add task solution

src/reduce.test.js

@@ -1,4 +1,5 @@
 'use strict';
+/* eslint-disable max-len */
 
 const { reduce } = require('./reduce');
 
@@ -11,9 +12,87 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  it('should not mutate the original array', () => {
+    const array = [1, 2, 3, 4, 5];
+    const copy = [...array];
 
+    array.reduce2((prev, curr) => prev + curr, 0);
+
+    expect(array).toEqual(copy);
+  });
+
+  it('should return a value', () => {
+    const arr = [1, 2, 3];
+
+    const result = arr.reduce2((acc, x) => acc + x, 0);
+
+    expect(result).toBeDefined();
+  });
+
+  it('should reduce array without initial value', () => {
+    const arr = [1, 2, 3];
+    const result = arr.reduce2((acc, curr) => acc + curr);
+
+    expect(result).toBe(6);
+  });
+
+  it('should return the same type as initial value', () => {
+    const arr = [1, 2, 3];
+
+    const result = arr.reduce2((acc, x) => {
+      acc.push(x);
+
+      return acc;
+    }, []);
+
+    expect(Array.isArray(result)).toBe(true);
+  });
+
+  it.skip('should throw TypeError when reducing empty array without initial value', () => {
+    const arr = [];
+
+    expect(() => arr.reduce2((acc, curr) => acc + curr)).toThrow(TypeError);
+  });
+
+  it('should work with the single element array correctly', () => {
+    const arr = [42];
+    const result1 = arr.reduce2((acc, curr) => acc + curr);
+    const result2 = arr.reduce2((acc, curr) => acc + curr, -42);
+
+    expect(result1).toBe(42);
+    expect(result2).toBe(0);
+  });
+
+  it('should call callback once per element', () => {
+    const arr = [1, 2, 3];
+    const cb = jest.fn((acc, x) => acc + x);
+
+    arr.reduce2(cb, 0);
+
+    expect(cb).toHaveBeenCalledTimes(arr.length);
   });
 
-  // Add tests here
+  it('should pass accumulator, current value, index and array', () => {
+    const arr = [10, 20];
+    const cb = jest.fn((acc, x) => acc + x);
+
+    arr.reduce2(cb, 0);
+
+    expect(cb).toHaveBeenNthCalledWith(
+      1,
+      0,
+      10,
+      0,
+      arr
+    );
+  });
+
+  it('should not call callback for empty array', () => {
+    const cb = jest.fn();
+
+    const result = [].reduce2(cb, 100);
+
+    expect(cb).not.toHaveBeenCalled();
+    expect(result).toBe(100);
+  });
 });