XMLHttpRequest (XHR) is an API that can be used by JavaScript and other web browser scripting languages to transfer XML and other text data between a web server and a browser. Though it can do synchronous fetches, it is virtually always asynchronous, due to the greater UI responsiveness. The data returned from XMLHttpRequest calls will often be provided by back-end databases. Besides XML, XMLHttpRequest can be used to fetch data in other formats such as HTML, JSON or plain text.