XMLHttpRequest (also AJAX, short for "asynchronous JavaScript and XML") is a set of Web development techniques using many Web technologies on the client-side to create asynchronous Web applications.

XMLHttpRequest applications can send data to and retrieve from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, XMLHttpRequest allows for Single-Page Applications. In practice, modern implementations commonly most Single-Page Applications substitute JSON for XML due to the advantages of being native to JavaScript.

Fetch API provides an interface for fetching resources and provides a better alternative that can be easily used by other technologies such as Service Workers.

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