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docs/step2-04/README.md

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+# Step 2.4
+
+[Lessons](../) | [Exercise](./exercise/) | [Demo](./demo/)
+
+Testing Typescript code with jest. jest is a test framework made by Facebook and is very popular in the React and the wider JS ecosystem. We will work on implementing simple unit tests here in this exercise.
+
+https://jestjs.io/
+
+- Multi-threaded and isolated test runner
+- Provides a "fake" browser environment if needed (window, document, DOM, etc).
+- Snapshots: show API or large object changes along side code changes in pull requests
+- Code coverage is integrated (--coverage)
+- Very clear error messages of where the test failures occur
+
+# Demo
+
+## jest basics
+
+In this repo, we can start an inner loop development of tests with the command: `npm test`
+
+Take a look at code inside `demo/src`:
+
+1. `index.ts` is exports a few functions for a counter as well as a test for squaring numbers but demonstrates out jest uses mocks
+
+2. `multiply.ts` is a contrived example of a function that is exported
+
+3. `index.spec.ts` is the test file: note how tests are re-run on save to test file changes as well as source code changes under `src`
+
+## testing React applications
+
+You can also test React Components with `jest` with the help of a partner library called `enzyme`. Take a look at the test below:
+
+```ts
+import { mount } from 'enzyme';
+
+describe('Foo Component Tests', () => {
+  it('allows us to set props', () => {
+    const wrapper = mount(<Foo bar="baz" />);
+    expect(wrapper.props().bar).toBe('baz');
+    wrapper.setProps({ bar: 'foo' });
+    expect(wrapper.props().bar).toBe('foo');
+  });
+});
+```
+
+`mount` does a full mount of the component. You can use the `enzyme` wrapper to simulate clicks, etc:
+
+```ts
+wrapper.find('button').simulate('click');
+```
+
+# Exercise
+
+## Basic Testing
+
+1. Run the tests by running `npm test` at the root of the bootcamp project
+
+2. Look at the `stack.ts` for a sample implementation of a stack
+
+3. Follow the instructions inside the `stack.spec.ts` file to complete the two tests
+
+## Enzyme Testing
+
+1. Open up `exercise/src/TestMe.spec.tsx`
+
+2. Fill in the blank for the missing test using `enzyme` concepts introduced from the demo
+
+3. Run tests with `npm test`