name: jquery-4
description: |
Migrate jQuery 3.x to 4.0.0 safely in WordPress and legacy web projects. Covers all breaking changes: removed APIs ($.isArray, $.trim, $.parseJSON, $.type), focus event order changes, slim build differences, ES modules migration, and Trusted Types support.

Use when: upgrading jQuery to 4.0, fixing "$.isArray is not a function" errors, WordPress jQuery migration, updating legacy JavaScript, or troubleshooting focus/blur event order issues.

jQuery 4.0 Migration

Status: Production Ready
Last Updated: 2026-01-25
Dependencies: None
Latest Versions: [email protected], [email protected]

Quick Start (5 Minutes)

1. Add jQuery Migrate Plugin for Safe Testing

Before upgrading, add the migrate plugin to identify compatibility issues:

<!-- Development: Shows console warnings for deprecated features -->
<script src="https://code.jquery.com/jquery-4.0.0.js"></script>
<script src="https://code.jquery.com/jquery-migrate-4.0.2.js"></script>

Why this matters:
- Logs specific warnings when deprecated/removed features execute
- Identifies code that needs updating before it breaks
- Provides backward compatibility shims during transition

2. Install via npm (if using build tools)

npm install [email protected]
# Or with migrate plugin for testing
npm install [email protected] [email protected]

3. Check for Breaking Changes

Run your application and check console for migrate plugin warnings. Each warning indicates code that needs updating.

Breaking Changes Reference

Removed jQuery Utility Functions

These functions were deprecated and are now removed. Use native JavaScript equivalents:

camelCase replacement:

// Native replacement for $.camelCase
function camelCase(str) {
  return str.replace(/-([a-z])/g, (match, letter) => letter.toUpperCase());
}

Removed Prototype Methods

Three internal array methods removed from jQuery objects:

// OLD - No longer works in jQuery 4.0
$elems.push(elem);
$elems.sort(compareFn);
$elems.splice(index, count);

// NEW - Use array methods with call/apply
[].push.call($elems, elem);
[].sort.call($elems, compareFn);
[].splice.call($elems, index, count);

// Or convert to array first
const arr = $.makeArray($elems);
arr.push(elem);

Focus/Blur Event Order Changed

jQuery 4.0 follows the W3C specification for focus event order:

// jQuery 3.x order (non-standard):
// focusout → blur → focusin → focus

// jQuery 4.0 order (W3C standard):
// blur → focusout → focus → focusin

Impact: If your code depends on specific event ordering, test thoroughly.

// Example: code that may need adjustment
$input.on('blur focusout focus focusin', function(e) {
  console.log(e.type); // Order changed in 4.0
});

Removed from Slim Build

The slim build (jquery-4.0.0.slim.min.js) no longer includes:
- Deferreds and Callbacks (use native Promises instead)
- AJAX functionality
- Animation effects

// If using slim build, replace Deferreds with Promises
// OLD - Deferred
const deferred = $.Deferred();
deferred.resolve(value);
deferred.promise();

// NEW - Native Promise
const promise = new Promise((resolve, reject) => {
  resolve(value);
});

toggleClass Changes

The toggleClass(boolean) and toggleClass(undefined) signatures are removed:

// OLD - No longer works
$elem.toggleClass(true);   // Added all classes
$elem.toggleClass(false);  // Removed all classes

// NEW - Be explicit
$elem.addClass('class1 class2');    // Add classes
$elem.removeClass('class1 class2'); // Remove classes

// Or use toggleClass with class names
$elem.toggleClass('active', true);  // Force add
$elem.toggleClass('active', false); // Force remove

AJAX Script Execution

Scripts fetched via AJAX no longer auto-execute unless dataType is specified:

// OLD - Scripts auto-executed
$.get('script.js');

// NEW - Must specify dataType for auto-execution
$.get({
  url: 'script.js',
  dataType: 'script'
});

// Or use $.getScript (still works)
$.getScript('script.js');

Removed CSS Properties

WordPress-Specific Migration

1. Check WordPress jQuery Version

# Check current jQuery version in WordPress
wp eval "echo wp_scripts()->registered['jquery-core']->ver;"

2. WordPress jQuery Migration Path

WordPress themes/plugins should:

// Dequeue old jQuery and enqueue 4.0 (testing only)
function upgrade_jquery_for_testing() {
  if (!is_admin()) {
    wp_deregister_script('jquery-core');
    wp_deregister_script('jquery');

    wp_register_script('jquery-core',
      'https://code.jquery.com/jquery-4.0.0.min.js',
      array(), '4.0.0', true);

    wp_register_script('jquery', false, array('jquery-core'), '4.0.0', true);

    // Add migrate plugin for debugging
    wp_enqueue_script('jquery-migrate',
      'https://code.jquery.com/jquery-migrate-4.0.2.min.js',
      array('jquery'), '4.0.2', true);
  }
}
add_action('wp_enqueue_scripts', 'upgrade_jquery_for_testing', 1);

3. Common WordPress Plugin Issues

Many WordPress plugins use removed jQuery methods:

// Common pattern in older plugins - BROKEN in 4.0
if ($.isArray(data)) { ... }
var json = $.parseJSON(response);
var cleaned = $.trim(userInput);

// Fix: Update to native methods
if (Array.isArray(data)) { ... }
var json = JSON.parse(response);
var cleaned = userInput.trim();

Migration Patterns

Pattern 1: Type Checking Migration

// OLD jQuery type checking
if ($.type(value) === 'array') { ... }
if ($.type(value) === 'function') { ... }
if ($.type(value) === 'object') { ... }
if ($.type(value) === 'string') { ... }
if ($.type(value) === 'number') { ... }

// NEW Native type checking
if (Array.isArray(value)) { ... }
if (typeof value === 'function') { ... }
if (value !== null && typeof value === 'object' && !Array.isArray(value)) { ... }
if (typeof value === 'string') { ... }
if (typeof value === 'number') { ... }

Pattern 2: Utility Function Polyfills

If you need quick compatibility without changing all code:

// Polyfill removed methods (temporary migration aid)
if (typeof $.isArray === 'undefined') {
  $.isArray = Array.isArray;
}
if (typeof $.parseJSON === 'undefined') {
  $.parseJSON = JSON.parse;
}
if (typeof $.trim === 'undefined') {
  $.trim = function(str) {
    return str == null ? '' : String.prototype.trim.call(str);
  };
}
if (typeof $.now === 'undefined') {
  $.now = Date.now;
}
if (typeof $.isFunction === 'undefined') {
  $.isFunction = function(fn) {
    return typeof fn === 'function';
  };
}
if (typeof $.isNumeric === 'undefined') {
  $.isNumeric = function(val) {
    return !isNaN(parseFloat(val)) && isFinite(val);
  };
}

CRITICAL: This is a temporary measure. Update your code to use native methods.

Pattern 3: ES Modules Import

jQuery 4.0 supports ES modules:

// ES Module import (new in 4.0)
import $ from 'jquery';

// Or with named export
import { $ } from 'jquery';

// In package.json, ensure module resolution
{
  "type": "module"
}

Pattern 4: Trusted Types Compliance

For CSP with Trusted Types:

// jQuery 4.0 accepts TrustedHTML in DOM manipulation
import DOMPurify from 'dompurify';

// Create trusted HTML
const clean = DOMPurify.sanitize(untrustedHTML, {RETURN_TRUSTED_TYPE: true});

// Safe to use with jQuery 4.0
$('#container').html(clean);

Critical Rules

Always Do

• Add jquery-migrate plugin BEFORE upgrading production
• Test focus/blur event handlers thoroughly
• Replace removed utility functions with native equivalents
• Specify dataType: 'script' for AJAX script loading
• Use ES module imports when possible for modern projects

Never Do

• Upgrade production without testing with migrate plugin first
• Assume WordPress plugins are jQuery 4.0 compatible
• Use slim build if you need AJAX or Deferreds
• Rely on $.type() - use native type checking
• Use toggleClass(boolean) signature

Known Issues Prevention

This skill prevents 8 documented issues:

Issue #1: $.isArray is not a function

Error: TypeError: $.isArray is not a function
Source: https://github.com/jquery/jquery/issues/5411
Why It Happens: Method removed in jQuery 4.0
Prevention: Use Array.isArray() instead

Issue #2: $.parseJSON is not a function

Error: TypeError: $.parseJSON is not a function
Source: https://jquery.com/upgrade-guide/4.0/
Why It Happens: Deprecated since 3.0, removed in 4.0
Prevention: Use JSON.parse() instead

Issue #3: $.trim is not a function

Error: TypeError: $.trim is not a function
Source: https://jquery.com/upgrade-guide/4.0/
Why It Happens: Native String.prototype.trim available everywhere
Prevention: Use str.trim() or String.prototype.trim.call(str)

Issue #4: Focus events fire in wrong order

Error: Unexpected behavior in form validation
Source: https://blog.jquery.com/2026/01/17/jquery-4-0-0/
Why It Happens: jQuery 4.0 follows W3C spec, not legacy order
Prevention: Test and update event handlers that depend on order

Issue #5: Deferreds undefined in slim build

Error: TypeError: $.Deferred is not a function
Source: https://blog.jquery.com/2026/01/17/jquery-4-0-0/
Why It Happens: Removed from slim build in 4.0
Prevention: Use full build or native Promises

Issue #6: Scripts not executing from AJAX

Error: Script loaded but not executed
Source: https://jquery.com/upgrade-guide/4.0/
Why It Happens: Auto-execution disabled without explicit dataType
Prevention: Add dataType: 'script' to AJAX options

Issue #7: toggleClass not working

Error: toggleClass(true) has no effect
Source: https://jquery.com/upgrade-guide/4.0/
Why It Happens: Boolean signature removed
Prevention: Use addClass/removeClass or toggleClass with class names

Issue #8: WordPress plugin conflicts

Error: Various "is not a function" errors
Source: Common in WordPress ecosystem
Why It Happens: Plugins using removed jQuery methods
Prevention: Audit plugins with jquery-migrate before upgrading

Browser Support

Supported in jQuery 4.0

• Chrome (last 3 versions)
• Firefox (last 2 versions + ESR)
• Safari (last 3 versions)
• Edge (Chromium-based)
• iOS Safari (last 3 versions)
• Android Chrome (last 3 versions)

Dropped Support

• IE 10 and older (IE 11 supported until jQuery 5.0)
• Edge Legacy (EdgeHTML)
• Very old mobile browsers

Slim vs Full Build Comparison

Use slim build when: Static sites, no AJAX needs, using native fetch/Promises

Use full build when: WordPress, AJAX-heavy apps, need $.animate or Deferreds

Migration Checklist

• [ ] Add [email protected] to development
• [ ] Run full site test, check console for warnings
• [ ] Replace $.isArray() with Array.isArray()
• [ ] Replace $.parseJSON() with JSON.parse()
• [ ] Replace $.trim() with str.trim()
• [ ] Replace $.now() with Date.now()
• [ ] Replace $.type() with native type checking
• [ ] Replace $.isFunction() with typeof check
• [ ] Replace $.isNumeric() with isNaN/isFinite check
• [ ] Update toggleClass(boolean) usage
• [ ] Add dataType to script AJAX calls
• [ ] Test focus/blur event order
• [ ] Audit WordPress plugins if applicable
• [ ] Remove jquery-migrate after fixing all issues
• [ ] Upgrade to [email protected] in production

Official Documentation

• jQuery 4.0.0 Release: https://blog.jquery.com/2026/01/17/jquery-4-0-0/
• Upgrade Guide: https://jquery.com/upgrade-guide/4.0/
• jQuery Migrate Plugin: https://github.com/jquery/jquery-migrate
• API Documentation: https://api.jquery.com/
• npm Package: https://www.npmjs.com/package/jquery

Package Versions (Verified 2026-01-25)

{
  "dependencies": {
    "jquery": "^4.0.0"
  },
  "devDependencies": {
    "jquery-migrate": "^4.0.2"
  }
}

Troubleshooting

Problem: WordPress admin breaks after jQuery upgrade

Solution: Only upgrade frontend jQuery. Admin uses its own version. Use conditional logic to avoid affecting wp-admin.

Problem: Third-party plugins stop working

Solution: Keep jquery-migrate loaded until plugins are updated. Check plugin changelogs for jQuery 4.0 compatibility updates.

Problem: AJAX requests work but scripts don't execute

Solution: Add dataType: 'script' to $.ajax options or use $.getScript() for script loading.

Problem: Form validation fires at wrong times

Solution: Review focus/blur/focusin/focusout handlers. jQuery 4.0 fires: blur → focusout → focus → focusin (W3C order).

Questions? Issues?

1. Check console for jquery-migrate warnings
2. Review upgrade guide: https://jquery.com/upgrade-guide/4.0/
3. Check jQuery GitHub issues: https://github.com/jquery/jquery/issues