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Easily diff HTML with transitions and tagged template strings, website is a WIP

diffHTML: A JavaScript View Layer

Latest stable version: 0.9.2

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Inspired by React and motivated by the Web, this is a lowish-level tool which aims to help web developers write components for the web. By focusing on the markup representing how your application state should look, diffHTML will figure out how to modify the page with a minimal amount of operations. This tool is designed and optimized around HTML and standards features within JavaScript.

It works by parsing your HTML markup into a lightweight JSON-serializable Virtual DOM heirarchy. I refer to these as Virtual Trees or VTree. These element (and attribute) objects are pooled to provide consistent memory management and garbage collection. diffHTML maintains a single VTree root that mirrors a mounted element in the DOM, it reconciles all future renders into this tree and the DOM.


  • Helps you build components using HTML and JavaScript
  • Provides a tagged template function to inline your markup with dynamic interpolation into your component code
  • Your markup is compiled to a VTree and patched efficiently to the DOM
  • Transitions API to hook into element and attribute state changes
  • Object pooling to avoid GC thrashing and expensive childNode/attribute/uuid generation


The latest built version is available for quick download from the master branch.

Or you can use npm:

npm install diffhtml

The module can be required via Node or browser environments. It is exported as a global named diff unless loaded as a module.

You can import only what you need if you’re using ES6 modules with a transpiler:

import { innerHTML } from 'diffhtml';

innerHTML(document.body, 'Hello world!');

diffHTML is authored using many modern browser features, such as Set, which are not available in all browsers.

If you wish to use diffHTML in older browsers, make sure you have the Babel polyfill loaded first:


Module format locations

The codebase is authored with JavaScript (ES6 enhancements) and transpiled into ES5 (CJS) into the dist/cjs folder on every NPM publish. If you want to reference the ES5 files, point your entry to diffhtml/dist/cjs. If you want ES6, point to diffhtml/lib. If you want the UMD build you can simply point to diffhtml as it is the default or reference dist/diffhtml.js in your browser.

Quick start

Before diving into all the API details, the easiest way to understand using diffHTML is to replace the usage of innerHTML.

For example, the following example destroys and recreates the span every time the render method is called:

Assume the following markup:


The following code:

function render(string) {
  document.querySelector('span').innerHTML = string;

render('Hello world!');
render('Foo bar baz!');

We could rewrite this with diffHTML to leverage the Virtual DOM like this:

function render(string) {
  diff.innerHTML(document.querySelector('span'), string);


The exposed API provides the following methods:

The follow error types are exposed so you can test exceptions:

  • TransitionStateError - Happens when errors occur during transitions.
  • DOMException - Happens whenever a DOM manipulation fails.

This is an optional argument that can be passed to any diff method. The inner property can only be used with the element method.

  • inner - Boolean that determines if innerHTML is used.
Diff an element with markup

This method will take in a string of markup that matches the element root you are diffing against. This allows you to change attributes and text on the main element. This also allows you to change the document.documentElement.

You cannot override the inner options property here.

diff.outerHTML(document.body, '<body class="test"><h1>Hello world!</h1></body>');
Diff an element’s children with markup

This method also takes in a string of markup, but unlike outerHTML this is children-only markup that will be nested inside the element passed.

You cannot override the inner options property here.

diff.innerHTML(document.body, '<h1>Hello world!</h1>');
Diff an element to another element

Unlike the previous two methods, this will take in two elements and diff them together.

The inner options property can be set here to change between inner/outerHTML.

var newBody = document.createElement('body');

newBody.innerHTML = '<h1>Hello world!</h1>';
newBody.setAttribute('class', 'test');

diff.element(document.body, newBody);

With inner set:

var h1 = document.createElement('h1');

h1.innerHTML = 'Hello world!';

diff.element(document.body, h1, { inner: true });
Release element

Use this method if you need to clean up memory allocations and anything else internal to diffHTML associated with your element. This is very useful for unit testing and general cleanup when you’re done with an element.

var h1 = document.createElement('h1');

h1.innerHTML = 'Hello world!';

diff.element(document.body, h1, { inner: true });
Add a transition state callback

Adds a global transition listener. With many elements this could be an expensive operation, so try to limit the amount of listeners added if you’re concerned about performance.

Since the callback triggers with various elements, most of which you probably don’t care about, you’ll want to filter. A good way of filtering is to use the DOM matches method. It’s fairly well supported (http://caniuse.com/#feat=matchesselector) and may suit many projects. If you need backwards compatibility, consider using jQuery’s is.

You can do fun, highly specific, filters:

addTransitionState('attached', function(element) {
  // Fade in the main container after it's attached into the DOM.
  if (element.matches('body main.container')) {
    $(element).stop(true, true).fadeIn();

If you like these transitions and want to declaratively assign them in tagged templates, check out the diffhtml-inline-transitions plugin.

Available states

Format is: name[callbackArgs]

  • attached[element] For when an element is attached to the DOM.
  • detached[element] For when an element leaves the DOM.
  • replaced[oldElement, newElement] For when elements are swapped
  • attributeChanged[element, attributeName, oldValue, newValue] For when attributes are changed.
  • textChanged[element, oldValue, newValue] For when text has changed in either TextNodes or SVG text elements.
A note about detached/replaced element accuracy

When rendering Nodes that contain lists of identical elements, you may not receive the elements you expect in the detached and replaced transition state hooks. This is a known limitation of string diffing and allows for better performance. By default if no key is specified, the last element will be removed and the subsequent elements from the one that was removed will be mutated via replace.

This isn’t really ideal. At all.

What you should do here is add a key attribute with a unique value that persists between renders.

For example, when the following markup…


…is changed into…


The transformative operations are:

  1. Remove the last element
  2. Replace the text of the second element to ‘out’

What we intended, however, was to simply remove the second item. And to achieve that, decorate your markup like so…

  <li key="1">Test</li>
  <li key="2">This</li>
  <li key="3">Out</li>

…and update with matching attributes…

  <li key="1">Test</li>
  <li key="3">Out</li>

Now the transformative operations are:

  1. Remove the second element
Remove a transition state callback

Removes a global transition listener.

When invoked with no arguments, this method will remove all transition callbacks. When invoked with the name argument it will remove all transition state callbacks matching the name, and so on for the callback.

// Removes all registered transition states.

// Removes states by name.

// Removes states by name and callback reference.
diff.removeTransitionState('attached', callbackReference);


You can use the diff.html tagged template helper to build up dynamic trees in a way that looks very similar to JSX.

For instance the following example:

const fixture = document.createElement('div');

function showUnixTime() {
  fixture.querySelector('span').innerHTML = Date.now();

diff.outerHTML(fixture, `
    <button>Show current unix time</button>

fixture.addEventListener('click', showUnixTime);

Could be rewritten with the helper as:

const fixture = document.createElement('div');

function showUnixTime() {
  fixture.querySelector('span').innerHTML = Date.now();

diff.outerHTML(fixture, html`
  <div onclick=${showUnixTime}>
    <button>Show current unix time</button>

So this feature allows for inline binding of any DOM event, and sending dynamic property data to any element.

Tagged templates also have no problem consuming other tagged templates (even from arrays), so you will be able to do:

const fixture = document.createElement('div');

const listItems = ['diff', 'HTML', '♥'];

diff.outerHtml(fixture, html`
    ${listItems.map(item => html`<li>${item.text}</li>`)}


Click above to learn what prollyfill “means”.

I’d love to see this project become a browser standard in the future. To enable how I’d envision it working, simply invoke the following method on the diff object:


Disclaimer: By calling this method, you are agreeing that it’s okay for diffHTML to modify your browser’s HTMLElement constructor, Element.prototype, the document object, and run some logic on your page load event.


Scans for changes in attributes and text on the parent, and all child nodes.

document.querySelector('main').diffOuterHTML = '<new markup to diff/>';

Only scans for changes in child nodes.

document.querySelector('main').diffInnerHTML = '<new child markup to diff/>';

Compares the two elements for changes like outerHTML, if you pass { inner: true } as the second argument it will act like innerHTML.

var newElement = document.createElement('main');
newElement.innerHTML = '<div></div>';


Cleans up after diffHTML and removes the associated worker.

var newElement = document.createElement('main');
newElement.innerHTML = '<div></div>';


More information and a demo are available on http://www.diffhtml.org/

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