♻️ Idiomorph

Idiomorph is a javascript library for morphing one DOM tree to another. It is inspired by other libraries that
pioneered this functionality:

* morphdom - the original DOM morphing library
* nanomorph - an updated take on morphdom

Both morphdom and nanomorph use the id property of a node to match up elements within a given set of sibling nodes. When
an id match is found, the existing element is not removed from the DOM, but is instead morphed in place to the new content.
This preserves the node in the DOM, and allows state (such as focus) to be retained.

However, in both these algorithms, the structure of the _children_ of sibling nodes is not considered when morphing two
nodes: only the ids of the nodes are considered. This is due to performance: it is not feasible to recurse through all
the children of siblings when matching things up.

id sets

Idiomorph takes a different approach: before node-matching occurs, both the new content and the old content
are processed to create _id sets_, a mapping of elements to _a set of all ids found within that element_. That is, the
set of all ids in all children of the element, plus the element's id, if any.

Id sets can be computed relatively efficiently via a query selector + a bottom up algorithm.

Given an id set, you can now adopt a broader sense of "matching" than simply using id matching: if the intersection between
the id sets of element 1 and element 2 is non-empty, they match. This allows Idiomorph to relatively quickly match elements
based on structural information from children, who contribute to a parent's id set, which allows for better overall matching
when compared with simple id-based matching.

A testimonial:

> We are indeed using idiomorph and we'll include it officially as part of Turbo 8. We
> started with morphdom, but eventually switched to idiomorph as we found it way more suitable. It just worked great
> with all the tests we threw at it, while morphdom was incredibly picky about "ids" to match nodes. Also, we noticed
> it's at least as fast.
>
> -- Jorge Marubia / 37Signals

Installing

Idiomorph is a small (3.3k min/gz'd), dependency free JavaScript library. The /dist/idiomorph.js file can be included
directly in a browser:

``html

`

For production systems we recommend downloading and vendoring the library.

If you are using JavaScript Modules, we provide
three additional files:

* dist/idiomorph.amd.js - for AMD-style modules
* dist/idiomorph.cjs.js - for CommonJS-style modules
* dist/idiomorph.esm.js - for ESM-style modules

Idiomorph can be installed via NPM or your favorite dependency management system under the idiomorph dependency
name.

`js
require("idiomorph"); // CommonJS
import "idiomorph"; // ESM
`

Usage

Idiomorph has a very simple API:

`js
Idiomorph.morph(existingNode, newNode);
`

This will morph the existingNode to have the same structure as the newNode. Note that this is a destructive operation
with respect to both the existingNode and the newNode.

You can also pass string content in as the second argument, and Idiomorph will parse the string into nodes:

`js
Idiomorph.morph(existingNode, "

If you wish to target the innerHTML rather than the outerHTML of the content, you can pass in a morphStyle
in a third config argument:

`js
Idiomorph.morph(existingNode, "

| option (with default) | meaning | example |
|-------------------------------|------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------|
| morphStyle: 'outerHTML' | The style of morphing to use, either outerHTML or innerHTML | Idiomorph.morph(..., {morphStyle:'innerHTML'}) |
| ignoreActive: false | If true, idiomorph will skip the active element | Idiomorph.morph(..., {ignoreActive:true}) |
| ignoreActiveValue: false | If true, idiomorph will not update the active element's value | Idiomorph.morph(..., {ignoreActiveValue:true}) |
| restoreFocus: true | If true, idiomorph will attempt to restore any lost focus and selection state after the morph. | Idiomorph.morph(..., {restoreFocus:true}) |
| head: {style: 'merge', ...} | Allows you to control how the head tag is merged. See the head section for more details | Idiomorph.morph(..., {head:{style:'merge'}}) |
| callbacks: {...} | Allows you to insert callbacks when events occur in the morph lifecycle. See the callback table below | Idiomorph.morph(..., {callbacks:{beforeNodeAdded:function(node){...}}) |

#### Callbacks

Idiomorph provides the following callbacks, which can be used to intercept and, for some callbacks, modify the swapping behavior
of the algorithm.

| callback | description | return value meaning |
|-----------------------------------------------------------|----------------------------------------------------------------------------------------------------------------|----------------------------------------------------|
| beforeNodeAdded(node) | Called before a new node is added to the DOM | return false to not add the node |
| afterNodeAdded(node) | Called after a new node is added to the DOM | none |
| beforeNodeMorphed(oldNode, newNode) | Called before a node is morphed in the DOM | return false to skip morphing the node |
| afterNodeMorphed(oldNode, newNode) | Called after a node is morphed in the DOM | none |
| beforeNodeRemoved(node) | Called before a node is removed from the DOM | return false to not remove the node |
| afterNodeRemoved(node) | Called after a node is removed from the DOM | none |
| beforeAttributeUpdated(attributeName, node, mutationType) | Called before an attribute on an element is updated or removed (mutationType is either "update" or "remove") | return false to not update or remove the attribute |

$3

The head tag is treated specially by idiomorph because:

* It typically only has one level of children within it
* Those children often do not have id attributes associated with them
* It is important to remove as few elements as possible from the head, in order to minimize network requests for things
like style sheets
* The order of elements in the head tag is (usually) not meaningful

Because of this, by default, idiomorph adopts a merge algorithm between two head tags, old and new:

* Elements that are in both old and new are ignored
* Elements that are in new but not in old are added to old
* Elements that are in old but not in new are removed from old

Thus the content of the two head tags will be the same, but the order of those elements will not be.

#### Attribute Based Fine-Grained Head Control

Sometimes you may want even more fine-grained control over head merging behavior. For example, you may want a script
tag to re-evaluate, even though it is in both old and new. To do this, you can add the attribute im-re-append='true'
to the script tag, and idiomorph will re-append the script tag even if it exists in both head tags, forcing re-evaluation
of the script.

Similarly, you may wish to preserve an element even if it is not in new. You can use the attribute im-preserve='true'
in this case to retain the element.

#### Additional Configuration

You are also able to override these behaviors, see the head config object in the source code.

You can set head.style to:

* merge - the default algorithm outlined above
* append - simply append all content in new to old
* morph - adopt the normal idiomorph morphing algorithm for the head
* none - ignore the head tag entirely

For example, if you wanted to merge a whole page using the morph algorithm for the head tag, you would do this:

`js
Idiomorph.morph(document.documentElement, newPageSource, {head:{style: 'morph'}})
`

The head object also offers callbacks for configuring head merging specifics.

$3

All the behaviors specified above can be set to a different default by mutating the Idiomorph.defaults object, including
the Idiomorph.defaults.callbacks and Idiomorph.defaults.head objects.

$3

Idiomorph was created to integrate with htmx and can be used as a swapping mechanism by including
the dist/idiomorph-ext.js file in your HTML:

`html



`html
import "idiomorph/htmx";
`

Note that this file includes both Idiomorph and the htmx extension.

#### Configuring Morphing Behavior in htmx

The Idiomorph extension for htmx supports three different syntaxes for specifying behavior:

* hx-swap='morph' - This will perform a morph on the outerHTML of the target
* hx-swap='morph:outerHTML' - This will perform a morph on the outerHTML of the target (explicit)
* hx-swap='morph:innerHTML' - This will perform a morph on the innerHTML of the target (i.e. the children)
* hx-swap='morph:' - In this form, can be any valid JavaScript expression. The results of the expression
will be passed into the Idiomorph.morph() method as the configuration.

The last form gives you access to all the configuration options of Idiomorph. So, for example, if you wanted to ignore
the input value in a given morph, you could use the following swap specification:

`html
hx-swap="morph:{ignoreActiveValue:true}"
hx-target="closest form">
Morph The Closest Form But Ignore The Active Input Value

`

Performance

Idiomorph is not designed to be as fast as either morphdom or nanomorph. Rather, its goals are:

* Better DOM tree matching
* Relatively simple code

Performance is a consideration, but better matching is the reason Idiomorph was created. Our benchmarks indicate that
it is approximately equal to 10% slower than morphdom for large DOM morphs, and equal to or faster than morphdom for
smaller morphs. See the Performance document for more details.

Example Morph

Here is a simple example of some HTML in which Idiomorph does a better job of matching up than morphdom:

Initial HTML
`html


`html


`html


`html


Note that the iframe has an id on it, but the first-level divs do not have ids on them. This means
that morphdom is unable to tell that the video element has moved up, and the first div should be discarded, rather than morphed into, to preserve the video element.

Idiomorph, however, has an id-set for the top level divs, which includes the id of the embedded child, and can see that the video has moved to be a child of the first element in the top level children, so it correctly discards the first div and merges the video content with the second node.

You can see visually that idiomorph is able to keep the video running because of this, whereas morphdom is not:

!Rick Roll Demo

To keep things stable with morphdom, you would need to add ids to at least one of the top level divs.

Here is a diagram explaining how the two algorithms differ in this case:

!Comparison Diagram

Usage in the wild

* Datastar - uses idiomorph as its default merging strategy and embeds a Typescript port as part of its backend integration layer.
* Turbo - uses idiomorph to perform full page refreshing.