0.3.0 • Public • Published

♻️ 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.


Idiomorph is a small (1.7k min/gz'd), dependency free JavaScript library, and can be installed via NPM or your favorite dependency management system under the Idiomorph dependency name. You can also include it via a CDN like unpkg to load it directly in a browser:

<script src=""></script>

Or you can download the source to your local project.

Idiomorph has a very simple usage:

  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:

  Idiomorph.morph(existingNode, "<div>New Content</div>");

And it will be parsed and merged into the new content.

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:

  Idiomorph.morph(existingNode, "<div>New Content</div>", {morphStyle:'innerHTML'});

This will replace the inner content of the existing node with the new content.


Idiomorph supports the following options:

option meaning example
morphstyle The style of morphing to use, either innerHTML or outerHTML Idiomorph.morph(..., {morphStyle:'innerHTML'})
ignoreActive If set to true, idiomorph will skip the active element Idiomorph.morph(..., {ignoreActive:true})
ignoreActiveValue If set to true, idiomorph will not update the active element's value Idiomorph.morph(..., {ignoreActiveValue:true})
head 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 life cycle, see the callback table below Idiomorph.morph(..., {callbacks:{beforeNodeAdded:function(node){...}})


| callback | description | return value meaning |

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. mutationType is either "updated" or "removed" return false to not update or remove the attribute

The head tag

The head tag is treated specially by idiomorph because:

  • It typically only has one level of children within it
  • Those children often to 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 the 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 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:

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

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

Setting Defaults

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


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:

<script src=""></script>
<div hx-ext="morph">
    <button hx-get="/example" hx-swap="morph:innerHTML">
        Morph My Inner HTML

    <button hx-get="/example" hx-swap="morph:outerHTML">
        Morph My Outer HTML
    <button hx-get="/example" hx-swap="morph">
        Morph My Outer HTML

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:<expr>' - In this form, <expr> 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:

  <button hx-get="/example" 
          hx-target="closest form">
      Morph The Closest Form But Ignore The Active Input Value


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. Initial tests 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.

Example Morph

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

Initial HTML

        <p id="p1">A</p>
        <p id="p2">B</p>

Final HTML

        <p id="p2">B</p>
        <p id="p1">A</p>

Here we have a common situation: a parent div, with children divs and grand-children divs that have ids on them. This is a common situation when laying out code in HTML: parent divs often do not have ids on them (rather they have classes, for layout reasons) and the "leaf" nodes have ids associated with them.

Given this example, morphdom will detach both #p1 and #p2 from the DOM because, when it is considering the order of the children, it does not see that the #p2 grandchild is now within the first child.

Idiomorph, on the other hand, has an id set for the (id-less) children, which includes the ids of the grandchildren. Therefore, it is able to detect the fact that the #p2 grandchild is now a child of the first id-less child. Because of this information it is able to only move/detach one grandchild node, #p1. (This is unavoidable, since they changed order)

So, you can see, by computing id sets for nodes, idiomorph is able to achieve better DOM matching, with fewer node detachments.


You can see a practical demo of Idiomorph out-performing morphdom (with respect to DOM stability, not performance) here:

For both algorithms, this HTML:

        <iframe id="video" width="422" height="240" src=""
                title="Rick Astley - Never Gonna Give You Up (Official Music Video)" frameborder="0"
                allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"

is morphed into this HTML:

        <iframe id="video" width="422" height="240" src=""
                title="Rick Astley - Never Gonna Give You Up (Official Music Video)" frameborder="0"
                allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"

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



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