Htmx in a Nutshell

Htmx is a library that allows you to access modern browser features directly from HTML, rather than using javascript.

To understand htmx, first lets take a look at an anchor tag:

  <a href="/blog">Blog</a>

This anchor tag tells a browser:

"When a user clicks on this link, issue an HTTP GET request to '/blog' and load the response content into the browser window".

With that in mind, consider the following bit of HTML:

  <div hx-post="/clicked"
    Click Me!

This tells htmx:

"When a user clicks on this div, issue an HTTP POST request to '/clicked' and use the content from the response to replace the element with the id parent-div in the DOM"

Htmx extends and generalizes the core idea of HTML as a hypertext, opening up many more possibilities directly within the language:

  • Now any element, not just anchors and forms, can issue an HTTP request
  • Now any event, not just clicks or form submissions, can trigger requests
  • Now any HTTP verb, not just GET and POST, can be used
  • Now any element, not just the entire window, can be the target for update by the request

Note that when you are using htmx, on the server side you typically respond with HTML, not JSON. This keeps you firmly within the original web programming model, using Hypertext As The Engine Of Application State without even needing to really understand that concept.

It's worth mentioning that, if you prefer, you can use the data- prefix when using htmx:

  <a data-hx-post="/click">Click Me!</a>


Htmx is a dependency-free javascript library.

It can be used via NPM as "" or downloaded or included from unpkg or your other favorite NPM-based CDN:

    <script src=""></script>


The core of htmx is a set of attributes that allow you to issue AJAX requests directly from HTML:

Attribute Description
hx-get Issues a GET request to the given URL
hx-post Issues a POST request to the given URL
hx-put Issues a PUT request to the given URL
hx-patch Issues a PATCH request to the given URL
hx-delete Issues a DELETE request to the given URL

Each of these attributes takes a URL to issue an AJAX request to. The element will issue a request of the specified type to the given URL when the element is triggered:

  <div hx-put="/messages">
    Put To Messages

This tells the browser:

When a user clicks on this div, issue a PUT request to the URL /messages and load the response into the div

Triggering Requests

By default, AJAX requests are triggered by the "natural" event of an element:

  • input, textarea & select are triggered on the change event
  • form is triggered on the submit event
  • everything else is triggered by the click event

If you want different behavior you can use the hx-trigger attribute to specify which event will cause the request.

Here is a div that posts to /mouse_entered when a mouse enters it:

   <div hx-post="/mouse_entered" hx-trigger="mouseenter">
      [Here Mouse, Mouse!]

If you want a request to only happen once, you can use the once modifier for the trigger:

   <div hx-post="/mouse_entered" hx-trigger="mouseenter once">
     [Here Mouse, Mouse!]

There are few other modifiers you can use for trigger:

  • changed - only issue a request if the value of the element has changed
  • delay:<time interval> - wait the given amount of time (e.g. 1s) before issuing the request. If the event triggers again, the countdown is reset.
  • throttle:<time interval> - wait the given amount of time (e.g. 1s) before issuing the request. Unlike delay if a new event occurs before the time limit is hit the event will be discarded, so the request will trigger at the end of the time period.

You can use these attributes to implement many common UX patterns, such as Active Search:

   <input type="text" name="q" 
          hx-trigger="keyup changed delay:500ms" 
    <div id="search-results"></div>

This input will issue a request 500 milliseconds after a key up event if the input has been changed and inserts the results into the div with the id search-results.

Multiple triggers can be specified in the hx-trigger attribute, separated by commas.

Special Events

htmx provides a few special events for use in hx-trigger:

  • load - fires once when the element is first loaded
  • revealed - fires once when an element first scrolls into the viewport

You can also use custom events to trigger requests if you have an advanced use case.


If you want an element to poll the given URL rather than wait for an event, you can use the every syntax with the hx-trigger attribute:

  <div hx-get="/news" hx-trigger="every 2s">

This tells htmx

Every 2 seconds, issue a GET to /news and load the response into the div

If you want to stop polling from a server response you can respond with the HTTP response code 286 and the element will cancel the polling.

Load Polling

Another technique that can be used to achieve polling in htmx is "load polling", where an element specifies an load trigger along with a delay, and replaces itself with the response:

<div hx-get="/messages" 
     hx-trigger="load delay:1s"

If the /messages end point keeps returning a div set up this way, it will keep "polling" back to the URL every second.

Load polling can be useful in situations where a poll has an end point at which point the polling terminates, such as when you are showing the user a progress bar.

Request Indicators

When an AJAX request is issued it is often good to let the user know that something is happening since the browser will not give them any feedback. You can accomplish this in htmx by using htmx-indicator class.

The htmx-indicator class is defined so that the opacity of any element with this class is 0 by default, making it invisible but present in the DOM.

When htmx issues a request, it will put a htmx-request class onto an element (either the requesting element or another element, if specified). The htmx-request class will cause a child element with the htmx-indicator class on it to transition to an opacity of 1, showing the indicator.

  <button hx-get="/click">
      Click Me!
     <img class="htmx-indicator" src="/spinner.gif"/>

Here we have a button. When it is clicked the htmx-request class will be added to it, which will reveal the spinner gif element. (I like SVG spinners these days.)

While the htmx-indicator class uses opacity to hide and show the progress indicator, if you would prefer another mechanism you can create your own CSS transition like so:

    .htmx-request .my-indicator{

If you want the htmx-request class added to a different element, you can use the hx-indicator attribute with a CSS selector to do so:

      <button hx-get="/click" hx-indicator="#indicator">
        Click Me!
      <img id="indicator" class="htmx-indicator" src="/spinner.gif"/>  

Here we call out the indicator explicitly by id. Note that we could have placed the class on the parent div as well and had the same effect.


If you want the response to be loaded into a different element other than the one that made the request, you can use the hx-target attribute, which takes a CSS selector. Looking back at our Live Search example:

   <input type="text" name="q" 
          hx-trigger="keyup delay:500ms changed" 
    <div id="search-results"></div>

You can see that the results from the search are going to be loaded into div#search-results, rather than into the input tag.


htmx offers a few different ways to swap the HTML returned into the DOM. By default, the content replaces the innerHTML of the target element. You can modify this by using the hx-swap attribute with any of the following values:

Name Description
innerHTML the default, puts the content inside the target element
outerHTML replaces the entire target element with the returned content
afterbegin prepends the content before the first child inside the target
beforebegin prepends the content before the target in the targets parent element
beforeend appends the content after the last child inside the target
afterend appends the content after the target in the targets parent element
none does not append content from response (out of band items will still be processed)

Out of Band Swaps

If you want to swap content from a response directly into the DOM by using the id attribute you can use the hx-swap-oob attribute in the response html:

  <div id="message" hx-swap-oob="true">Swap me directly!</div>
  Additional Content

In this response, div#message would be swapped directly into the matching DOM element, while the additional content would be swapped into the target in the normal manner.

You can use this technique to "piggy-back" updates on other requests.

Note that out of band elements must be in the top level of the response, and not children of the top level elements.

Selecting Content To Swap

If you want to select a subset of the response HTML to swap into the target, you can use the hx-select attribute, which takes a CSS selector and selects the matching elements from the response.


By default, an element that causes a request will include its value if it has one. If the element is a form it will include the values of all inputs within it.

Additionally, if the element causes a non-GET request, the values of all the inputs of the nearest enclosing form will be included.

If you wish to include the values of other elements, you can use the hx-include attribute with a CSS selector of all the elements whose values you want to include in the request.

If you wish to filter out some parameters you can use the hx-params attribute.

Finally, if you want to programatically modify the parameters, you can use the htmx:configRequest event.

File Upload

If you wish to upload files via an htmx request, you can set the hx-encoding attribute to multipart/form-data. This will use a FormData object to submit the request, which will properly include the file in the request.

Note that depending on your server-side technology, you may have to handle requests with this type of body content very differently.

Note that htmx fires a htmx:xhr:progress event periodically based on the standard progress event during upload, which you can hook into to show the progress of the upload.


You can also include dynamically computed variables in the parameters of a request by using the hx-vars attribute. This attributes allows you to use name-expression pairs to include additional computed variables in a request.


Htmx supports "boosting" regular HTML anchors and forms with the hx-boost attribute. This attribute will convert all anchor tags and forms into AJAX requests that, by default, target the body of the page.

Here is an example:

<div hx-boost="true">
    <a href="/blog">Blog</a>

The anchor tag in this div will issue an AJAX GET request to /blog and swap the response into the body tag.

This functionality is somewhat similar to Turbolinks and allows you to use htmx for progressive enhancement.

Web Sockets & SSE

Htmx has experimental support for declarative use of both WebSockets and Server Sent Events.

These features are under active development, so please let us know if you are willing to experiment with them.


If you wish to establish a WebSocket connection in htmx, you use the hx-ws attribute:

  <div hx-ws="connect wss:/chatroom">
    <div id="chat_room">
    <form hx-ws="send:submit">
        <input name="chat_message">

The source delcaration established the connection, and the send declaration tells the form to submit values to the socket on submit.

More details can be found on the hx-ws attribute page

Server Sent Events

Server Sent Events are a way for servers to send events to browsers. It provides a higher-level mechanism for communication between the server and the browser than websockets.

If you want an element to respond to a Server Sent Event via htmx, you need to do two things:

  1. Define an SSE source. To do this, add a hx-sse attribute on a parent element with a connect <url> declaration that specifies the URL from which Server Sent Events will be received.

  2. Define elements that are descendents of this element that are triggered by server sent events using the hx-trigger="sse:<event_name>" syntax

Here is an example:

    <body hx-sse="connect /news_updates">
        <div hx-trigger="sse:new_news" hx-get="/news"></div>

Depending on your implementation, this may be more efficient than the polling example above since the server would notify the div if there was new news to get, rather than the steady requests that a poll causes.

History Support

Htmx provides a simple mechanism for interacting with the browser history API:

If you want a given element to push its request URL into the browser navigation bar and add the current state of the page to the browser's history, include the hx-push-url attribute:

    <a hx-get="/blog" hx-push-url="true">Blog</a>

When a user clicks on this link, htmx will snapshot the current DOM and store it before it makes a request to /blog. It then does the swap and pushes a new location onto the history stack.

When a user hits the back button, htmx will retrieve the old content from storage and swap it back into the target, simulating "going back" to the previous state.

Specifying History Snapshot Element

By default, htmx will use the body to take and restore the history snapshop from. This is usually the right thing, but if you want to use a narrower element for snapshotting you can use the hx-history-elt attribute to specify a different one.

Careful: this element will need to be on all pages or restoring from history won't work reliably.

Requests & Responses

Htmx expects responses to the AJAX requests it makes to be HTML, typically HTML fragments (although a full HTML document, matched with a hx-select tag can be useful too). Htmx will then swap the returned HTML into the document at the target specified and with the swap strategy specified.

Sometimes you might want to do nothing in the swap, but still perhaps trigger a client side event (see below). For this situation you can return a 204 - No Content response code, and htmx will ignore the content of the response.

In the event of an error response from the server (e.g. a 404 or a 501), htmx will trigger the htmx:responseError event, which you can handle.

In the event of a connection error, the htmx:sendError event will be triggered.

Request Headers

htmx includes a number of useful headers in requests:

Header Description
HX-Request will be set to "true"
HX-Trigger will be set to the id of the element that triggered the request
HX-Trigger-Name will be set to the name of the element that triggered the request
HX-Target will be set to the id of the target element
HX-Current-URL will be set to the URL of the browser
HX-Prompt will be set to the value entered by the user when prompted via hx-prompt
HX-Event-Target the id of the original target of the event that triggered the request
HX-Active-Element the id of the current active element
HX-Active-Element-Name the name of the current active element
HX-Active-Element-Value the value of the current active element

Response Headers

htmx supports two special response headers:

  • HX-Trigger - can be used to trigger client side events, see the documentation for examples.
  • HX-Push - can be used to push a new URL into the browsers address bar

Request Order of Operations

The order of operations in a htmx request are:

  • The element is triggered and begins a request
    • Values are gathered for the request
    • The htmx-request class is applied to the appropriate elements
    • The request is then issued asynchronously via AJAX
      • Upon getting a response the target element is marked with the htmx-swapping class
      • An optional swap delay is applied (see the hx-swap attribute)
      • The actual content swap is done
        • the htmx-swapping class is removed from the target
        • the htmx-settling class is applied to the target
        • A settle delay is done (default: 100ms)
        • The DOM is settled
        • the htmx-settling class is removed from the target

You can use the htmx-swapping and htmx-settling classes to create CSS transitions between pages.


Htmx integrates with the HTML5 Validation API and will not issue a request if a validatable input is invalid. This is true for both AJAX requests as well as WebSocket sends.

Htmx fires events around validation that can be used to hook in custom validation and error handling:

  • htmx:validation:validate - called before an elements checkValidity() method is called. May be used to add in custom validation logic
  • htmx:validation:failed - called when checkValidity() returns false, indicating an invalid input
  • htmx:validation:halted - called when a request is not issued due to validation errors. Specific errors may be found in the event.details.errors object

Validation Example

Here is an example of an input that uses the htmx:validation:validate event to require that an input have the value foo, using hyperscript:

<form hx-post="/test">
  <input _="on htmx:validation:validate 
               if my.value != 'foo' 
                  call me.setCustomValidity('Please enter the value foo')   
                  call me.setCustomValidity('')" 

Note that all client side validations must be re-done on the server side, as they can always be bypassed.


Htmx allows you to use CSS transitions in many situations using only HTML and CSS.

Please see the Animation Guide for more details on the options available.


Htmx has an extension mechanism that allows you to customize the libraries' behavior. Extensions are defined in javascript and then used via the hx-ext attribute:

  <button hx-post="/example" hx-ext="debug">This button uses the debug extension</button>

If you are interested in adding your own extension to htmx, please see the extension docs

Included Extensions

Htmx includes some extensions that are tested against the htmx code base. Here are a few:

Extension Description
json-enc use JSON encoding in the body of requests, rather than the default x-www-form-urlencoded
morphdom-swap an extension for using the morphdom library as the swapping mechanism in htmx.
client-side-templates support for client side template processing of JSON responses
path-deps an extension for expressing path-based dependencies similar to intercoolerjs
class-tools an extension for manipulating timed addition and removal of classes on HTML elements

See the extensions page for a complete list.

Events & Logging

Htmx has an extensive events mechanism, which doubles as the logging system.

If you want to register for a given htmx event you can use the following javascript:

  htmx.on("htmx:load", function(evt) {

This event is fired every time an element is loaded into the DOM by htmx, and is effectively the load event. In fact this is so common, you can use the helper function:

  htmx.onLoad(function(target) {

This does the same thing as the first example, but is a little cleaner.

The full set of events can be seen on the reference page.


If you set a logger at htmx.logger, every event will be logged. This can be very useful for troubleshooting:

    htmx.logger = function(elt, event, data) {
        if(console) {
            console.log(event, elt, data);

Htmx includes a helper method:


if you want to log everything while developing.


NOTE: hyperscript is in very early alpha

Hyperscript is a small scripting language designed to be expressive, making it ideal for embedding directly in HTML, handling custom events, etc. The language is inspired by HyperTalk, javascript, gosu and others.

You can explore the language more fully on its main website:

Events & Hyperscript

Hyperscript was designed to help address features and functionality from intercooler.js that are not implemented in htmx directly, in a more flexible and open manner. One of its prime features is the ability to respond to arbitrary events on a DOM element, using the on syntax:

<div _="on htmx:afterSettle log 'Settled!'">

This will log Settled! to the console when the htmx:afterSettle event is triggered.

intercooler.js features & hyperscript implementations

Below are some examples of intercooler features and the hyperscript equivalent.


Intercooler provided the ic-remove-after attribute for removing an element after a given amount of time.

In hyperscript this is implemented like so:

<div _="on load wait 5s then remove me">Here is a temporary message!</div>

Intercooler provided the ic-post-errors-to attribute for posting errors that occured during requests and responses.

In hyperscript similar functionality is implemented like so:

<body _="on htmx:error(errorInfo) ajax POST errorInfo to /errors">

Intercooler provided the ic-switch-class attribute, which let you switch a class between siblings.

In hyperscript you can implement similar functionality like so:

<div hx-target="#content" _="on htmx:beforeOnLoad take .active from .tabs for">
    <a class="tabs active" hx-get="/tabl1" >Tab 1</a>
    <a class="tabs" hx-get="/tabl2">Tab 2</a>
    <a class="tabs" hx-get="/tabl3">Tab 3</a>
<div id="content">Tab 1 Content</div>

Intercooler provided more response headers than htmx does: X-IC-Refresh, X-IC-Redirect etc. Htmx omits these headers in favor of the general HX-Trigger, combined with some client side code.

Let's implement the X-IC-Redirect header using the HX-Trigger response header and some hyperscript.

First, let's trigger an event with a response header that looks like this:


Then we would write the following hyperscript:

<body _="on redirect(url) set window.location to url">

Configuring htmx

Htmx allows you to configure a few defaults:

Config Variable Info
htmx.config.historyEnabled defaults to true, really only useful for testing
htmx.config.historyCacheSize defaults to 10
htmx.config.defaultSwapStyle defaults to innerHTML
htmx.config.defaultSwapDelay defaults to 0
htmx.config.defaultSettleDelay defaults to 100
htmx.config.includeIndicatorStyles defaults to true (determines if the indicator styles are loaded)
htmx.config.indicatorClass defaults to htmx-indicator
htmx.config.requestClass defaults to htmx-request
htmx.config.settlingClass defaults to htmx-settling
htmx.config.swappingClass defaults to htmx-swapping

You can set them directly in javascript, or you can use a meta tag:

    <meta name="htmx-config" content='{"defaultSwapStyle":"outerHTML"}'>


And that's it! Have fun with htmx: you can accomplish quite a bit without a lot of code.