBe(expected);
+    });
   });
 
-  // Add tests here
+  describe('if initialValue is not provided', () => {
+    it('should execute correct sum', () => {
+      const actual = testArr.reduce2(mockReducer);
+      const expected = 10 + 20 + 30;
+
+      expect(mockReducer).toHaveBeenCalledTimes(2);
+      expect(mockReducer).toHaveBeenCalledWith(10, 20, 1, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(30, 30, 2, [10, 20, 30]);
+      expect(actual).toBe(expected);
+    });
+
+    it('should handle single-element array', () => {
+      const actual = [10].reduce2(mockReducer);
+      const expected = 10;
+
+      expect(mockReducer).toHaveBeenCalledTimes(0);
+      expect(actual).toBe(expected);
+    });
+
+    it('should execute correct multiply', () => {
+      mockReducer = jest.fn((acc, value) => acc * value);
+
+      const actual = testArr.reduce2(mockReducer);
+      const expected = 10 * 20 * 30;
+
+      expect(mockReducer).toHaveBeenCalledTimes(2);
+      expect(mockReducer).toHaveBeenCalledWith(10, 20, 1, [10, 20, 30]);
+      expect(mockReducer).toHaveBeenCalledWith(200, 30, 2, [10, 20, 30]);
+      expect(actual).toBe(expected);
+    });
+
+    it('should execute correct concatenate', () => {
+      const actual = ['Mate ', 'Academy ', '!'].reduce2(mockReducer);
+      const expected = 'Mate Academy !';
+
+      expect(mockReducer).toHaveBeenCalledTimes(2);
+
+      expect(mockReducer).toHaveBeenCalledWith('Mate ',
+        'Academy ', 1, ['Mate ', 'Academy ', '!']);
+
+      expect(mockReducer).toHaveBeenCalledWith('Mate Academy ',
+        '!', 2, ['Mate ', 'Academy ', '!']);
+
+      expect(actual).toBe(expected);
+    });
+
+    it('should handle empty array and throw TypeError', () => {
+      expect(() => [].reduce2(mockReducer)).toThrow(TypeError);
+    });
+  });
 });