How to Setup Jest for LWC Testing

Install/Setup guide for Jest:

1. Download nodejs here: Node JS Install or use chocolately and nodist if you know what you're doing (this approach is a bit better imo): NodeJS Nodist Install Instructions

2. Install Yarn

3. Install sfdx-lwc-jest via the CLI using the command: yarn add -D @salesforce/sfdx-lwc-jest

4. You may need to change your execution policy since jest doesn't register as a known Windows Package currently. Open powershell as an admin and enter the following command: Set-ExecutionPolicy Unrestricted

    a) When prompted, type the value A and then hit enter
   

5. Create a folder inside your lightning web component (or Aura Component) folder called __tests__

    a) If your javascript or the apex it calls does callouts to external systems, also make a folder 
       called __mocks__ to put mock files/classes in
   

6. Create tests in the __tests__ folder and name them [fileYouAreTesting].test.js

7. Use the cli command npm run test:unit to run tests inside your __tests__ folders

   a) Use the command npm run test:unit:watch to have jest automatically run tests when they get updated. 
   

More info on sfdx-lwc-jest here