Outlook JavaScript Add-ins

The Outlook API is also referred to the Mailbox API
Add-in Design Guidelines - docs.microsoft.com/en-us/outlook/add-ins/outlook-addin-design
Github Documentation - github.com/OfficeDev/office-js-docs-reference
Microsoft API Reference - docs.microsoft.com/en-us/javascript/api/overview?view=common-js-preview&viewFallbackFrom=office-js
Stack Overflow - stackoverflow.com/questions/tagged/office-js

with VS Code

You can use the Yeoman Generator to create an Outlook JavaScript Add-in.

with Visual Studio

You can use Visual Studio to create an Outlook JavaScript Add-in.

with NotePad

You can even use NotePad to create an Outlook JavaScript Add-in.