doing step 4 demo notes

step2-04/README.md

@@ -2,14 +2,30 @@
 
 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.
 
-We won't go too deeply into mocks in this exercise, but feel free to play around with that yourself by reading up on this:
+https://jestjs.io/
 
-https://jestjs.io/docs/en/mock-functions
+- 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
+
+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`
 
 # Exercise
 
-1. copy the generic `Stack<T>` code you have developed in Step 2.1
+1. Run the tests by running `npm test` at the root of the bootcamp project
 
-2. 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