Asynchronous Javascript And XML (AJAX)

"Although X in Ajax stands for XML, JSON is used more than XML nowadays because of its many advantages such as being lighter and a part of JavaScript. Both JSON and XML are used for packaging information in the Ajax model."

MDN page || "Ajax: A New Approach to Web Applications": Jesse James Garrett, Feb 2005, introducing Ajax and concepts

Getting Started
Using XMLHttpRequest
fetch API
Server-sent events: Traditionally, a web page has to send a request to the server to receive new data; that is, the page requests data from the server. With server-sent events, it's possible for a server to send new data to a web page at any time, by pushing messages to the web page. These incoming messages can be treated as Events + data inside the web page. See also: Using server-sent events.
Sending/receiving binary data
FileReader API: lets web applications asynchronously read the contents of files (or raw data buffers) stored on the user's computer, using File or Blob objects to specify the file or data to read. File objects may be obtained from a FileList object returned as a result of a user selecting files using the element, from a drag and drop operation's DataTransfer object, or from the mozGetAsFile() API on an HTMLCanvasElement.

Asynchronous Javascript And XML (AJAX)

A term coined that describes an approach to using HTML or XHTML, CSS, JavaScript, DOM, XML, XSLT, and most importantly the XMLHttpRequest object to make quick, incremental updates to the UI without reloading the entire browser page.