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

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:

index.ts is exports a few functions for a counter as well as a test for squaring numbers but demonstrates out jest uses mocks
multiply.ts is a contrived example of a function that is exported
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

Run the tests by running npm test at the root of the bootcamp project
Look at the stack.ts for a sample implementation of a stack
Follow the instructions inside the stack.spec.ts file to complete the two tests