XMLHttpRequest
XMLHttpRequest (XHR) is a JavaScript API that enables web applications to make HTTP requests to servers asynchronously without reloading the page, allowing for dynamic content updates. It is a foundational technology for AJAX (Asynchronous JavaScript and XML) and supports various data formats like XML, JSON, HTML, and plain text. Although largely superseded by the Fetch API in modern development, it remains widely used in legacy systems and browsers.
Developers should learn XMLHttpRequest for maintaining or updating older web applications that rely on AJAX, as it provides backward compatibility with legacy codebases. It is also useful for understanding the evolution of web APIs and for scenarios where fine-grained control over HTTP requests (e.g., handling progress events or aborting requests) is needed, though the Fetch API is generally preferred for new projects due to its promise-based design and cleaner syntax.