Ajax

Ajax refers to a group of web development techniques that enable asynchronous communication between a web browser and a server. In an Ajax workflow, a page can request small chunks of data and update specific parts of the page without requiring a full page reload, leading to more responsive and interactive applications.

The term was coined in 2005 by Jesse James Garrett to describe this approach as part of the Web 2.0 movement. Originally it emphasized asynchronous JavaScript and XML, but in practice many Ajax interactions use JSON or other data formats rather than XML.

How it works: the browser issues an HTTP request to a server endpoint using JavaScript, typically via the XMLHttpRequest object or the modern Fetch API. The server returns data, often in JSON, which is then parsed and injected into the page's DOM.

Key components include client-side JavaScript, a server-side endpoint capable of returning lightweight data, and a data format such as JSON. Cross-origin requests are governed by the same-origin policy and CORS, which developers must configure to allow legitimate Ajax calls.

Benefits include improved responsiveness and reduced bandwidth because only parts of the page are updated. Drawbacks include added complexity, potential accessibility concerns, and search engine indexing challenges for dynamic content; frameworks and progressive enhancement strategies are commonly used to mitigate these issues.

In modern web development, Ajax is frequently subsumed under asynchronous data loading patterns in single-page applications. The Fetch API and Web APIs have largely supplanted older XMLHttpRequest usage, while real-time interactions often employ WebSockets or Server-Sent Events as alternatives.