jQuery 4.0.0 shipped last week. I didn't expect to write about jQuery in 2026, but here we are.

The project has been around since 2006. That's twenty years. Most JavaScript libraries don't survive five. The fact that jQuery made it to version 4 after a decade of "you don't need jQuery" blog posts is impressive on its own.

What jQuery 4 Actually Changed

The release drops IE support entirely. No more IE10, no more IE11. This is the most consequential change because it unlocks everything else.

Without legacy browser baggage, the team removed a lot of code that existed purely for compatibility:

• Browser detection hacks for IE quirks
• Event handling workarounds for old DOM APIs
• Animation fallbacks for browsers without CSS transitions
• jQuery.isArray() and jQuery.isFunction() (use native Array.isArray() and typeof instead)

The other big change: the slim build got slimmer. It now removes Deferreds, Callbacks, and the queue module in addition to AJAX and effects.

The Slim Build: jQuery Admitting the Browser Won

Here's what slim removes and what you use instead:

The size difference:

jQuery 4.0.0 (gzipped)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Full  ████████████████████████████████████████ 27.6 KB
Slim  ████████████████████████████░░░░░░░░░░░░ 19.5 KB
                                    ↑
                                8.1 KB saved (29%)

8KB doesn't sound like much in 2026. But those 8KB represent code that duplicates what browsers already do natively. If you're already using fetch() and CSS transitions, you're shipping dead code.

When to Use Slim vs Full

Use slim when:

• You're already using fetch() and Promises
• jQuery is just for DOM stuff and event handling
• Bundle size matters (shared widgets, WordPress plugins, marketing pages)

Stick with full when:

• You have a lot of $.ajax() code and no appetite to refactor
• You're maintaining a theme in an ecosystem where jQuery is preloaded
• You need the animation queue for complex sequenced effects

The Migration Path

jQuery 4 doesn't force a rewrite. The practical approach:

1. Stop adding new $.ajax() calls. New code uses fetch().
2. Stop adding new jQuery animations. New code uses CSS transitions.
3. Replace $.Deferred() with native Promise when you touch old code.
4. Eventually swap jquery.min.js for jquery.slim.min.js.

// Old: jQuery AJAX
$.ajax({
  url: '/api/users',
  method: 'GET',
  dataType: 'json'
}).then(data => console.log(data));

// New: Native fetch
const data = await fetch('/api/users').then(r => r.json());

/* Old: jQuery fadeOut */
/* $('#box').fadeOut(300); */

/* New: CSS transition */
.box {
  opacity: 1;
  transition: opacity 0.3s ease;
}
.box.hidden {
  opacity: 0;
}

Twenty Years Later

jQuery's survival says something about web development. We've gone through Angular, React, Vue, Svelte, and countless build tools. jQuery just kept working.

The slim build in v4 is jQuery finally saying "use the platform" for the stuff the platform does well. Keep jQuery for the DOM manipulation it's still good at. Use fetch() for network requests. Use CSS for animations.

That's a more honest posture than most libraries manage. Most would rather pretend they're still the best solution for everything.

Related posts:

• Why jquery.slim.js?url Behaves Differently in Vite - Import suffixes and what they mean
• Switching from Webpack to Vite in 2025 - Modern build tooling