added small exercise for jest testing

step2-04/demo/src/index.spec.ts

@@ -0,0 +1,32 @@
+import { square } from '.';
+
+describe('jest example', () => {
+  beforeEach(() => {
+    jest.resetModules();
+  });
+
+  it('should be able to give the square of two numbers', () => {
+    console.log('test');
+    expect(square(5)).toBe(25);
+  });
+
+  it('should increment counter', () => {
+    const { increment } = require('.');
+    expect(increment()).toBe(1);
+  });
+
+  it('should decrement counter', () => {
+    const { decrement } = require('.');
+    expect(decrement()).toBe(-1);
+  });
+
+  it('should retrieve count', () => {
+    const { decrement, getCount, increment } = require('.');
+    increment();
+    increment();
+    decrement();
+    increment();
+
+    expect(getCount()).toBe(2);
+  });
+});

step2-04/demo/src/index.ts

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+let counter = 0;
+
+export function getCount() {
+  return counter;
+}
+
+export function increment() {
+  return ++counter;
+}
+
+export function decrement() {
+  return --counter;
+}
+
+export function square(x: number) {
+  return x * x;
+}

step2-04/demo/src/store/index.ts

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+export type FilterTypes = 'all' | 'active' | 'completed';
+
+export interface TodoItem {
+  label: string;
+  completed: boolean;
+}
+
+export interface Store {
+  todos: {
+    [id: string]: TodoItem;
+  };
+
+  filter: FilterTypes;
+}