jQuery 4.0.0 has officially launched, marking the first major update in nearly a decade. Released on January 14, 2026, this version introduces significant changes, including the removal of legacy code and deprecated APIs. Notably, support for Internet Explorer 10 and older is gone, with plans to phase out IE 11 support in the next major release. Other outdated browsers, such as Edge Legacy and certain old versions of Firefox and iOS, are also no longer supported. Users needing compatibility with these browsers should remain on jQuery 3.x. The update enhances security by adding support for Trusted Types, which helps manage HTML input in a way that complies with Content Security Policy directives. jQuery's source code has been migrated to ES modules, improving compatibility with modern development tools. The slim version of jQuery has been optimized further, dropping Deferreds and Callbacks, resulting in a reduced file size of around 19.5k bytes gzipped. Developers can leverage native Promises instead of Deferreds in most cases. Several deprecated APIs have been removed, including jQuery.isArray and jQuery.parseJSON, as these have native equivalents in modern browsers. The removal of these functions contributes to a file size reduction of over 3k bytes gzipped. Additionally, the handling of focus events aligns with the W3C specification, which now ensures consistency across supported browsers. The jQuery team has provided an upgrade guide and a Migrate plugin to help users transition to this new version smoothly.