Ajax allows you to collect data to update parts of the DOM of an HTML page without the need for a full page refresh. Ajax also lets you work asynchronously, meaning your code continues to run while the targeted part of your web page is trying to reload (compared to synchronously, which blocks your code from running until that part of your page is done reloading).
Fetch API Standard.
- Ajax - Getting started
- Using Fetch API
- AJAX on Wikipedia
- Synchronous vs. Asynchronous Communications (September 1, 2014)