htmx

name	htmx
description	Use when writing, reviewing, or debugging htmx code -- building hypermedia-driven UIs with hx-get, hx-post, hx-swap, hx-trigger, hx-target, hx-boost, hx-partial, hx-status, hx-action, morph swaps, SSE, WebSockets, htmx events, htmx extensions, or any project using htmx attributes. Covers htmx 4.x API.

htmx 4.x Complete Reference

htmx gives access to AJAX, CSS Transitions, WebSockets, and Server-Sent Events directly in HTML using attributes. No JavaScript needed for most interactive patterns.

htmx 4 uses fetch() (not XMLHttpRequest), has explicit attribute inheritance, swaps all HTTP responses by default, and includes a built-in morph algorithm.

Installation

<!-- CDN -->
<script src="https://cdn.jsdelivr.net/npm/htmx.org@next/dist/htmx.min.js"></script>

<!-- NPM -->
npm install htmx.org@next

Extensions are loaded as separate scripts (no hx-ext attribute needed):

<script src="https://cdn.jsdelivr.net/npm/htmx.org@next/dist/htmx.min.js"></script>
<script src="/path/to/ext/hx-sse.js"></script>

Restrict allowed extensions via config:

<meta name="htmx-config" content='{"extensions": "sse, ws"}'>

Core Pattern

Any element can issue HTTP requests and swap content into the DOM:

<button hx-get="/api/data" hx-target="#result" hx-swap="innerHTML" hx-trigger="click">
  Load
</button>
<div id="result"></div>

HTTP Verbs

Attribute	Method
hx-get	GET
hx-post	POST
hx-put	PUT
hx-patch	PATCH
hx-delete	DELETE

New in v4: hx-action + hx-method to separate URL from method:

<button hx-action="/api/item/1" hx-method="PATCH">Update</button>

Note: hx-get and hx-delete do NOT include enclosing form inputs. Use hx-include="closest form" if needed.

Targeting

• hx-target="#id" -- CSS selector for swap target
• hx-target="this" -- the element itself (default for non-boosted)
• hx-target="closest div" -- nearest ancestor matching selector
• hx-target="find .child" -- first descendant matching selector
• hx-target="next .sibling" / hx-target="previous .sibling"
• hx-target="body" / hx-target="document" / hx-target="window"
• hx-target="host" -- shadow DOM host element
• hx-select="#content" -- swap only matching portion of response

Swapping (hx-swap)

Styles:

Style	Effect
innerHTML	Replace children (default)
outerHTML	Replace entire element
beforebegin / before	Insert before element
afterbegin / prepend	Insert as first child
beforeend / append	Insert as last child
afterend / after	Insert after element
innerMorph	Morph children (preserves DOM state)
outerMorph	Morph entire element
textContent	Set text content (no HTML parsing)
delete	Remove target element
none	No swap

Modifiers (space-separated after value, key:value syntax):

• swap:<delay> / settle:<delay> -- timing for CSS transitions
• scroll:top / scroll:bottom -- scroll target after swap
• scrollTarget:<selector> -- element to scroll
• show:top / show:bottom -- scroll target into viewport
• showTarget:<selector> -- element to scroll into view
• transition:true|false -- enable/disable View Transitions for this swap
• ignoreTitle:true -- don't update document title
• focusScroll:true -- scroll focused element into view
• target:<selector> -- override target for this swap

<div hx-get="/data" hx-swap="innerHTML swap:300ms settle:100ms scroll:top transition:true">

Triggers (hx-trigger)

Default triggers: input/textarea/select -> change, form -> submit, input[type=submit] -> click, everything else -> click

Special triggers:

• load -- on element initialization
• revealed -- when scrolled into viewport (fires once)
• intersect -- IntersectionObserver-based (opts.root:<selector>, opts.threshold:<0.0-1.0>)
• every <interval> -- polling (every 1s, every 5s)

Modifiers (space-separated key:value syntax):

• once -- fire only once
• changed -- fire only if value changed
• delay:<time> -- debounce (delay:500ms)
• throttle:<time> -- throttle (throttle:1s)
• from:<selector> -- listen on different element (from:body, from:document, from:window, from:closest form)
• target:<selector> -- filter by event.target
• consume -- stop event propagation

Filters: hx-trigger="click[ctrlKey]", hx-trigger="keyup[key=='Enter']"

Multiple triggers: hx-trigger="load, click delay:1s, keyup changed delay:500ms"

Attribute Inheritance

htmx 4 uses explicit inheritance. Attributes do NOT inherit to children by default.

Use :inherited suffix to opt in:

<div hx-confirm:inherited="Are you sure?">
    <button hx-delete="/item/1">Delete</button> <!-- inherits hx-confirm -->
</div>

<div hx-target:inherited="#result">
    <button hx-get="/a">A</button> <!-- targets #result -->
    <button hx-get="/b">B</button> <!-- targets #result -->
</div>

Use :append to extend an inherited value:

<div hx-include:inherited="#global-fields">
    <form hx-include:inherited:append=".extra">...</form>
</div>

Revert to v2 implicit inheritance: htmx.config.implicitInheritance = true

Synchronization (hx-sync)

Control concurrent requests: hx-sync="<selector>:<strategy>"

Strategy	Behavior
queue first	Queue first request only (default)
drop	Ignore if request in flight
abort	Drop if in flight; abort this if new request arrives
replace	Abort current request, replace with this one
queue last	Queue last request only
queue all	Queue all requests

<input hx-get="/search" hx-trigger="keyup changed delay:300ms" hx-sync="this:replace">

Multi-Target Responses

<hx-partial> (new in v4)

Target multiple elements from one response. Cleaner alternative to OOB swaps:

<!-- Server response -->
<hx-partial hx-target="#messages" hx-swap="beforeend">
    <div>New message</div>
</hx-partial>

<hx-partial hx-target="#count">
    <span>5</span>
</hx-partial>

Each <hx-partial> has its own hx-target and hx-swap. The main content swaps first, then partials in document order.

Out-of-Band Swaps (hx-swap-oob)

<!-- Server response -->
<div id="main">Main content</div>
<div id="sidebar" hx-swap-oob="true">Updated sidebar</div>
<div id="notifications" hx-swap-oob="afterbegin">New notification</div>

Client-side with hx-select-oob:

<button hx-get="/data" hx-select="#main" hx-select-oob="#sidebar,#notifications:afterbegin">

Note: In v4, main content swaps first, then OOB/partials (reversed from v2).

Status Code Handling (hx-status)

htmx 4 swaps ALL responses by default (only 204 and 304 skip swap).

Control behavior per HTTP status code:

<form hx-post="/save"
      hx-status:422="swap:innerHTML target:#errors select:#validation-errors"
      hx-status:5xx="swap:none push:false">
</form>

Available config keys: swap:, target:, select:, push:, replace:, transition:

Wildcards: exact (404), single-digit (50x), range (5xx). Evaluated by specificity.

Revert to v2 behavior: htmx.config.noSwap = [204, 304, '4xx', '5xx']

Request Configuration

• hx-vals='{"key": "value"}' -- add JSON values
• hx-vals="js:{key: computeValue()}" -- dynamic values via js: prefix
• hx-headers='{"X-Custom": "value"}' -- custom headers (supports js: prefix)
• hx-include="[name='token']" -- include extra inputs
• hx-encoding="multipart/form-data" -- for file uploads
• hx-config="credentials:include, timeout:5000" -- per-element request config
• hx-validate="true" -- force HTML5 form validation

Indicators

<button hx-get="/api" hx-indicator="#spinner">Load</button>
<span id="spinner" class="htmx-indicator">Loading...</span>

CSS class htmx-request is added during requests. Default CSS fades in .htmx-indicator elements.

hx-disable="this" -- disable element during request (was hx-disabled-elt in v2).

Boosting

hx-boost="true" on a container converts <a> and <form> children to AJAX with history support.

Advanced config overrides:

<div hx-boost="swap:outerMorph target:#content push:true">

History

• hx-push-url="true" / hx-push-url="/custom-url" -- push to browser history
• hx-replace-url="true" -- replace current URL (no history entry)
• hx-preserve -- keep element unchanged across swaps (requires id)

No localStorage cache in v4. Back/forward triggers a re-fetch (or location.reload() with htmx.config.history = "reload").

Confirmation

<button hx-delete="/item/1" hx-confirm="Are you sure?">Delete</button>

<!-- Async/custom confirm with js: prefix -->
<button hx-delete="/item/1" hx-confirm="js:myCustomDialog('Delete this?')">Delete</button>

The htmx:confirm event fires with issueRequest() / dropRequest() callbacks for fully custom dialogs.

Ignore Processing

hx-ignore disables htmx processing on an element and its children (was hx-disable in v2).

Inline Event Handling (hx-on)

<button hx-get="/api" hx-on:htmx:before:request="showSpinner()">
<div hx-on:click="this.classList.toggle('active')">

Note: The hx-on:: double-colon shorthand from v2 no longer works. Use full event names.

Etag / Conditional Requests

Use the ptag extension for per-element conditional polling:

<div hx-get="/news" hx-trigger="every 3s" hx-ptag="initial-value">Latest News</div>

Server responds with HX-PTag header. Next request sends HX-PTag request header. Server returns 304 if unchanged.

JSX Compatibility

Replace : in attribute names with a custom character:

htmx.config.metaCharacter = "-";
// hx-confirm-inherited instead of hx-confirm:inherited

CSS Transitions

htmx applies CSS classes during the swap lifecycle:

Class	When
htmx-request	During request (on element or indicator)
htmx-swapping	Before swap, removed after swap
htmx-settling	After swap, removed after settle
htmx-added	On new content before settle

.fade-me-in.htmx-added { opacity: 0; }
.fade-me-in { opacity: 1; transition: opacity 300ms ease-in; }

View Transitions API (disabled by default): htmx.config.transitions = true

Events

All events follow pattern: htmx:phase:action[:sub-action]

Key events: htmx:config:request, htmx:before:request, htmx:after:request, htmx:before:swap, htmx:after:swap, htmx:before:settle, htmx:after:settle, htmx:error, htmx:finally:request

All errors consolidated in htmx:error. All events provide a ctx object.

Consult references/events-and-api.md for the complete list.

HTTP Headers

Consult references/headers-and-config.md for complete request/response headers and config options.

Key request headers: HX-Request, HX-Source (tagName#id), HX-Target (tagName#id), HX-Request-Type (full/partial), HX-Current-URL, Accept: text/html

Key response headers: HX-Redirect, HX-Refresh, HX-Retarget, HX-Reswap, HX-Reselect, HX-Trigger, HX-Push-Url, HX-Replace-Url

JavaScript API

Method	Purpose
htmx.ajax(method, url, config)	Programmatic AJAX request
htmx.on(elt, event, handler)	Add event listener (returns callback)
htmx.trigger(elt, event, detail)	Trigger event
htmx.process(elt)	Initialize htmx on dynamic content
htmx.find(selector) / htmx.findAll(selector)	DOM queries
htmx.swap(ctx)	Programmatic swap
htmx.onLoad(callback)	Run on new content (listens htmx:after:process)
htmx.takeClass(elt, class, container)	Remove class from siblings, add to element
htmx.forEvent(event, timeout, on)	Promise that resolves on event
htmx.timeout(time)	Promise that resolves after delay
htmx.parseInterval(str)	Parse time string (500ms, 2s, 1m)
htmx.registerExtension(name, ext)	Register extension

Removed from v2: htmx.addClass/removeClass/toggleClass/closest/remove/off/logAll/logNone/defineExtension -- use native DOM equivalents.

Extensions

htmx 4 ships with core extensions. Consult references/extensions.md for full documentation.

Extension	Description
hx-sse.js	Server-Sent Events (fetch-based, supports headers/body)
hx-ws.js	WebSocket bidirectional communication
hx-head.js	Merge <head> content from responses
hx-alpine-compat.js	Alpine.js compatibility
htmx-2-compat.js	htmx 2.x backward compatibility layer
hx-optimistic.js	Optimistic UI from templates
hx-preload.js	Prefetch on mouseover/mousedown
hx-upsert.js	Update existing elements by ID, insert new
hx-download.js	File download with progress events
hx-browser-indicator.js	Browser native loading indicator
hx-ptag.js	Per-element conditional polling tags
hx-targets.js	Multi-error targeting by HTTP status

Error Handling

Consult references/error-handling.md for patterns covering status codes, network errors, and timeouts.

Additional Resources

• references/extensions.md -- Complete extension docs (SSE, WebSocket, download, upsert, optimistic, all extensions)
• references/events-and-api.md -- All events with ctx properties, JS API details
• references/headers-and-config.md -- HTTP headers, htmx.config options
• references/error-handling.md -- Error handling patterns, hx-status, network errors