Extending HTML As Hypermedia

In the previous chapter we introduced a simple Web 1.0-style hypermedia application to manage contacts. Our application supported the normal CRUD operations for contacts, as well as a simple mechanism for searching contacts. Our application was built using nothing but forms and anchor tags, the traditional hypermedia controls used to interact with servers. The application exchanges hypermedia (HTML) with the server over HTTP, issuing GET and POST HTTP requests and receiving back full HTML documents in response.

It is a basic web application, but it is also definitely a Hypermedia-Driven Application. It is robust, it leverages the web’s native technologies, and it is simple to understand.

So what’s not to like about the application?

Unfortunately, our application has a few issues common to web 1.0 style applications:

• From a user experience perspective: there is a noticeable refresh when you move between pages of the application, or when you create, update or delete a contact. This is because every user interaction (link click or form submission) requires a full page refresh, with a whole new HTML document to process after each action.

• From a technical perspective, all the updates are done with the POST HTTP method. This, despite the fact that more logical actions and HTTP request types like PUT and DELETE exist and would make more sense for some of the operations we implemented. After all, if we wanted to delete a resource, wouldn’t it make more sense to use an HTTP DELETE request to do so? Somewhat ironically, since we have used pure HTML, we are unable to access the full expressive power of HTTP, which was designed specifically for HTML.

The first point, in particular, is noticeable in Web 1.0 style applications like ours and is what is responsible for giving them the reputation for being “clunky” when compared with their more sophisticated JavaScript-based Single Page Application cousins.

We could address this issue by adopting a Single Page Application framework, and updating our server-side to provide JSON-based responses. Single Page Applications eliminate the clunkiness of web 1.0 applications by updating a web page without refreshing it: they can mutate parts of the Document Object Model (DOM) of the existing page without needing to replace (and re-render) the entire page.

There are a few different styles of SPA, but, as we discussed in Chapter 1, the most common approach today is to tie the DOM to a JavaScript model and then let an SPA framework like React or Vue reactively update the DOM when a JavaScript model is updated: you make a change to a JavaScript object that is stored locally in memory in the browser, and the web page “magically” updates its state to reflect the change in the model.

In this style of application, communication with the server is typically done via a JSON Data API, with the application sacrificing the advantages of hypermedia in order to provide a better, smoother user experience.

Many web developers today would not even consider the hypermedia approach due to the perceived “legacy” feel of these web 1.0 style applications.

Now, the second more technical issue we mentioned may strike you as a bit pedantic, and we are the first to admit that conversations around REST and which HTTP Action is right for a given operation can become very tedious. But still, it’s odd that, when using plain HTML, it is impossible to use all the functionality of HTTP!

Just seems wrong, doesn’t it?

A Close Look At A Hyperlink

It turns out that we can boost the interactivity of our application and address both of these issues without resorting to the SPA approach. We can do so by using a hypermedia-oriented JavaScript library, htmx. The authors of this book built htmx specifically to extend HTML as a hypermedia and address the issues with legacy HTML applications we mentioned above (as well as a few others.)

Before we get into how htmx allows us to improve the UX of our Web 1.0 style application, let’s revisit the hyperlink/anchor tag from Chapter

1. Recall, a hyperlink is what is known as a hypermedia control,

a mechanism that describes some sort of interaction with a server by encoding information about that interaction directly and completely within the control itself.

Consider again this simple anchor tag which, when interpreted by a browser, creates a hyperlink to the website for this book:

<a href="https://hypermedia.systems/">
  Hypermedia Systems
</a>

Let’s break down exactly what happens with this link:

• The browser will render the text “Hypermedia Systems” to the screen, likely with a decoration indicating it is clickable.

• Then, when a user clicks on the text…​

• The browser will issue an HTTP GET to https://hypermedia.systems…​

• The browser will load the HTML body of the HTTP response into the browser window, replacing the current document.

So we have four aspects of a simple hypermedia link like this, with the last three aspects supplying the mechanism that distinguishes a hyperlink from “normal” text and, thus, makes this a hypermedia control.

Now, let’s take a moment and think about how we can generalize these last three aspects of a hyperlink.

Why Only Anchors & Forms?

Consider: what makes anchor tags (and forms) so special?

Why can’t other elements issue HTTP requests as well?

For example, why shouldn’t button elements be able to issue HTTP requests? It seems arbitrary to have to wrap a form tag around a button just to make deleting contacts work in our application, for example.

Maybe: other elements should be able to issue HTTP requests as well. Maybe other elements should be able to act as hypermedia controls on their own.

This is our first opportunity to generalize HTML as a hypermedia.

Why Only Click & Submit Events?

Next, let’s consider the event that triggers the request to the server on our link: a click event.

Well, what’s so special about clicking (in the case of anchors) or submitting (in the case of forms) things? Those are just two of many, many events that are fired by the DOM, after all. Events like mouse down, or key up, or blur are all events you might want to use to issue an HTTP request.

Why shouldn’t these other events be able to trigger requests as well?

This gives us our second opportunity to expand the expressiveness of HTML:

Why Only GET & POST?

Getting a bit more technical in our thinking leads us to the problem we noted earlier: plain HTML only give us access to the GET and POST actions of HTTP.

HTTP stands for Hypertext Transfer Protocol, and yet the format it was explicitly designed for, HTML, only supports two of the five developer-facing request types. You have to use JavaScript and issue an AJAX request to get at the other three: DELETE, PUT and PATCH.

Let’s recall what these different HTTP request types are designed to represent:

• GET corresponds with “getting” a representation for a resource from a URL: it is a pure read, with no mutation of the resource.

• POST submits an entity (or data) to the given resource, often creating or mutating the resource and causing a state change.

• PUT submits an entity (or data) to the given resource for update or replacement, again likely causing a state change.

• PATCH is similar to PUT but implies a partial update and state change rather than a complete replacement of the entity.

• DELETE deletes the given resource.

These operations correspond closely to the CRUD operations we discussed in Chapter 2. By giving us access to only two of the five, HTML hamstrings our ability to take full advantage of HTTP.

This gives us our third opportunity to expand the expressiveness of HTML:

Why Only Replace The Entire Screen?

As a final observation, consider the last aspect of a hyperlink: it replaces the entire screen when a user clicks on it.

It turns out that this technical detail is the primary culprit for poor user experience in Web 1.0 Applications. A full page refresh can cause a flash of unstyled content, where content “jumps” on the screen as it transitions from its initial to its styled final form. It also destroys the scroll state of the user by scrolling to the top of the page, removes focus from a focused element and so forth.

But, if you think about it, there is no rule saying that hypermedia exchanges must replace the entire document.

This gives us our fourth, final and perhaps most important opportunity to generalize HTML:

This is actually a very old concept in hypermedia. Ted Nelson, in his 1980 book “Literary Machines” coined the term transclusion to capture this idea: the inclusion of content into an existing document via a hypermedia reference. If HTML supported this style of “dynamic transclusion,” then Hypermedia-Driven Applications could function much more like a Single Page Application, where only part of the DOM is updated by a given user interaction or network request.

Extending HTML as a Hypermedia with Htmx

These four opportunities present us a way to extend HTML well beyond its current abilities, but in a way that is entirely within the hypermedia model of the web. The fundamentals of HTML, HTTP, the browser, and so on, won’t be changed dramatically. Rather, these generalizations of existing functionality already found within HTML would simply let us accomplish more using HTML.

Htmx is a JavaScript library that extends HTML in exactly this manner, and it will be the focus of the next few chapters of this book. Again, htmx is not the only JavaScript library that takes this hypermedia-oriented approach (other excellent examples are Unpoly and Hotwire), but htmx is the purest in its pursuit of extending HTML as a hypermedia.

Installing and Using Htmx

From a practical “getting started” perspective, htmx is a simple, dependency-free and stand-alone JavaScript library that can be added to a web application by simply including it via a script tag in your head element.

Because of this simple installation model, you can take advantage of tools like public CDNs to install the library.

Below is an example using the popular unpkg Content Delivery Network (CDN) to install version 1.9.2 of the library. We use an integrity hash to ensure that the delivered JavaScript content matches what we expect. This SHA can be found on the htmx website.

We also mark the script as crossorigin="anonymous" so no credentials will be sent to the CDN.

<head>
<script src="https://unpkg.com/htmx.org@1.9.2"
  integrity="sha384-L6OqL9pRWyyFU3+/bjdSri+iIphTN/
    bvYyM37tICVyOJkWZLpP2vGn6VUEXgzg6h"
  crossorigin="anonymous"></script>
</head>

If you are used to modern JavaScript development, with complex build systems and large numbers of dependencies, it may be a pleasant surprise to find that that’s all it takes to install htmx.

This is in the spirit of the early web, when you could simply include a script tag and things would “just work.”

If you don’t want to use a CDN, you can download htmx to your local system and adjust the script tag to point to wherever you keep your static assets. Or, you may have a build system that automatically installs dependencies. In this case you can use the Node Package Manager (npm) name for the library: htmx.org and install it in the usual manner that your build system supports.

Once htmx has been installed, you can begin using it immediately.

No JavaScript Required…​

And here we get to the interesting part of htmx: htmx does not require you, the user of htmx, to actually write any JavaScript.

Instead, you will use attributes placed directly on elements in your HTML to drive more dynamic behavior. Htmx extends HTML as a hypermedia, and it is designed to make that extension feel as natural and consistent as possible with existing HTML concepts. Just as an anchor tag uses an href attribute to specify the URL to retrieve, and forms use an action attribute to specify the URL to submit the form to, htmx uses HTML attributes to specify the URL that an HTTP request should be issued to.

Triggering HTTP Requests

Let’s look at the first feature of htmx: the ability for any element in a web page to issue HTTP requests. This is the core functionality provided by htmx, and it consists of five attributes that can be used to issue the five different developer-facing types of HTTP requests:

• hx-get - issues an HTTP GET request.

• hx-post - issues an HTTP POST request.

• hx-put - issues an HTTP PUT request.

• hx-patch - issues an HTTP PATCH request.

• hx-delete - issues an HTTP DELETE request.

Each of these attributes, when placed on an element, tells the htmx library: “When a user clicks (or whatever) this element, issue an HTTP request of the specified type.”

The values of these attributes are similar to the values of both href on anchors and action on forms: you specify the URL you wish to issue the given HTTP request type to. Typically, this is done via a server-relative path.

For example, if we wanted a button to issue a GET request to /contacts then we would write the following HTML:

<button hx-get="/contacts"> <1>
  Get The Contacts
</button>

1. A simple button that issues an HTTP GET to /contacts.

The htmx library will see the hx-get attribute on this button, and hook up some JavaScript logic to issue an HTTP GET AJAX request to the /contacts path when the user clicks on it.

Very easy to understand and very consistent with the rest of HTML.

It’s All Just HTML

With the request issued by the button above, we get to perhaps the most important thing to understand about htmx: it expects the response to this AJAX request to be HTML. Htmx is an extension of HTML. A native hypermedia control like an anchor tag will typically get an HTML response to an HTTP request it creates. Similarly, htmx expects the server to respond to the requests that it makes with HTML.

This may surprise web developers who are used to responding to an AJAX request with JSON, which is far and away the most common response format for such requests. But AJAX requests are just HTTP requests and there is no rule saying they must use JSON. Recall again that AJAX stands for Asynchronous JavaScript & XML, so JSON is already a step away from the format originally envisioned for this API: XML.

Htmx simply goes another direction and expects HTML.

Htmx vs. “Plain” HTML Responses

There is an important difference between the HTTP responses to “normal” anchor or form driven HTTP requests and to htmx-powered requests: in the case of htmx triggered requests, responses can be partial bits of HTML.

In htmx-powered interactions, as you will see, we are often not replacing the entire document. Rather we are using “transclusion” to include content within an existing document. Because of this, it is often not necessary or desirable to transfer an entire HTML document from the server to the browser.

This fact can be used to save bandwidth as well as resource loading time. Less overall content is transferred from the server to the client, and it isn’t necessary to reprocess a head tag with style sheets, script tags, and so forth.

When the “Get Contacts” button is clicked, a partial HTML response might look something like this:

<ul>
  <li><a href="mailto:joe@example.com">Joe</a></li>
  <li><a href="mailto:sarah@example.com">Sarah</a></li>
  <li><a href="mailto:fred@example.com">Fred</a></li>
</ul>

This is just an unordered list of contacts with some clickable elements in it. Note that there is no opening html tag, no head tag, and so forth: it is a raw HTML list, without any decoration around it. A response in a real application might contain more sophisticated HTML than this simple list, but even if it were more complicated it wouldn’t need to be an entire page of HTML: it could just be the “inner” content of the HTML representation for this resource.

Now, this simple list response is perfect for htmx. Htmx will simply take the returned content and then swap it in to the DOM in place of some element in the page. (More on exactly where it will be placed in the DOM in a moment.) Swapping in HTML content in this manner is fast and efficient because it leverages the existing native HTML parser in the browser, rather than requiring a significant amount of client-side JavaScript to be executed.

This small HTML response shows how htmx stays within the hypermedia paradigm: just like a “normal” hypermedia control in a “normal” web application, we see hypermedia being transferred to the client in a stateless and uniform manner.

This button just gives us a slightly more sophisticated mechanism for building a web application using hypermedia.

Targeting Other Elements

Now, given that htmx has issued a request and gotten back some HTML as a response, and that we are going to swap this content into the existing page (rather than replacing the entire page), the question becomes: where should this new content be placed?

It turns out that the default htmx behavior is to simply put the returned content inside the element that triggered the request. That’s not a good thing in the case of our button: we will end up with a list of contacts awkwardly embedded within the button element. That will look pretty silly and is obviously not what we want.

Fortunately htmx provides another attribute, hx-target which can be used to specify exactly where in the DOM the new content should be placed. The value of the hx-target attribute is a Cascading Style Sheet (CSS) selector that allows you to specify the element to put the new hypermedia content into.

Let’s add a div tag that encloses the button with the id main. We will then target this div with the response:

<div id="main"> <1>

  <button hx-get="/contacts" hx-target="#main"> <2>
    Get The Contacts
  </button>

</div>

1. A div element that wraps the button.

2. The hx-target attribute that specifies the target of the response.

We have added hx-target="#main" to our button, where #main is a CSS selector that says “The thing with the ID ‘main’.”

By using CSS selectors, htmx builds on top of familiar and standard HTML concepts. This keeps the additional conceptual load for working with htmx to a minimum.

Given this new configuration, what would the HTML on the client look like after a user clicks on this button and a response has been received and processed?

It would look something like this:

<div id="main">
  <ul>
    <li><a href="mailto:joe@example.com">Joe</a></li>
    <li><a href="mailto:sarah@example.com">Sarah</a></li>
    <li><a href="mailto:fred@example.com">Fred</a></li>
  </ul>
</div>

The response HTML has been swapped into the div, replacing the button that triggered the request. Transclusion! And this has happened “in the background” via AJAX, without a clunky page refresh.

Swap Styles

Now, perhaps we don’t want to load the content from the server response into the div, as child elements. Perhaps, for whatever reason, we wish to replace the entire div with the response. To handle this, htmx provides another attribute, hx-swap, that allows you to specify exactly how the content should be swapped into the DOM.

The hx-swap attribute supports the following values:

• innerHTML - The default, replace the inner html of the target element.

• outerHTML - Replace the entire target element with the response.

• beforebegin - Insert the response before the target element.

• afterbegin - Insert the response before the first child of the target element.

• beforeend - Insert the response after the last child of the target element.

• afterend - Insert the response after the target element.

• delete - Deletes the target element regardless of the response.

• none - No swap will be performed.

The first two values, innerHTML and outerHTML, are taken from the standard DOM properties that allow you to replace content within an element or in place of an entire element respectively.

The next four values are taken from the Element.insertAdjacentHTML() DOM API, which allow you to place an element or elements around a given element in various ways.

The last two values, delete and none are specific to htmx. The first option will remove the target element from the DOM, while the second option will do nothing (you may want to only work with response headers, an advanced technique we will look at later in the book.)

Again, you can see htmx stays as close as possible to existing web standards in order to minimize the conceptual load necessary for its use.

So let’s consider that case where, rather than replacing the innerHTML content of the main div above, we want to replace the entire div with the HTML response.

To do so would require only a small change to our button, adding a new hx-swap attribute:

<div id="main">

  <button hx-get="/contacts" hx-target="#main" hx-swap="outerHTML"> <1>
    Get The Contacts
  </button>

</div>

1. The hx-swap attribute specifies how to swap in new content.

Now, when a response is received, the entire div will be replaced with the hypermedia content:

<ul>
  <li><a href="mailto:joe@example.com">Joe</a></li>
  <li><a href="mailto:sarah@example.com">Sarah</a></li>
  <li><a href="mailto:fred@example.com">Fred</a></li>
</ul>

You can see that, with this change, the target div has been entirely removed from the DOM, and the list that was returned as the response has replaced it.

Later in the book we will see additional uses for hx-swap, for example when we implement infinite scrolling in our contact management application.

Note that with the hx-get, hx-post, hx-put, hx-patch and hx-delete attributes, we have addressed two of the four opportunities for improvement that we enumerated regarding plain HTML:

• Opportunity 1: We can now issue an HTTP request with any element (in this case we are using a button).

• Opportunity 3: We can issue any sort of HTTP request we want, PUT, PATCH and DELETE, in particular.

And, with hx-target and hx-swap we have addressed a third shortcoming: the requirement that the entire page be replaced.

• Opportunity 4: We can now replace any element we want in our page via transclusion, and we can do so in any manner we want.

So, with only seven relatively simple additional attributes, we have addressed most of the shortcomings of HTML as a hypermedia that we identified earlier.

What’s next? Recall the one other opportunity we noted: the fact that only a click event (on an anchor) or a submit event (on a form) can trigger an HTTP request. Let’s look at how we can address that limitation.

Using Events

Thus far we have been using a button to issue a request with htmx. You have probably intuitively understood that the button would issue its request when you clicked on the button since, well, that’s what you do with buttons: you click on them.

And, yes, by default when an hx-get or another request-driving annotation from htmx is placed on a button, the request will be issued when the button is clicked.

However, htmx generalizes this notion of an event triggering a request by using, you guessed it, another attribute: hx-trigger. The hx-trigger attribute allows you to specify one or more events that will cause the element to trigger an HTTP request.

Often you don’t need to use hx-trigger because the default triggering event will be what you want. The default triggering event depends on the element type, and should be fairly intuitive:

• Requests on input, textarea & select elements are triggered by the change event.

• Requests on form elements are triggered on the submit event.

• Requests on all other elements are triggered by the click event.

To demonstrate how hx-trigger works, consider the following situation: we want to trigger the request on our button when the mouse enters it. Now, this is certainly not a good UX pattern, but bear with us: we are just using this as an example.

To respond to a mouse entering the button, we would add the following attribute to our button:

<div id="main">

  <button hx-get="/contacts" hx-target="#main" hx-swap="outerHTML"
    hx-trigger="mouseenter"> <1>
    Get The Contacts
  </button>

</div>

1. Issue a request on the… mouseenter event.

Now, with this hx-trigger attribute in place, whenever the mouse enters this button, a request will be triggered. Silly, but it works.

Let’s try something a bit more realistic and potentially useful: let’s add support for a keyboard shortcut for loading the contacts, Ctrl-L (for “Load”). To do this we will need to take advantage of additional syntax that the hx-trigger attribute supports: event filters and additional arguments.

Event filters are a mechanism for determining if a given event should trigger a request or not. They are applied to an event by adding square brackets after it: someEvent[someFilter]. The filter itself is a JavaScript expression that will be evaluated when the given event occurs. If the result is truthy, in the JavaScript sense, it will trigger the request. If not, the request will not be triggered.

In the case of keyboard shortcuts, we want to catch the keyup event in addition to the click event:

<div id="main">

  <button hx-get="/contacts" hx-target="#main" hx-swap="outerHTML"
    hx-trigger="click, keyup"> <1>
    Get The Contacts
  </button>

</div>

1. A trigger with two events.

Note that we have a comma separated list of events that can trigger this element, allowing us to respond to more than one potential triggering event. We still want to respond to the click event and load the contacts, in addition to handling the Ctrl-L keyboard shortcut.

Unfortunately there are two problems with our keyup addition: As it stands, it will trigger requests on any keyup event that occurs. And, worse, it will only trigger when a keyup occurs within this button. The user would need to tab onto the button to make it active and then begin typing.

Let’s fix these two issues. To fix the first one, we will use a trigger filter to test that Control key and the “L” key are pressed together:

<div id="main">

  <button hx-get="/contacts" hx-target="#main" hx-swap="outerHTML"
    hx-trigger="click, keyup[ctrlKey && key == 'l']"> <1>
    Get The Contacts
  </button>

</div>

1. keyup now has a filter, so the control key and L must be pressed.

The trigger filter in this case is ctrlKey && key == 'l'. This can be read as “A key up event, where the ctrlKey property is true and the key property is equal to l.” Note that the properties ctrlKey and key are resolved against the event rather than the global name space, so you can easily filter on the properties of a given event. You can use any expression you like for a filter, however: calling a global JavaScript function, for example, is perfectly acceptable.

OK, so this filter limits the keyup events that will trigger the request to only Ctrl-L presses. However, we still have the problem that, as it stands, only keyup events within the button will trigger the request.

If you are not familiar with the JavaScript event bubbling model: events typically “bubble” up to parent elements. So an event like keyup will be triggered first on the focused element, and then on its parent (enclosing) element, and so on, until it reaches the top level document object that is the root of all other elements.

To support a global keyboard shortcut that works regardless of what element has focus, we will take advantage of event bubbling and a feature that the hx-trigger attribute supports: the ability to listen to other elements for events. The syntax for doing this is the from: modifier, which is added after an event name and that allows you to specify a specific element to listen for the given event on using a CSS selector.

In this case, we want to listen to the body element, which is the parent element of all visible elements on the page.

Here is what our updated hx-trigger attribute looks like:

<div id="main">

  <button hx-get="/contacts" hx-target="#main" hx-swap="outerHTML"
    hx-trigger="click, keyup[ctrlKey && key == 'l'] from:body"> <1>
    Get The Contacts
  </button>

</div>

1. Listen to the ‘keyup” event on the body tag.

Now, in addition to clicks, the button will listen for keyup events on the body of the page. So it will issue a request when it is clicked on and also whenever someone hits Ctrl-L within the body of the page.

And now we have a nice keyboard shortcut for our Hypermedia-Driven Application.

The hx-trigger attribute supports many more modifiers, and it is more elaborate than other htmx attributes. This is because events, in general, are complicated and require a lot of details to get just right. The default trigger will often suffice, however, and you typically don’t need to reach for complicated hx-trigger features when using htmx.

Even with more sophisticated trigger specifications like the keyboard shortcut we just added, the overall feel of htmx is declarative rather than imperative. That keeps htmx-powered applications “feeling like” standard web 1.0 applications in a way that adding significant amounts of JavaScript does not.

Htmx: HTML eXtended

And hey, check it out! With hx-trigger we have addressed the final opportunity for improvement of HTML that we outlined at the start of this chapter:

• Opportunity 2: We can use any event to trigger an HTTP request.

That’s a grand total of eight, count ‘em, eight attributes that all fall squarely within the same conceptual model as normal HTML and that, by extending HTML as a hypermedia, open up a whole new world of user interaction possibilities within it.

Here is a table summarizing those opportunities and which htmx attributes address them:

Any element should be able to make HTTP requests

hx-get, hx-post, hx-put, hx-patch, hx-delete

Any event should be able to trigger an HTTP request

hx-trigger

Any HTTP Action should be available

hx-put, hx-patch, hx-delete

Any place on the page should be replaceable (transclusion)

hx-target, hx-swap

Passing Request Parameters

So far we have just looked at a situation where a button makes a simple GET request. This is conceptually very close to what an anchor tag might do. But there is that other native hypermedia control in HTML-based applications: forms. Forms are used to pass additional information beyond just a URL up to the server in a request.

This information is captured via input and input-like elements within the form via the various types of input tags available in HTML.

Htmx allows you include this additional information in a way that mirrors HTML itself.

Enclosing Forms

The simplest way to pass input values with a request in htmx is to enclose the element making a request within a form tag.

Let’s take our original search form and convert it to use htmx instead:

<form action="/contacts" method="get" class="tool-bar"> <1>
  <label for="search">Search Term</label>
  <input id="search" type="search" name="q" 
    value="{{ request.args.get('q') or '' }}"
    placeholder="Search Contacts"/>
  <button hx-post="/contacts" hx-target="#main"> <2>
    Search
  </button>
</form>

1. When an htmx-powered element is withing an ancestor form tag, all input values within that form will be submitted for non-GET requests

2. We have switched from an input of type submit to a button and added the hx-post attribute

Now, when a user clicks on this button, the value of the input with the id search will be included in the request. This is by virtue of the fact that there is a form tag enclosing both the button and the input: when an htmx-driven request is triggered, htmx will look up the DOM hierarchy for an enclosing form, and, if one is found, it will include all values from within that form. (This is sometimes referred to as “serializing” the form.)

You might have noticed that the button was switched from a GET request to a POST request. This is because, by default, htmx does not include the closest enclosing form for GET requests, but it does include the form for all other types of requests.

This may seem a little strange, but it avoids junking up URLs that are used within forms when dealing with history entries, which we will discuss in a bit. You can always include an enclosing form’s values with an element that uses a GET by using the hx-include attribute, which we will discuss next.

Note also that we could have added the hx-post attribute to the form, rather than to the button but that would create a somewhat awkward duplication of the search URL in the action and hx-post attributes. This can be avoided by using the hx-boost attribute, which we discuss in the next chapter.

Including Inputs

While enclosing all the inputs you want included in a request within a form is the most common approach for serializing inputs for htmx requests, it isn’t always possible or desirable: form tags can have layout consequences and simply cannot be placed in some spots in HTML documents. A good example of the latter situation is in table row (tr) elements: the form tag is not a valid child or parent of table rows, so you can’t place a form within or around a row of data in a table.

To address this issue, htmx provides a mechanism for including input values in requests: the hx-include attribute. The hx-include attribute allows you to select input values that you wish to include in a request via CSS selectors.

Here is the above example reworked to include the input, dropping the form:

<div id="main">

  <label for="search">Search Contacts:</label>
  <input id="search" name="q"  type="search" 
    value="{{ request.args.get('q') or '' }}"
    placeholder="Search Contacts"/>
  <button hx-post="/contacts" hx-target="#main" hx-include="#search"> <1>
    Search
  </button>

</div>

1. hx-include can be used to include values directly in a request.

The hx-include attribute takes a CSS selector value and allows you to specify exactly which values to send along with the request. This can be useful if it is difficult to colocate an element issuing a request with all the desired inputs.

It is also useful when you do, in fact, want to submit values with a GET request and overcome the default behavior of htmx.

Relative CSS selectors

The hx-include attribute and, in fact, most attributes that take a CSS selector, also support relative CSS selectors. These allow you to specify a CSS selector relative to the element it is declared on. Here are some examples:

closest

Find the closest parent

element matching the given selector, e.g., closest form.

next

Find the next element (scanning

forward) matching the given selector, e.g., next input.

previous

Find the previous element

(scanning backwards) matching the given selector, e.g., previous input.

find

Find the next element within this

element matching the given selector, e.g., find input.

this

The current element.

Using relative CSS selectors often allows you to avoid generating ids for elements, since you can take advantage of their local structural layout instead.

Inline Values

A final way to include values in htmx-driven requests is to use the hx-vals attribute, which allows you to include “static” values in the request. This can be useful if you have additional information that you want to include in requests, but you don’t want to have this information embedded in, for example, hidden inputs (which would be the standard mechanism for including additional, hidden information in HTML.)

Here is an example of hx-vals:

<button hx-get="/contacts" hx-vals='{"state":"MT"}'> <1>
  Get The Contacts In Montana
</button>

1. hx-vals, a JSON value to include in the request.

The parameter state with the value MT will be included in the GET request, resulting in a path and parameters that looks like this: /contacts?state=MT. Note that we switched the hx-vals attribute to use single quotes around its value. This is because JSON strictly requires double quotes and, therefore, to avoid escaping we needed to use the single-quote form for the attribute value.

You can also prefix hx-vals with a js: and pass values evaluated at the time of the request, which can be useful for including things like a dynamically maintained variable, or value from a third party JavaScript library.

For example, if the state variable were maintained dynamically, via some JavaScript, and there existed a JavaScript function, getCurrentState(), that returned the currently selected state, it could be included dynamically in htmx requests like so:

<button hx-get="/contacts"
  hx-vals='js:{"state":getCurrentState()}'> <1>
  Get The Contacts In The Selected State
</button>

1. With the js: prefix, this expression will evaluate at submit time.

These three mechanisms, using form tags, using the hx-include attribute and using the hx-vals attribute, allow you to include values in your hypermedia requests with htmx in a manner that should feel very familiar and in keeping with the spirit of HTML, while also giving you the flexibility to achieve what you want.

History Support

We have a final piece of functionality to close out our overview of htmx: browser history support. When you use normal HTML links and forms, your browser will keep track of all the pages that you have visited. You can then use the back button to navigate back to a previous page and, once you have done this, you can use a forward button to go forward to the original page you were on.

This notion of history was one of the killer features of the early web. Unfortunately it turns out that history becomes tricky when you move to the Single Page Application paradigm. An AJAX request does not, by itself, register a web page in your browser’s history, which is a good thing: an AJAX request may have nothing to do with the state of the web page (perhaps it is just recording some activity in the browser), so it wouldn’t be appropriate to create a new history entry for the interaction.

However, there are likely to be a lot of AJAX driven interactions in a Single Page Application where it is appropriate to create a history entry. There is a JavaScript API to work with browser history, but this API is deeply annoying and difficult to work with, and thus often ignored by JavaScript developers.

If you have ever used a Single Page Application and accidentally clicked the back button, only to lose your entire application state and have to start over, you have seen this problem in action.

In htmx, as with Single Page Application frameworks, you will often need to explicitly work with the history API. Fortunately, since htmx sticks so close to the native model of the web and since it is declarative, getting web history right is typically much easier to do in an htmx-based application.

Consider the button we have been looking at to load contacts:

<button hx-get="/contacts" hx-target="#main">
  Get The Contacts
</button>

As it stands, if you click this button it will retrieve the content from /contacts and load it into the element with the id main, but it will not create a new history entry.

If we wanted it to create a history entry when this request happened, we would add a new attribute to the button, the hx-push-url attribute:

<button hx-get="/contacts" hx-target="#main" hx-push-url="true"> <1>
  Get The Contacts
</button>

1. hx-push-url will create an entry in history when the button is clicked.

Now, when the button is clicked, the /contacts path will be put into the browser’s navigation bar and a history entry will be created for it. Furthermore, if the user clicks the back button, the original content for the page will be restored, along with the original URL.

Now, the name hx-push-url for this attribute might sound a little obscure, but it is based on the JavaScript API, history.pushState(). This notion of “pushing” derives from the fact that history entries are modeled as a stack, and so you are “pushing” new entries onto the top of the stack of history entries.

With this relatively simple, declarative mechanism, htmx allows you to integrate with the back button in a way that mimics the “normal” behavior of HTML.

Now, there is one additional thing we need to handle to get history “just right”: we have “pushed” the /contacts path into the browsers location bar successfully, and the back button works. But what if someone refreshes their browser while on the /contacts page?

In this case, you will need to handle the htmx-based “partial” response as well as the non-htmx “full page” response. You can do this using HTTP headers, a topic we will go into in detail later in the book.

Conclusion

So that’s our whirlwind introduction to htmx. We’ve only seen about ten attributes from the library, but you can see a hint of just how powerful these attributes can be. Htmx enables a much more sophisticated web application than is possible in plain HTML, with minimal additional conceptual load compared to most JavaScript-based approaches.

Htmx aims to incrementally improve HTML as a hypermedia in a manner that is conceptually coherent with the underlying markup language. Like any technical choice, this is not without trade-offs: by staying so close to HTML, htmx does not give developers a lot of infrastructure that many might feel should be there “by default”.

By staying closer to the native model of the web, htmx aims to strike a balance between simplicity and functionality, deferring to other libraries for more elaborate frontend extensions on top of the existing web platform. The good news is that htmx plays well with others, so when these needs arise it is often easy enough to bring in another library to handle them.