diff --git a/src/reduce.js b/src/reduce.js
index 38be21c..2afd99d 100644
--- a/src/reduce.js
+++ b/src/reduce.js
@@ -2,24 +2,46 @@
 
 /**
  * @param {function} callback
- * @param {*} startValue
- *
+ * @param {*} initialValue
  * @returns {*}
  */
-function reduce(callback, startValue) {
-  let prev = startValue;
-  let startIndex = 0;
+function reduce(callback, initialValue) {
+  if (this == null) {
+    throw new TypeError('Array.prototype.reduce called on null or undefined');
+  }
+
+  if (typeof callback !== 'function') {
+    throw new TypeError(callback + ' is not a function');
+  }
+
+  const arr = Object(this);
+  const len = arr.length >>> 0;
+
+  let k = 0;
+  let accumulator;
+
+  if (arguments.length > 1) {
+    accumulator = initialValue;
+  } else {
+    // шукаємо перший існуючий елемент
+    while (k < len && !(k in arr)) {
+      k++;
+    }
+
+    if (k >= len) {
+      throw new TypeError('Reduce of empty array with no initial value');
+    }
 
-  if (arguments.length < 2) {
-    startIndex = 1;
-    prev = this[0];
+    accumulator = arr[k++];
   }
 
-  for (let i = startIndex; i < this.length; i++) {
-    prev = callback(prev, this[i], i, this);
+  for (; k < len; k++) {
+    if (k in arr) {
+      accumulator = callback(accumulator, arr[k], k, arr);
+    }
   }
 
-  return prev;
+  return accumulator;
 }
 
 module.exports = { reduce };
diff --git a/src/reduce.test.js b/src/reduce.test.js
index 47a892f..b296d71 100644
--- a/src/reduce.test.js
+++ b/src/reduce.test.js
@@ -1,19 +1,144 @@
 'use strict';
 
+// eslint-disable-next-line no-extend-native
 const { reduce } = require('./reduce');
 
 describe('reduce', () => {
   beforeAll(() => {
-    Array.prototype.reduce2 = reduce; // eslint-disable-line
+    // eslint-disable-next-line no-extend-native
+    Array.prototype.reduce2 = reduce;
   });
 
   afterAll(() => {
     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 provided', () => {
+    const array = [1, 2, 3, 4];
+    array.reduce2(callback, 0);
+    expect(callback).toHaveBeenCalledTimes(array.length);
+  });
+
+  it('should run callback array`s length - 1 times if no initialValue', () => {
+    const array = [1, 2, 3, 4];
+    array.reduce2(callback);
+    expect(callback).toHaveBeenCalledTimes(array.length - 1);
+  });
+
+  it('should not run callback if array is empty and initialValue is provided', () => {
+    const array = [];
+    array.reduce2(callback, 0);
+    expect(callback).toHaveBeenCalledTimes(0);
+  });
+
+  it('should run callback with correct arguments when initialValue is provided', () => {
+    const array = [1, 2, 3, 4];
+    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);
+  });
+
+  it('should return initial value if array is empty', () => {
+    const array = [];
+    const initialValue = 0;
+    const result = array.reduce2(callback, initialValue);
+
+    expect(result).toBe(initialValue);
+  });
+
+  it('should throw if array is empty and no initial value', () => {
+    const array = [];
+    expect(() => array.reduce2(callback)).toThrow(TypeError);
+  });
+
+  it('should skip empty slots in sparse arrays', () => {
+    // eslint-disable-next-line no-sparse-arrays
+    const array = [1, , 3];
+    const result = array.reduce2((acc, cur) => acc + cur, 0);
+    expect(result).toBe(4);
+  });
+
+  it('should handle leading holes without initialValue', () => {
+    // eslint-disable-next-line no-sparse-arrays
+    const array = [, , 3, 4];
+    const result = array.reduce2((acc, cur) => acc + cur);
+    expect(result).toBe(7); // accumulator = 3, then adds 4
+  });
+
+  it('should throw on all-holes array without initialValue', () => {
+    const array = new Array(3); // [ , , , ]
+    expect(() => array.reduce2(callback)).toThrow(TypeError);
+  });
+
+  it('should throw if callback is not a function', () => {
+    const array = [1, 2];
+    expect(() => array.reduce2(null)).toThrow(TypeError);
+  });
+
+  it('should have undefined as callback this in strict mode', () => {
+    const array = [1];
+    let recordedThis;
+    array.reduce2(function (acc, cur) {
+      recordedThis = this;
+      return acc + cur;
+    }, 0);
+    expect(recordedThis).toBeUndefined();
+  });
+
+  it('should support array-like objects via call', () => {
+    const obj = { 0: 1, 1: 2, length: 2 };
+    const result = reduce.call(obj, (acc, cur) => acc + cur, 0);
+    expect(result).toBe(3);
+  });
+
+  it('should throw if called on null/undefined', () => {
+    expect(() => reduce.call(null, callback, 0)).toThrow(TypeError);
+    expect(() => reduce.call(undefined, callback, 0)).toThrow(TypeError);
+  });
+
+  it('should ignore non-numeric properties', () => {
+    const array = [1, 2, 3];
+    // @ts-ignore
+    array.foo = 10;
+    // element beyond initial length
+    array[100] = 100;
+
+    const result = array.reduce2((acc, cur) => acc + cur, 0);
+    expect(result).toBe(6); // only 1+2+3
+  });
+
+  it('should work with string concatenation', () => {
+    const array = ['a', 'b', 'c'];
+    const result = array.reduce2((acc, cur) => acc + cur, '');
+    expect(result).toBe('abc');
+  });
+
+  it('should work with objects as accumulator', () => {
+    const array = ['x', 'y'];
+    const result = array.reduce2((acc, cur, i) => {
+      acc[i] = cur;
+      return acc;
+    }, {});
+    expect(result).toEqual({ 0: 'x', 1: 'y' });
+  });
 });