add article on the fetch()ening

www/content/essays/the-fetchening.md

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+title = "The fetch()ening"
+description = """\
+  You know, technically, I never said anything about a version *four*"""
+date = 2025-11-01
+authors = ["Carson Gross"]
+[taxonomies]
+tag = ["posts"]
++++
+
+![Stop trying to make fetch() happen](/img/fetch.png)
+
+OK, I said there would never be a version three of htmx.  
+
+But, _technically_, I never said anything about a version *four*...
+
+## htmx 4: The fetch()ening
+
+In [The Future of htmx](@/essays/hypermedia-driven-applications.md) I said the following:
+
+> We are going to work to ensure that htmx is extremely stable in both API & implementation. This means accepting and
+documenting the [quirks](https://htmx.org/quirks/) of the current implementation.
+
+Earlier this year, on a whim, I created [fixi.js](https://github.com/bigskysoftware/fixi), a hyperminimalist implementation
+of the ideas in htmx.  That work gave me a chance to get a lot more familiar  with the `fetch()` and, especially, the 
+[async](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) infrastructure 
+available in JavaScript.
+
+In doing that work I began to wonder if that, while the htmx [API](https://htmx.org/reference/#attributes) 
+is (at least reasonably) correct, maybe there was room for a more dramatic change of the implementation that took 
+advantage of these features in order to simplify the library.
+
+Further, changing from ye olde [`XMLHttpRequest`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest)
+(a legacy of htmx 1.0 IE support) to [`fetch()`](https://developer.mozilla.org/en-US/docs/Web/API/Window/fetch) would
+be a pretty violent change, guaranteed to break at least *some* stuff.  
+
+So I began thinking: if we are going to consider moving to fetch, then maybe we should _also_ that this update as a
+chance address at least _some_ of the [quirks](https://htmx.org/quirks/) that htmx has acquired over its lifetime.
+
+So, eventually & reluctantly, I have changed my mind: there _will_ be another major version of htmx.
+
+However, in order to keep my word that there will not be a htmx 3.0, the next release will instead be htmx 4.0.
+
+## Project Goals
+
+With htmx 4.0 we are rebuilding the internals of htmx, based on the lessons learned from 
+fixi.js and [five+ years](https://www.npmjs.com/package/htmx.org/v/0.0.1) of supporting htmx.  
+
+There are three major simplifying changes:
+
+### The fetch()ening
+
+The biggest internal change is that `fetch()` will replace `XMLHttpRequest` as the core ajax infrastructure.  This
+won't actually have a huge effect on most usages of htmx _except_ that the events model will necessarily change due
+to the differences between `fetch()` and `XMLHttpRequest`.
+
+### The Long National Nightmare of Implicit Attribute Inheritance Ends
+
+I feel that my biggest mistake in htmx 1.0 & 2.0 was making attribute inheritance _implicit_.  I was inspired by CSS in
+doing this, and the results have been roughly the same as CSS: powerful & maddening.
+
+In htmx 4.0, attribute inheritance will be *explicit* rather than *implicit*, via the `:inherited` modifier: 
+
+```html
+  <div hx-target:inherited="#output">
+    <button hx-post="/up">Like</button>
+    <button hx-post="/down">Dislike</button>
+  </div>
+  <output id="output">Pick a button...</output>
+```
+
+Here the `hx-target` attribute is explicitly declared as `inherited` on the enclosing `div` and, if it wasn't, the 
+`button` elements would not inherit the target from it.
+
+This will be the most significant upgrade change to deal with for most htmx users.
+
+### The Tyranny Of Locally Cached History Ends
+
+Another constant source of pain for both us and for htmx users is history support.  htmx 2.0 stores history in local 
+cache to make navigation faster.  Unfortunately, snapshotting the DOM is often brittle because of third-party 
+modifications, hidden state, etc.  There is a terrible simplicity to the web 1.0 model of blowing everything away and
+starting over.  There are also security concerns storing history information in session storage.
+
+In htmx 2.0, we often end up recommending that people facing history-related issues simply disable the cache entirely,
+and that usually fixes the problems.
+
+In htmx 4.0, history support will no longer snapshot the DOM and keep it locally.  It will, rather, issue a network 
+request for the restored content.  This is the behavior of 2.0 on a history cache-miss, and it works reliably with 
+little effort on behalf of htmx users.
+
+We will offer an extension that enables history caching like in htmx 2.0, but it will be opt-in, rather than the default.
+
+This tremendously simplifies the htmx codebase and should make the out-of-the-box behavior much more plug-and-play.
+
+## What Stays The Same?
+
+Most things.
+
+The [core](https://dl.acm.org/doi/10.1145/3648188.3675127) functionality of htmx will remain the same, `hx-get`, `hx-post`,
+`hx-target`, `hx-boost`, `hx-swap`, `hx-trigger`, etc.
+
+Except for adding an `:inherited` modifier on a few attributes, many htmx projects will "just work" with htmx 4.  
+
+These changes will make the long term maintenance & sustainability of the project much stronger.  It will also take
+pressure off the 2.0 releases, which can now focus on stability rather than contemplating new features.
+
+## Upgrading
+
+That said, htmx 2.0 users _will_ face an upgrade project when moving to 4.0 in a way that they did not have to in moving
+from 1.0 to 2.0.  
+
+I am sorry about that, and want to offer three things to address it:
+
+* htmx 2.0 (like htmx 1.0 & intercooler.js 1.0) will be supported _in perpetuity_, so there is absolutely _no_ pressure to 
+  upgrade your application: if htmx 2.0 is satisfying your hypermedia needs, you can stick with it.
+* We will create extensions that revert htmx 4 to htmx 2 behaviors as much as is feasible (e.g. Supporting the old implicit
+  attribute inheritance model, at least)
+* We will roll htmx 4.0 out slowly, over a multi-year period.  As with the htmx 1.0 -> 2.0 upgrade, there will be a long
+  period where htmx 2.x is `latest` and htmx 3.x is `next`
+
+## New Features
+
+Of course, it isn't all bad.  Beyond simplifying the implementation of htmx significantly, switching to fetch also gives 
+us the opportunity to add some nice new features to htmx
+
+### Streaming Responses & SSE in Core
+
+By switching to `fetch()`, we can take advantage of its support for 
+[readable streams](https://developer.mozilla.org/en-US/docs/Web/API/Streams_API/Using_readable_streams), which
+allow for a stream of content to be swapped into the DOM, rather than a single response.
+
+htmx 1.0 had Server Sent Event support integrated into the library.  In htmx 2.0 we pulled this functionality out as an 
+extension.  It turns out that SSE is just a specialized version of a streaming response, so in adding streaming 
+support, it's an almost-free free two-fer to add that back into core as well.
+
+This will make incremental response swapping much cleaner and well-supported in htmx.
+
+### Morphing Swap in Core
+
+[Three years ago](https://github.com/bigskysoftware/idiomorph/commit/7760e89d9f198b43aa7d39cc4f940f606771f47b) I had
+an idea for a DOM morphing algorithm that improved on the initial algorithm pioneered by [morphdom](https://github.com/patrick-steele-idem/morphdom).
+
+The idea was to use "id sets" to make smarter decisions regarding which nodes to preserve and which nodes to delete when
+merging changes into the DOM, and I called this idea "idiomorph".  Idiomorph has gone on to be adopted by many other
+web project such as [Hotwire](https://hotwired.dev/).
+
+We strongly considered including it in htmx 2.0, but I decided not too because it worked well as an extension and
+htmx 2.0 had already grown larger than I wanted.
+
+In 4.0, with the complexity savings we achieved by moving to `fetch()`, we can now comfortably fit a `morphInner` and
+`morphOuter` swap into core, thanks to the excellent work of Michael West.
+
+### Explicit <partial> Tag Support
+
+htmx has, since very early on, supported a concept of "Out-of-band" swaps: content that is removed from the main HTML
+response and swapped into the DOM elsewhere.  I have always been a bit ambivalent about them, because they move away
+from [Locality of Behavior](https://htmx.org/essays/locality-of-behaviour/), but there is no doubt that they are useful
+and often crucial for achieving certain UI patterns.
+
+Out-of-band swaps started off very simply: if you marked an element as `hx-swap-oob='true'`, htmx would swap the element
+as the outer HTML of any existing element already in the DOM with that id.  Easy-peasy.
+
+However, over time, people started asking for different functionality around Out-of-band swaps: prepending, appending, 
+etc. and the feature began acquiring some fairly baroque syntax to handle all these needs.
+
+We have come to the conclusion that the problem is that there are really _two_ use cases, both currently trying to be
+filled by Out-of-band swaps:
+
+* A simple, id-based replacement
+* A more elaborate swap of partial content
+
+Therefore, we are introducing the notion of `<partial>`s in htmx 4.0
+
+A partial element is, under the covers, a template element and, thus, can contain any sort of content you like.  It 
+specifies on itself all the standard htmx options regarding swapping, `hx-target` and `hx-swap` in particular, allowing
+you full access to all the standard swapping behavior of htmx without using a specialized syntax.  This tremendously
+simplifies the mental model for these sorts of needs, and dovetails well with the streaming support we intend to offer.
+
+Out-of-band swaps will be retained in htmx 4.0, but will go back to their initial, simple focus of simply replacing 
+an existing element by id.
+
+### Improved View Transitions Support
+
+htmx 2.0 has have [View Transition](https://developer.mozilla.org/en-US/docs/Web/API/View_Transition_API) support since
+[April of 2023](https://github.com/bigskysoftware/htmx/blob/master/CHANGELOG.md#190---2023-04-11).  In the interceding
+two years, support for the feature has grown across browsers (c'mon, safari, you can do it) and we've gain experience
+with the feature.
+
+One thing that has become apparent to us while using them is that, to use them in a stable manner, it is important
+to establish a queue of transitions, so each can complete before the other begins.  If you don't do this, you can get
+visually ugly transition cancellations.
+
+So, in htmx 4.0 we have added this queue which will ensure that all view transitions complete smoothly.
+
+CSS transitions will continue to work as before as well, although the swapping model is again made much simpler by the 
+async runtime.
+
+We may enable View Transitions by default, the jury is still out on that.
+
+### Stabilized Event Ordering
+
+A wonderful thing about `fetch()` and the async support in general is that it is _much_ easier to guarantee a stable
+order of events.  By linearizing asynchronous code and allowing us to use standard language features like try/catch,
+the event model of htmx should be much more predictable and comprehensible.
+
+We are going to adopt a new standard for event naming to make things even clearer:
+
+`htmx:<phase>:<system>[:<optional-sub-action>]`
+
+So, for example, `htmx:before:request` will be triggered before a request is made.
+
+### Improved Extension Support
+
+Another opportunity we have is to take advantage of the `async` behavior of `fetch()` for much better performance in our
+preload extension (where we issue a speculative (`GET` only!) request in anticipation of an actual trigger).  We have 
+also added an optimistic update extension to the core extensions, again made easy by the new async features.
+
+In general, we have opened up the internals of the htmx request/response/swap cycle much more fully to extension developers,
+up to and including allowing them to replace the `fetch()` implementation used by htmx for a particular request.  There
+should not be a need for any hacks to get the behavior you want out of htmx now: the events and the open "context" object
+should provide the ability to do almost anything.
+
+### Improved `hx-on` Support
+
+In htmx 2.0, I somewhat reluctantly added the [`hx-on`](https://htmx.org/attributes/hx-on/) attributes to support light
+scripting inline on elements.  I added this because HTML does not allow you to listen for arbitrary events via `on` 
+attributes: only standard DOM events like `onclick` can be responded to.
+
+We hemmed and hawed about the syntax and so, unfortunately, there are a few different ways to do it.
+
+In htmx 4.0 we will adopt a single standard for the `hx-on` attributes: `hx-on:<event name>`.  Additionally, we are
+working to improve the htmx JavaScript API (especially around async operation support) and will make those features
+available in `hx-on`:
+
+```html
+<button hx-post="/like"
+        hx-on:htmx:after:swap="await timeout('3s'); ctx.newContent[0].remove()">
+    Get A Response Then Remove It 3 Seconds Later
+</button>
+```
+
+htmx will never support a fully featured scripting mechanism in core, we recommend something like 
+[Alpine.js](https://alpinejs.dev/) for that, but our hope is that we can provide a relatively minimalist API that
+allows for easy, light async scripting of the DOM.
+
+I should note that htmx 4.0 will continue to work with `eval()` disabled, but you will need to forego a few features like
+`hx-on` if you choose to do so.
+
+### A Better But Familiar htmx
+
+All in all, our hope is that htmx 4.0 will feel an awful lot like 2.0, but with better features and, we hope, with fewer bugs.
+
+## Timeline
+
+As always, software takes as long as it takes.  
+
+However, our current planned timeline is:
+
+* An alpha release is available _today_:  `htmx@4.0.0-alpha1`
+* A 4.0.0 release should be available in early-to-mid 2026
+* 4.0 will be marked `latest` in early-2027ish
+
+You can track our progress (and see quite a bit of dust flying around) in the `four` branch on 
+[github](https://github.com/bigskysoftware/htmx/tree/four) and at:
+
+<https://four.htmx.org>
+
+Thank you for your patience and pardon our dust!
+
+> "Well, when events change, I change my mind. What do you do?" --Paul Samuelson or John Maynard Keynes