test: add tests for reduce method

src/reduce.js

@@ -7,12 +7,17 @@
  * @returns {*}
  */
 function reduce(callback, startValue) {
-  let prev = startValue;
+  let prev;
   let startIndex = 0;
 
   if (arguments.length < 2) {
+    if (this.length === 0) {
+      throw new TypeError('Reduce of empty array with no initial value');
+    }
     startIndex = 1;
     prev = this[0];
+  } else {
+    prev = startValue;
   }
 
   for (let i = startIndex; i < this.length; i++) {

src/reduce.test.js

@@ -1,6 +1,8 @@
 'use strict';
 
 const { reduce } = require('./reduce');
+const arr = [0, 1, 2, 3, 4];
+const sumCb = (acc, current, i, array) => acc + current;
 
 describe('reduce', () => {
   beforeAll(() => {
@@ -11,9 +13,53 @@ describe('reduce', () => {
     delete Array.prototype.reduce2;
   });
 
-  it('should ', () => {
+  it('should return a number', () => {
+    expect(typeof arr.reduce2(sumCb, 0)).toBe('number');
+  });
+
+  it('should return an accumulated value', () => {
+    expect(arr.reduce2(sumCb, 0)).toBe(10);
+  });
+
+  it('should return an initial value if array is empty', () => {
+    expect([].reduce2(sumCb, 0)).toBe(0);
+  });
+
+  it('should call cb for every item', () => {
+    const cb = jest.fn();
+
+    arr.reduce2(cb, 0);
+    expect(cb).toHaveBeenCalledTimes(5);
+  });
+
+  it('should call cb with all params', () => {
+    const cb = jest.fn(sumCb);
 
+    arr.reduce2(cb, 0);
+
+    expect(cb).nthCalledWith(1, 0, 0, 0, arr);
+    expect(cb).nthCalledWith(2, 0, 1, 1, arr);
+    expect(cb).nthCalledWith(3, 1, 2, 2, arr);
+    expect(cb).nthCalledWith(4, 3, 3, 3, arr);
+    expect(cb).nthCalledWith(5, 6, 4, 4, arr);
   });
 
-  // Add tests here
+  it('should use first element as accumulator if it is not provided', () => {
+    const cb = jest.fn();
+
+    arr.reduce2(cb);
+    expect(cb).nthCalledWith(1, arr[0], arr[1], 1, arr);
+  });
+
+  it('throws TypeError if array and initial value are empty', () => {
+    expect(() => [].reduce2(sumCb)).toThrow(TypeError);
+  });
+
+  it("doesn't call cb for [1] without init", () => {
+    const cb = jest.fn();
+
+    [1].reduce2(cb);
+
+    expect(cb).not.toHaveBeenCalled();
+  });
 });