add task solution for qa_array-method-reduce

src/reduce.js

@@ -7,12 +7,18 @@
  * @returns {*}
  */
 function reduce(callback, startValue) {
-  let prev = startValue;
+  let prev;
   let startIndex = 0;
 
   if (arguments.length < 2) {
-    startIndex = 1;
+    if (this.length === 0) {
+      throw new TypeError('Reduce of empty array with no initial value');
+    }
+
     prev = this[0];
+    startIndex = 1;
+  } else {
+    prev = startValue;
   }
 
   for (let i = startIndex; i < this.length; i++) {

src/reduce.test.js

@@ -1,3 +1,4 @@
+/* eslint-disable max-len */
 'use strict';
 
 const { reduce } = require('./reduce');
@@ -11,9 +12,53 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  it('should sum up numbers with initial value', () => {
+    const result = [1, 2, 3, 4].reduce2((acc, val) => acc + val, 10);
 
+    expect(result).toBe(20);
   });
 
-  // Add tests here
+  it('should sum up numbers without initial value', () => {
+    const result = [1, 2, 3, 4].reduce2((acc, val) => acc + val);
+
+    expect(result).toBe(10);
+  });
+
+  it('should concatenate strings', () => {
+    const result = ['a', 'b', 'c'].reduce2((acc, val) => acc + val, '');
+
+    expect(result).toBe('abc');
+  });
+
+  it('should multiply numbers without initial value', () => {
+    const result = [2, 3, 4].reduce2((acc, val) => acc * val);
+
+    expect(result).toBe(24);
+  });
+
+  it('should use array index and array as arguments', () => {
+    const spy = jest.fn((acc, val, i, array) => acc + val);
+    const arr = [1, 2, 3];
+
+    arr.reduce2(spy, 0);
+    expect(spy).toHaveBeenCalledWith(0, 1, 0, arr);
+    expect(spy).toHaveBeenCalledWith(1, 2, 1, arr);
+    expect(spy).toHaveBeenCalledWith(3, 3, 2, arr);
+  });
+
+  it('should work on array with single element and no initial value', () => {
+    const result = [42].reduce2((acc, val) => acc + val);
+
+    expect(result).toBe(42);
+  });
+
+  it('should throw error on empty array with no initial value', () => {
+    expect(() => [].reduce2((acc, val) => acc + val)).toThrow();
+  });
+
+  it('should return initial value on empty array if initial value provided', () => {
+    const result = [].reduce2((acc, val) => acc + val, 100);
+
+    expect(result).toBe(100);
+  });
 });