<!-- 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>

npm install jquery@4.0.0
# Or with migrate plugin for testing
npm install jquery@4.0.0 jquery-migrate@4.0.2

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

// 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);

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

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

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

// 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);
});

// 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

// 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');

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

// 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);

// 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();

// 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') { ... }

// 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);
  };
}

// 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"
}

// 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);

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