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: NodeJS Nodist Install Instructions

2. Install SFDX CLI

3. Run this command in your project to install jest for lwc: sfdx force:lightning:lwc:test:setup

    a) This command also adds the __tests__ folders to the .forceignore file which 
    makes sure its not synced to the server.
   

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) This can be automatically done by utilizing the sfdx force:lightning:lwc:test:create command
   
    b) 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. You will need to add the glob pattern
       **/__mocks__/** to your .forceignore file if you do this.
   

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

    a) This can be automatically done by utilizing the sfdx force:lightning:lwc:test:create command
   

7. Use the cli command sfdx force:lightning:lwc:test:run to run tests inside your __tests__ folders

   a) Use the command sfdx force:lightning:lwc:test:run --watch to have jest 
      automatically run tests when they get updated. 
   

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Useful Links:

sfdx-lwc-jest repo here

sfdx cli lwc command list

Salesforce Documentation on Writing Jest Tests