XMLHttpRequest

XMLHttpRequest is a browser API that enables JavaScript to perform HTTP requests to a server without reloading the page. It supports asynchronous operation and event-driven updates, enabling dynamic content loading and interaction via AJAX.

History and standardization: XMLHttpRequest originated as an ActiveX object used in Internet Explorer and was later standardized by web committees such as WHATWG and W3C. While the Fetch API introduced later provides a more modern approach to network requests, XMLHttpRequest remains widely supported for legacy code and continues to offer features like upload and download progress events.

Key parts of the API: The open method initializes a request, for example xhr.open(method, url, async, user, password). The send method dispatches the request, and setRequestHeader configures HTTP headers. The object exposes properties such as readyState, status, statusText, responseText, responseXML, and responseType. Events include onreadystatechange, onload, and onerror, with xhr.upload providing progress events for uploads. The response can be read as text, XML, or a typed value depending on the configured responseType. Asynchronous usage is the default and recommended; synchronous requests are discouraged in modern web development.

Cross-origin and security: Like other browser APIs, cross-origin requests are governed by the same-origin policy and may require CORS headers from the server to be accessible by the client. Synchronous requests, if used, block the user interface and are deprecated in many contexts. Modern development often favors the Fetch API for new code, while XMLHttpRequest persists for compatibility and specific use cases such as upload progress tracking.

Usage contexts: XMLHttpRequest is commonly used for dynamic content loading, form submissions, and long-running requests in web applications. It remains supported across major browsers and continues to be a foundational technology in many legacy codebases.