# Head Support
The `head-support` extension adds support for [head tags](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/head) in responses to htmx requests.
htmx began as a library focused on partial replacement of HTML within the `body` tag. The [`hx-boost`](https://four.htmx.org/reference/attributes/hx-boost) attribute moved htmx closer to full HTML-document support, and support for extracting the `title` tag was eventually added, but full head tag support has never been a feature of the core library. This extension addresses that.
## Installing
```html
<script src="/path/to/htmx.js"></script>
<script src="/path/to/ext/head-support.js"></script>
```
## Usage
All responses that htmx receives that contain a `head` tag in them (even if they are not complete HTML documents with a root `<html>` element) will be processed.
How the head tag is handled depends on the type of htmx request.
If the htmx request is from a **boosted element**, then the following merge algorithm is used:
- Elements that exist in the current head as exact textual matches will be left in place
- Elements that do not exist in the current head will be added at the end of the head tag
- Elements that exist in the current head, but not in the new head will be removed from the head
If the htmx request is from a **non-boosted element**, then all content will be _appended_ to the existing head element.
If you wish to override this behavior in either case, you can place the `hx-head` attribute on the new `<head>` tag, with either of the following two values:
- `merge` - follow the merging algorithm outlined above
- `append` - append the elements to the existing head
### Controlling Merge Behavior
You may also control merging behavior of individual elements:
- If you place `hx-head="re-eval"` on a head element, it will be re-added (removed and appended) to the head tag on every request, even if it already exists. This can be useful to execute a script on every htmx request, for example.
- If you place [`hx-preserve`](https://four.htmx.org/reference/attributes/hx-preserve)`="true"` on an element, it will never be removed from the head.
### Example
Consider the following head tag in an existing document:
```html
<head>
<link rel="stylesheet" href="https://the.missing.style">
<link rel="stylesheet" href="/css/site1.css">
<script src="/js/script1.js"></script>
<script src="/js/script2.js"></script>
</head>
```
If htmx receives a request containing this new head tag:
```html
<head>
<link rel="stylesheet" href="https://the.missing.style">
<link rel="stylesheet" href="/css/site2.css">
<script src="/js/script2.js"></script>
<script src="/js/script3.js"></script>
</head>
```
Then the following operations will occur:
- `<link rel="stylesheet" href="https://the.missing.style">` will be left alone
- `<link rel="stylesheet" href="/css/site1.css">` will be removed from the head
- `<link rel="stylesheet" href="/css/site2.css">` will be added to the head
- `<script src="/js/script1.js"></script>` will be removed from the head
- `<script src="/js/script2.js"></script>` will be left alone
- `<script src="/js/script3.js"></script>` will be added to the head
## Events
- `htmx:removingHeadElement` - triggered when a head element is about to be removed. The element is available in `event.detail.headElement`. Call `preventDefault()` to keep it.
- `htmx:addingHeadElement` - triggered when a head element is about to be added. The element is available in `event.detail.headElement`. Call `preventDefault()` to skip it.
- `htmx:afterHeadMerge` - triggered after a head tag merge has occurred, with `detail.added`, `detail.kept`, and `detail.removed` arrays.
- `htmx:beforeHeadMerge` - triggered before a head merge occurs.
Merge head tag information (styles, scripts) in htmx responses
The head-support extension adds support for head tags in responses to htmx requests.
htmx began as a library focused on partial replacement of HTML within the body tag. The hx-boost attribute moved htmx closer to full HTML-document support, and support for extracting the title tag was eventually added, but full head tag support has never been a feature of the core library. This extension addresses that.
All responses that htmx receives that contain a head tag in them (even if they are not complete HTML documents with a root <html> element) will be processed.
How the head tag is handled depends on the type of htmx request.
If the htmx request is from a boosted element, then the following merge algorithm is used:
Elements that exist in the current head as exact textual matches will be left in place
Elements that do not exist in the current head will be added at the end of the head tag
Elements that exist in the current head, but not in the new head will be removed from the head
If the htmx request is from a non-boosted element, then all content will be appended to the existing head element.
If you wish to override this behavior in either case, you can place the hx-head attribute on the new <head> tag, with either of the following two values:
merge - follow the merging algorithm outlined above
You may also control merging behavior of individual elements:
If you place hx-head="re-eval" on a head element, it will be re-added (removed and appended) to the head tag on every request, even if it already exists. This can be useful to execute a script on every htmx request, for example.
If you place hx-preserve="true" on an element, it will never be removed from the head.
htmx:removingHeadElement - triggered when a head element is about to be removed. The element is available in event.detail.headElement. Call preventDefault() to keep it.
htmx:addingHeadElement - triggered when a head element is about to be added. The element is available in event.detail.headElement. Call preventDefault() to skip it.
htmx:afterHeadMerge - triggered after a head tag merge has occurred, with detail.added, detail.kept, and detail.removed arrays.
htmx:beforeHeadMerge - triggered before a head merge occurs.