API extensions allow you to define HTTP functions in your app's backend that you can call from your frontend code.
You can only add API extensions in the Wix CLI, which handles the setup and hosting of the backend code for you, simplifying the development process. You can invoke HTTP functions added in API extensions from any frontend component in your CLI project.
Tip:
In many cases, using web methods to call backend code from your app's frontend may be a more suitable option than an API extension. Web methods require less boilerplate code and have simpler error handling. Use an API extension when you require HTTP-specific features, such as header codes.
Learn more about the advantages of using web methods over API extensions.