Another take on testing using Jest, Enzyme to build out a react-redux todo-list app.

To run tests, enter $ yarn test in the terminal.

Why do we test our code?

Make sure small parts of application are working as expected
Make sure new features don't break old features
Automate previous two points - checking existing code is less tedious simulate events like clicks, typing, form submit, fetch
Reusable - running into the same bug more than once?
Communicate ideas to other developers - 'self-document' your code by writing tests
highlight bugs at their source
proof that your code works

What do we need to test?

This is the trickiest part - you can test anything small or large.
Deciding what exactly to test can be painful, but will lead to a deeper understanding of the code you wind up writing.
Test the behavior of your app! If it's event-driven, simulate the events that trigger different functions.
Test functions that accept input and return an output
(in react) test components! Test their methods, whether they render, that they match a snapshot etc.
fetches || network requests
inputs/outputs
test changes to data structures

Jest - facebook's test runner, included with 'create-react-app'
Enzyme - Airbnb's test suite, allows for shallow rendering, snapshotting components and more
Rspec - Testing Ruby code (useful if your backend is built in rails!)

To add Enzyme: $ yarn add enzyme enzyme-adapter-react-16

Create setup file called setupTests.js. add the following:

import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() })

to configure enzyme-to-json (which makes snapshots more readable): $ yarn add enzyme-to-json and add the following to your package.json:

    "jest": {
      "snapshotSerializers": [
        "enzyme-to-json/serializer"
      ]
    },

to run tests: $ yarn test (or npm test)

export a 'non-connected' or 'non-combined' version of your components/reducers for easy testing
If a test seems simple, write it anyway! It could prevent sneaky bugs
Keep calm, and read the docs!