jQuery 4.0 Modern Coding Skill

This skill provides guidance for writing modern jQuery code compatible with jQuery 4.0.0+ (released January 2026).

When to Use This Skill

•Writing new jQuery code in projects using jQuery 4.0+
•Migrating legacy jQuery code from versions 1.x-3.x to 4.0
•Reviewing jQuery code for deprecated API usage
•Optimizing jQuery performance
•Integrating jQuery with TypeScript

Critical: Banned APIs in jQuery 4.0

The following APIs were removed in jQuery 4.0 and must never be used. Replace with native JavaScript equivalents:

Banned API	Replacement
`$.isArray()`	`Array.isArray()`
`$.parseJSON()`	`JSON.parse()`
`$.trim()`	`str.trim()`
`$.type()`	`typeof` / `instanceof`
`$.now()`	`Date.now()`
`$.isNumeric()`	`Number.isFinite()`
`$.isFunction()`	`typeof fn === 'function'`
`$.isWindow()`	`obj === window`
`$.camelCase()`	Custom function or lodash
`$.nodeName()`	`element.nodeName`

Also removed from prototype: $elems.push(), $elems.sort(), $elems.splice() — use [].method.call($elems, ...) instead.

Core Coding Patterns

Selector Best Practices

javascript

// ✅ Correct: Cache selectors, use $ prefix for jQuery objects
const $container = $('#container')
const $items = $container.find('.item')

// ❌ Wrong: Repeated selector calls, element type in class selector
$('div.container').addClass('active')
$('div.container').find('.item') // Re-queries DOM

Event Handling

javascript

// ✅ Correct: Use .on() with event delegation
$container.on('click', '.item', (e) => {
	e.preventDefault()
	console.log($(e.currentTarget).data('id'))
})

// ✅ Correct: DOMReady shorthand with arrow function
$(() => {
	initApp()
})

// ❌ Wrong: Deprecated event methods
$element.click(handler) // Use .on("click", handler)
$element.bind('click', handler) // Use .on()
$(document).ready(function () {}) // Use $(() => {})

AJAX with async/await

javascript

// ✅ Correct: async/await pattern
async function fetchData(endpoint) {
	try {
		const data = await $.ajax({
			url: endpoint,
			method: 'GET',
			dataType: 'json',
		})
		return data
	} catch (error) {
		console.error('API Error:', error)
		throw error
	}
}

// ❌ Wrong: JSONP (security risk, auto-promotion removed in 4.0)
$.ajax({ url: '/api', dataType: 'jsonp' }) // Use CORS instead

DOM Manipulation

javascript

// ✅ Correct: Batch operations
const html = items.map((item) => `<li>${item.name}</li>`).join('')
$list.append(html)

// ✅ Correct: Use prop() for boolean attributes
$checkbox.prop('checked', true)
$input.prop('disabled', false)

// ❌ Wrong: Loop-based append (causes reflow per iteration)
items.forEach((item) => $list.append(`<li>${item.name}</li>`))

Animation

Prefer CSS transitions with class manipulation over jQuery animations:

javascript

// ✅ Correct: CSS class-based animation
$element.addClass('fade-in')
// CSS: .fade-in { transition: opacity 0.3s; opacity: 1; }

// ⚠️ Use jQuery animation only for dynamic values
$element.animate({ scrollTop: position }, 300)

ES Modules Import

jQuery 4.0 supports ES modules natively:

javascript

import $ from 'jquery'

Detailed References

For comprehensive API migration tables, performance patterns, and TypeScript integration, see:

•references/api-migration.md - Complete deprecated API replacement guide
•references/patterns.md - Modern coding patterns with examples

Code Review Checklist

When reviewing jQuery code, verify:

•No banned APIs - Check for $.isArray, $.trim, $.parseJSON, etc.
•Cached selectors - Same selector should not be called multiple times
•Event delegation - Use .on() with delegation for dynamic elements
•Async/await for AJAX - Avoid callback-style $.ajax
•Native JS utilities - Prefer Array.isArray(), JSON.parse(), etc.
•CSS animations - Prefer class-based transitions over .animate()