Upgrading to jQuery 4.0 represents a major milestone for developers maintaining legacy code or modernizing existing JavaScript applications. After nearly a decade since the last major release, jQuery 4.0 brings significant changes that impact compatibility, browser support, APIs, and development workflows. Understanding these changes and planning your migration carefully will help ensure a smooth transition with minimal disruption. In this article, we walk through the most important things to consider when upgrading to jQuery 4.0.
What’s New in jQuery 4.0
Cleaned-up Core and Modern Features
jQuery 4.0 trims legacy code and adopts modern standards. The library’s source has moved from AMD to ES modules, improving compatibility with modern bundlers and workflows like Webpack, Rollup, and Vite. The published build still includes a UMD module, but developers importing directly as modules will find improved integration.
Improved Security and Standards Support
Security improvements are a central theme in jQuery 4.0. This includes Trusted Types support and behavior changes to AJAX requests that avoid automatic JSONP promotion, reducing potential cross-site scripting risk. It also aligns default event behavior with W3C specifications, such as focus and blur event order.
Breaking Changes and Removed APIs
Dropped Browser Support
jQuery 4.0 officially drops support for older browsers, including Internet Explorer 10 and below, Edge Legacy, and older mobile browsers. If your project still requires these platforms, staying on jQuery 3.x may be necessary.
Removed Deprecated Functions
Several long-deprecated jQuery utilities have been removed. Examples include:
- jQuery.isArray, jQuery.parseJSON, and jQuery.trim — use native Array.isArray, JSON.parse, and String.prototype.trim() instead.
- Methods such as jQuery.isWindow, jQuery.isNumeric, and internal utilities like jQuery.camelCase.
Replacing deprecated calls with modern JavaScript alternatives is essential before upgrading.
Behavior Changes in Core Methods
Some jQuery methods behave differently in version 4.0:
- toggleClass() no longer accepts boolean or undefined shortcuts; class toggling must now be explicit.
- CSS handling no longer automatically appends “px” to unitless values in most cases.
- Script loading via AJAX requires explicit dataType: "script".
These changes aim to make behavior more predictable and standards-aligned.
Planning Your Upgrade
Use jQuery Migrate to Detect Issues
The jQuery Migrate plugin is strongly recommended during your upgrade process. It logs warnings for deprecated or removed features used in your codebase, helping you identify and fix compatibility issues incrementally before removing the plugin entirely.
Test Across Environments
Because of changes to event handling and AJAX behavior, comprehensive testing is critical. Verify both functionality and user interactions across supported browsers to ensure your application behaves as expected after the upgrade.
Review Third-Party Dependencies
Plugins and other libraries that depend on older jQuery features may break under version 4.0. Update or replace third-party code that uses removed APIs or behaviors not present in the new release.
Final Thoughts
Upgrading to jQuery 4.0 is a valuable step toward a more modern and secure codebase, but it requires careful consideration of breaking changes, removed features, and browser support adjustments. By reviewing deprecated APIs, leveraging the jQuery Migrate tool, and thoroughly testing your application, you can minimize disruption and benefit from the improvements in this major release.