jQuery 4 vs. jQuery 3: A Technical Comparison

by jQueryScript, 09/04/2026 05:11:54 UTC

The release of jQuery 4 marks a significant shift in the library's philosophy. While version 3.x focused on maintaining broad compatibility across a fragmented browser ecosystem, version 4 focuses on cleanup and modernization.

This update introduces breaking changes that go beyond simple deprecations. The following sections detail the technical differences between the two versions.

jQuery 4 vs jQuery 3

This comparison uses jQuery 3.7.1 as the 3.x baseline and jQuery 4.0.0 as the 4.x baseline.

jQuery 4 vs. jQuery 3.7.1 at a Glance

	jQuery 3.7.1 vs. jQuery 4.0.0
Browser support	jQuery 3.7.1 keeps IE 9-10, Edge Legacy, iOS 7+, and Android 4.0+. jQuery 4.0.0 drops those legacy targets and keeps IE 11 plus current browser generations.
Source modules	AMD source modules in 3.x move to ES Modules in 4.0.0. The main browser build remains UMD.
Package exports	jQuery 4.0.0 exports UMD, ESM, Slim, and factory entry points through `package.json`.
Deprecated APIs	APIs deprecated across jQuery 3.x are removed in 4.0.0, including `jQuery.trim`, `jQuery.isArray`, `jQuery.type`, and `jQuery.now`.
AJAX and JSONP	JSON requests no longer auto-promote to JSONP, script execution must be explicit, and `FormData` receives native binary-data handling.
Focus events	jQuery 4.0.0 stops overriding the browser focus-event order and removes the old `focusin`/`focusout` shims.
CSS units	jQuery 3.x adds `px` to most numeric CSS values. jQuery 4.0.0 limits automatic `px` insertion to a smaller set of properties.
Trusted Types	jQuery 4.0.0 adds Trusted Types support to HTML manipulation methods.
Slim build	The 3.7.1 Slim build excludes Ajax and Effects. The 4.0.0 Slim build also excludes Deferred, Callbacks, and Queue modules.
File size	The full jQuery 4 build is more than 3KB smaller gzipped after deprecated APIs and older-browser compatibility code are removed.

1. Browser Support and File Size

The most structural change in jQuery 4 is the removal of legacy browser support. This cleanup allows for the removal of internal polyfills and workarounds. The full jQuery 4 build is more than 3KB smaller gzipped than 3.7.1 after removing deprecated APIs and compatibility code for older browsers. Sizzle should not be counted as a jQuery 4-only removal because jQuery 3.7.0 had already stopped depending on it as a separate project.

Browser	Support Difference
Internet Explorer	3.7.1: IE 9+. 4.0.0: IE 11 only.
Edge	3.7.1: Includes Edge Legacy. 4.0.0: Chromium Edge Current and Current - 1; Edge Legacy is removed.
Chrome, Firefox, Safari	3.7.1: Current and Current - 1. 4.0.0: Current and Current - 1.
Opera	3.7.1: Current. 4.0.0: Current.
Mobile Safari	3.7.1: iOS 7+. 4.0.0: Current, Current - 1, and Current - 2.
Android	3.7.1: Android 4.0+. 4.0.0: Current and Current - 1.

2. Core Architecture: ESM vs. AMD

jQuery 3.x: The source code uses AMD modules.
jQuery 4: The source code uses ES Modules (ESM). The main browser file remains a UMD build, while the npm package exposes UMD, ESM, Slim, and factory builds.

Package Exports and Node.js Factory

jQuery 4 adds an exports map to package.json. Browser projects that load the normal distribution file do not need a new initialization pattern, but Node.js code that creates a DOM with JSDOM has a different factory entry point.

// jQuery 3.x in Node.js without a global window
var $ = require("jquery")(window);

// jQuery 4.0.0
var $ = require("jquery/factory")(window);

3. AJAX: Security and Binary Data

The AJAX module changes several request behaviors that can affect existing integrations.

JSONP Auto-Promotion

jQuery 3.x: A dataType: "json" request with a JSONP callback placeholder can be promoted to JSONP automatically.
jQuery 4: JSONP requires an explicit dataType: "jsonp".

// jQuery 3.x could auto-promote this request to JSONP
$.ajax({
    url: "https://example.com/data?callback=?",
    dataType: "json"
});

// jQuery 4.0.0 requires an explicit JSONP request
$.ajax({
    url: "https://example.com/data?callback=?",
    dataType: "jsonp"
});

Binary Data Support

jQuery 3.x: Non-serializable Ajax data could be converted before Ajax prefilters ran, which required extra handling for values such as FormData.
jQuery 4: jQuery.ajax supports binary data such as FormData passed through the data option.

Script Requests

jQuery 3.x: Same-origin Ajax responses could execute as scripts even when dataType: "script" was not explicit.
jQuery 4: Script execution requires dataType: "script" or jQuery.getScript(). Async script requests use a <script> transport by default, with limited exceptions such as requests that set custom headers.

4. Event System: Focus Order and Shims

jQuery 4 stops normalizing several focus-event behaviors that current browsers now handle consistently.

Focus Event Order

jQuery 3.x: Uses the order focusout -> blur -> focusin -> focus.
jQuery 4: Stops overriding native focus behavior. Modern browsers follow blur -> focusout -> focus -> focusin; IE 11 keeps its native ordering.

Removal of Shims

jQuery 3.x: Includes compatibility shims for focusin and focusout.
jQuery 4: Removes those shims. Code that depends on the relative order of focus, blur, focusin, and focusout needs regression testing.

5. API Removals and Replacements

jQuery 4 removes deprecated utilities that had native JavaScript replacements or no longer serve the current browser matrix.

Removed API	Replacement or Migration Note
`jQuery.trim()`	`String.prototype.trim()`
`jQuery.isArray()`	`Array.isArray()`
`jQuery.parseJSON()`	`JSON.parse()`
`jQuery.isFunction()`	`typeof value === "function"`
`jQuery.isNumeric()`	`Number.isFinite()` or an application-specific numeric check.
`jQuery.isWindow()`	Window detection is discouraged. When required, use an explicit application check.
`jQuery.now()`	`Date.now()`
`jQuery.nodeName()`	`element.nodeName.toLowerCase()`
`jQuery.type()`	Use `typeof`, `Array.isArray()`, `instanceof`, or `Object.prototype.toString.call()` as appropriate.
`jQuery.unique()`	`jQuery.uniqueSort()`
`jQuery.camelCase()`	Internal utility removed. Use application-specific string handling when needed.
`jQuery.cssNumber`	Specify CSS units explicitly for properties that require them.
`jQuery.cssProps`	Vendor-prefix mapping is no longer required for the jQuery 4 browser matrix.
`jQuery.fx.interval`	No direct replacement is required for normal effects code.

Example: Replace a Deprecated Utility

var input = "  value  ";

// jQuery 3.7.1
var value = jQuery.trim(input);

// jQuery 4.0.0
var value = input.trim();

Prototype Array Methods

jQuery 3.x: The jQuery prototype includes push, sort, and splice as internal references to native Array methods.
jQuery 4: These methods are removed from the jQuery prototype. Code such as $elem.push(value) must use an Array method explicitly, for example [].push.call($elem, value).

6. CSS and Attributes

Automatic `px` Addition

jQuery 3.x: Adds px to most unitless numeric values passed to .css(), except properties listed in jQuery.cssNumber.
jQuery 4: Adds px only to a limited set of properties known to require it. Existing code should pass explicit units for CSS properties that require them.

`toggleClass` Signature

jQuery 3.x: Supports the deprecated single-argument toggleClass(boolean) and undocumented toggleClass(undefined) signatures.
jQuery 4: Removes those signatures. Pass the class name explicitly when changing class state.

Opacity Hook

jQuery 3.x: Includes an opacity CSS hook.
jQuery 4: Removes the hook. .css("opacity") on a detached element can now return an empty string in standards-compliant browsers.

7. Security: Trusted Types

jQuery 3.x: Does not natively accept Trusted Types across HTML manipulation methods.
jQuery 4: Adds Trusted Types support to manipulation methods such as .html() and .append(), which improves compatibility with sites that enforce require-trusted-types-for through Content Security Policy.

8. The Slim Build

The Slim build removes modules that a project does not need.

jQuery 3.7.1 Slim: Excludes Ajax and Effects.
jQuery 4.0.0 Slim: Excludes Ajax, Effects, Deferred, Callbacks, and Queue. Use the full build when existing code depends on jQuery.Deferred, jQuery.Callbacks, jQuery queues, Ajax, or Effects.

Selector Changes

jQuery 4 updates the native selector path to use the selection-context logic that previously existed in Sizzle. Context-sensitive queries can therefore return different results in edge cases, and relative selectors such as $container.find("> *") work through the native selector engine.

Selection context: A selector is evaluated against the starting context more strictly. Code that depended on descendants outside that context needs review.
Legacy custom pseudos: Old Sizzle-style custom pseudo extensions are removed. Plugins that register legacy pseudo selectors need compatibility testing with jQuery 4.

Advanced Compatibility Changes

Data keys: Data keys that match Object.prototype properties no longer override native prototype properties. Code that calls hasOwnProperty directly on a jQuery data object should use Object.hasOwn() or the in operator.
jQuery.event.special: This object no longer inherits from Object.prototype. Direct hasOwnProperty calls on it need the same adjustment.
jQuery.fn.init(): The undocumented third root parameter is removed. This mainly affects old plugins that called jQuery internals directly.
DOM manipulation: Internal fragment construction no longer depends on the previous string-concatenation behavior, and script comments or CDATA sections are no longer stripped by jQuery.
Deferred debugging: jQuery.Deferred.getStackHook was already renamed to jQuery.Deferred.getErrorHook in jQuery 3.7.0, so this is not a jQuery 3.7.1-to-4.0.0 difference.

jQuery 4 or jQuery 3.7.1?

jQuery 4.x is the current release branch. jQuery 3.x is limited to critical security patches and bug fixes, but it remains the compatibility choice for projects that cannot drop older browsers or older plugins.

Choose jQuery 4.0.0 When

The project runs on IE 11 or the current browser generations in the jQuery 4 support matrix.
Required plugins have been checked against jQuery 4.
The codebase does not depend on removed utilities, undocumented prototype methods, or legacy selector extensions.
A new jQuery-based project has no requirement for IE 9-10, Edge Legacy, older iOS releases, or Android Browser.

Keep jQuery 3.7.1 When

IE 9-10, Edge Legacy, iOS 7+, or Android 4.0+ remains part of the browser requirement.
A required third-party plugin has not been verified with jQuery 4.
The application depends on jQuery 3 behavior that has not yet passed regression testing under jQuery 4.

Migration Impact: jQuery 3.7.1 to 4.0.0

Audit removed APIs, implicit JSONP requests, script loading, focus-event order, CSS unit assumptions, custom pseudos, and use of push, sort, or splice on jQuery objects.
Use the full jQuery 4 build when Deferred, Callbacks, Queue, Ajax, or Effects modules are required.
Test third-party plugins that access jQuery internals, register custom selectors, or depend on browser compatibility outside the jQuery 4 matrix.
Use jQuery Migrate 4.x during development to find compatibility warnings in jQuery 3.x code and plugins, then retest after the migration layer is removed.