It is not possible to call our API directly via AJAX/XMLHttpRequest. This is due to security considerations - we don't want you to send the API key along via the user's browser because it would then become visible to the user.
Our API is intended for implementation into your backend. After all, your API key must remain secret to prevent others from using it to make API calls.
You can, however, use AJAX to call your own server, which, in turn, will then call our API. This way, you can offer the user AJAX features while your API key remains secret.