FEZ - Custom DOM Elements

Check the Demo site https://dux.github.io/fez/

FEZ is a small library (20kb minified) that allows writing of Custom DOM elements in a clean and easy-to-understand way.

It uses

• Goober to enable runtime SCSS (similar to styled components)
• Idiomorph to morph DOM from one state to another (as React or Stimulus/Turbo does it)

Latest version of libs are baked in Fez distro.

It uses minimal abstraction. You will learn to use it in 15 minutes, just look at examples, it includes all you need to know.

How to install

<script src="https://dux.github.io/fez/dist/fez.js"></script>

Little more details

Uses DOM as a source of truth and tries to be as close to vanilla JS as possible. There is nothing to learn or "fight", or overload or "monkey patch" or anything. It just works.

Although fastest, Modifying DOM state directly in React / Vue / etc. is considered an anti-pattern. For Fez this is just fine if you want to do it. Fez basically modifies DOM, you just have a few helpers to help you do it.

It replaces modern JS frameworks by using native Autonomous Custom Elements to create new HTML tags. This has been supported for years in all major browsers.

This article, Web Components Will Replace Your Frontend Framework, is from 2019. Join the future, ditch React, Angular and other never defined, always "evolving" monstrosities. Vanilla is the way :)

There is no some "internal state" that is by some magic reflected to DOM. No! All methods Fez use to manipulate DOM are just helpers around native DOM interface. Work on DOM raw, use built in node builder or full template mapping with morphing.

How it works

• define your custom component - Fez('ui-foo', class UiFoo extends FezBase)
• add HTML - <ui-foo bar="baz" id="node1"></ui-foo>
  • lib will call node1.fez.init() when node is added to DOM and connect your component to dom.
  • use Fez helper methods, or do all by yourself, all good.

That is all.

Example: Counter Component

Here's a simple counter component that demonstrates Fez's core features:

<!-- Define a counter component in ex-counter.fez.html -->
<script>
  // called when Fez node is connected to DOM
  init() {
    this.MAX = 6
    this.state.count = 0
  }

  isMax() {
    return this.state.count >= this.MAX
  }

  // is state is changed, template is re-rendered
  more() {
    this.state.count += this.isMax() ? 0 : 1
  }
</script>

<style>
  /* compiles from scss to css and injects class in head */
  /* body style */
  background-color: #f7f7f7;

  /* scoped to this component */
  :fez {
    zoom: 2;
    margin: 10px 0;

    button {
      position: relative;
      top: -3px;
    }

    span {
      padding: 0 5px;
    }
  }
</style>

<button onclick="fez.state.count -= 1" disabled={{ state.count == 1 }}>-</button>

<span>
  {{ state.count }}
</span>

<button onclick="fez.more()" disabled={{ isMax() }}>+</button>
{{if state.count > 0}}
  <span>&mdash;</span>
  {{if state.count == MAX }}
    MAX
  {{else}}
    {{#if state.count % 2 }}
      odd
    {{else}}
      even
    {{/if}}
  {{/if}}
{{/if}}

To use this component in your HTML:

<!-- Load Fez library -->
<script src="https://dux.github.io/fez/dist/fez.js"></script>

<!-- Load component via template tag -->
<template fez="/fez-libs/ex-counter.fez.html"></template>

<!-- Use the component -->
<ex-counter></ex-counter>

This example showcases:

• Reactive state: Changes to this.state automatically update the DOM
• Template syntax: {{ }} for expressions, @ as shorthand for this.
• Event handling: Direct DOM event handlers with access to component methods
• Conditional rendering: {{#if}}, {{:else}} blocks for dynamic UI
• Scoped styling: SCSS support with styles automatically scoped to component
• Component lifecycle: init() method called when component mounts

What can it do and why is it great?

Core Features

• Native Custom Elements - Creates and defines Custom HTML tags using the native browser interface for maximum performance
• Server-Side Friendly - Works seamlessly with server-generated HTML, any routing library, and progressive enhancement strategies
• Semantic HTML Output - Transforms custom elements to standard HTML nodes (e.g., <ui-button> → <button class="fez fez-button">), making components fully stylable with CSS
• Single-File Components - Define CSS, HTML, and JavaScript in one file, no build step required
• No Framework Magic - Plain vanilla JS classes with clear, documented methods. No hooks, runes, or complex abstractions
• Runtime SCSS - Style components using SCSS syntax via Goober, compiled at runtime
• Smart Memory Management - Automatic garbage collection cleans up disconnected nodes every 5 seconds

Advanced Templating & Styling

• Powerful Template Engine - Multiple syntaxes ({{ }} and [[ ]]), control flow (#if, #unless, #for, #each), and block templates
• Reactive State Management - Built-in reactive state object automatically triggers re-renders on property changes
• DOM Morphing - Uses Idiomorph for intelligent DOM updates that preserve element state and animations
• Preserve DOM Elements - Use fez-keep="unique-key" attribute to preserve DOM elements across re-renders (useful for animations, form inputs, or stateful elements)
• DOM Memoization - Use fez-memoize="key" attribute to memoize and restore DOM content by key (component-scoped) or <fez-memoize key="unique-key"> component for global memoization
• Style Macros - Define custom CSS shortcuts like Fez.cssMixin('mobile', '@media (max-width: 768px)') and use as :mobile { ... }
• Scoped & Global Styles - Components can define both scoped CSS (:fez { ... }) and global styles in the same component

Developer Experience

• Built-in Utilities - Helpful methods like formData(), setInterval() (auto-cleanup), onWindowResize(), and nextTick()
• Two-Way Data Binding - Use fez-bind directive for automatic form synchronization
• Advanced Slot System - Full <slot /> support with event listener preservation
• Publish/Subscribe - Built-in pub/sub system for component communication
• Global State Management - Automatic subscription-based global state with this.globalState proxy
• Dynamic Component Loading - Load components from URLs with <template fez="path/to/component.html">
• Auto HTML Correction - Fixes invalid self-closing tags (<fez-icon name="gear" /> → <fez-icon name="gear"></fez-icon>)

Performance & Integration

• Optimized Rendering - Batched microtask rendering for flicker-free component initialization
• Smart DOM Updates - Efficient DOM manipulation with minimal reflows
• Built-in Fetch with Caching - Fez.fetch() includes automatic response caching and JSON/FormData handling
• Global Component Access - Register components globally with GLOBAL = 'ComponentName' for easy access
• Rich Lifecycle Hooks - init, onMount, beforeRender, afterRender, onDestroy, onPropsChange, onStateChange, onGlobalStateChange
• Development Mode - Enable detailed logging with Fez.DEV = true

Why It's Great

• Zero Build Step - Just include the script and start coding
• 20KB Minified - Tiny footprint with powerful features
• Framework Agnostic - Use alongside React, Vue, or any other framework
• Progressive Enhancement - Perfect for modernizing legacy applications one component at a time
• Native Performance - Leverages browser's native Custom Elements API
• Intuitive API - If you know vanilla JavaScript, you already know Fez

Full available interface

Fez Static Methods

Fez('#foo')                  // find fez node with id="foo"
Fez('ui-tabs', this)         // find first parent node ui-tabs
Fez('ui-tabs', (fez)=> ... ) // loop over all ui-tabs nodes

// define custom DOM node name -> <foo-bar>...
Fez('foo-bar', class {
  // set element node name, set as property or method, defaults to DIV
  // why? because Fez renames custom dom nodes to regular HTML nodes
  NAME = 'span'
  NAME(node) { ... }
  // alternative: static nodeName = 'span'

  // set element style, set as property or method
  CSS = `scss string... `

  // define static HTML. calling `this.render()` (no arguments) will refresh current node.
  // if you pair it with `reactiveStore()`, to auto update on props change, you will have Svelte or Vue style reactive behaviour.
  HTML = `...`

  // Make it globally accessible as `window.Dialog`
  // The component is automatically appended to the document body as a singleton. See `demo/fez/ui-dialog.fez` for a complete example.
  GLOBAL = 'Dialog'
  GLOBAL = true // just append node to document, do not create window reference

  // called when fez element is connected to dom, before first render
  // here you still have your slot available in this.root
  init(props) { ... }

  // execute after init and first render
  onMount() { ... }

  // execute before or after every render
  beforeRender() { ... }
  afterRender() { ... }

  // if you want to monitor new or changed node attributes
  // monitors all original node attributes
  // <ui-icon name="home" color="red" />
  onPropsChange(attrName, attrValue) { ... }

  // called when local component state changes
  onStateChange(key, value, oldValue) { ... }

  // called when global state changes (only if component uses key in question that key)
  onGlobalStateChange(key, value) { ... }

  // called when component is destroyed
  onDestroy() { ... }

  /* used inside lifecycle methods (init(), onMount(), ... */

  // copy original attributes from attr hash to root node
  this.copy('href', 'onclick', 'style')

  // set style property to root node. look at a clock example
  // shortcut to this.root.style.setProperty(key, value)
  this.setStyle('--color', 'red')

  // clasic interval, that runs only while node is attached
  this.setInterval(func, tick) { ... }

  // get closest form data, as object. Searches for first parent or child FORM element
  this.formData()

  // mounted DOM node root. Only in init() point to original <slot /> data, in onMount() to rendered data.
  this.root

  // mounted DOM node root wrapped in $, only if jQuery is available
  this.$root

  // node attributes, converted to properties
  this.props

  // gets single node attribute or property
  this.prop('onclick')

  // shortcut for this.root.querySelector(selector)
  this.find(selector)

  // gets value for FORM fields or node innerHTML
  this.val(selector)
  // set value to a node, uses value or innerHTML
  this.val(selector, value)

  // you can publish globally, and subscribe locally
  Fez.publish('channel', foo)
  this.subscribe('channel', (foo) => { ... })

  // gets root childNodes
  this.childNodes()
  this.childNodes(func)  // pass function to loop forEach on selection, mask nodes out of position

  // check if the this.root node is attached to dom
  this.isConnected

  // this.state has reactiveStore() attached by default. any change will trigger this.render()
  this.state.foo = 123

  // generic window event handler with automatic cleanup
  // eventName: 'resize', 'scroll', 'mousemove', etc.
  // delay: throttle delay in ms (default: 200ms)
  this.on(eventName, func, delay)

  // window resize event with cleanup (shorthand for this.on('resize', func, delay))
  // runs immediately on init and then throttled
  this.onWindowResize(func, delay)

  // window scroll event with cleanup (shorthand for this.on('scroll', func, delay))
  // runs immediately on init and then throttled
  this.onWindowScroll(func, delay)

  // requestAnimationFrame wrapper with deduplication
  this.nextTick(func, name)

  // get unique ID for root node, set one if needed
  this.rootId()

  // get/set attributes on root node
  this.attr(name, value)

  // hide the custom element wrapper and move children to parent
  this.fezHide()

  // automatic form submission handling if there is FORM as parent or child node
  this.onSubmit(formData) { ... }

  // render template and attach result dom to root. uses Idiomorph for DOM morph
  this.render()
  this.render(this.find('.body'), someHtmlTemplate) // you can render to another root too
})

/* Utility methods */

// define custom style macro
// Fez.cssMixin('mobile', '@media (max-width:  768px)')
// :mobile { ... } -> @media (max-width:  768px) { ... }
Fez.cssMixin(name, value)

// add global scss
Fez.globalCss(`
  .some-class {
    color: red;
    &.foo { ... }
    .foo { ... }
  }
  ...
`)

// internal, get unique ID for a string, poor mans MD5 / SHA1
Fez.fnv1('some string')

// get dom node containing passed html
Fez.domRoot(htmlData || htmlNode)

// activates node by adding class to node, and removing it from siblings
Fez.activateNode(node, className = 'active')

// get generated css class name, from scss source string
Fez.css(text)

// get generated css class name without global attachment
Fez.cssClass(text)

// display information about registered components in console
Fez.info()

// low-level DOM morphing function
Fez.morphdom(target, newNode, opts)

// HTML escaping utility
Fez.htmlEscape(text)

// create HTML tags with encoded props
Fez.tag(tag, opts, html)

// execute function until it returns true
Fez.untilTrue(func, pingRate)

// resolve and execute a function from string or function reference
// useful for event handlers that can be either functions or strings
// Fez.resolveFunction('alert("hi")', element) - creates function and calls with element as this
// Fez.resolveFunction(myFunc, element) - calls myFunc with element as this
Fez.resolveFunction(pointer, context)

// add scripts/styles to document head
// Load JavaScript from URL: Fez.head({ js: 'path/to/script.js' })
// Load JavaScript with attributes: Fez.head({ js: 'path/to/script.js', type: 'module', async: true })
// Load JavaScript with callback: Fez.head({ js: 'path/to/script.js' }, () => console.log('loaded'))
// Load JavaScript module and auto-import to window: Fez.head({ js: 'path/to/module.js', module: 'MyModule' })
// Load CSS: Fez.head({ css: 'path/to/styles.css' })
// Load CSS with attributes: Fez.head({ css: 'path/to/styles.css', media: 'print' })
// Execute inline script: Fez.head({ script: 'console.log("Hello world")' })
Fez.head(config, callback)

Fez script loading and definition

  <!-- Remote loading for a component via URL in fez attribute -->
  <!-- Component name is extracted from filename (ui-button) -->
  <!-- If remote HTML contains template/xmp tags with fez attributes, they are compiled -->
  <!-- Otherwise, the entire content is compiled as the component -->
  <script fez="path/to/ui-button.fez.html"></script>

  <!-- prefix with : to calc before node mount -->
  <foo-bar :size="document.getElementById('icon-range').value"></foo-bar>

  <!-- pass JSON props via data-props -->
  <foo-bar data-props='{"name": "John", "age": 30}'></foo-bar>

  <!-- pass JSON template via data-json-template -->
  <script type="text/template">{...}</script>
  <foo-bar data-json-template="true"></foo-bar>

Component structure

All parts are optional

<!-- Head elements support (inline only in XML tags) -->
<xmp tag="some-tag">
  <head>
    <!-- everything in head will be copied to document head-->
    <script>console.log('Added to document head, first script to execute.')</script>
  </head>

  <script>
    class {
      init(props) { ... }     // when fez node is initialized, before template render
      onMount(props) { ... }  // called after first template render
    }
  </script>
  <script> // class can be omitted if only functions are passed
    init(props) { ... }
  </script>

  <style>
    b {
      color: red; /* will be global style*/
    }

    :fez {
      /* component styles */
    }
  </style>
  <style>
    color: red; /* if "body {" or ":fez {" is not found, style is considered local component style */
  </style>

  <div> ... <!-- any other html after head, script or style is considered template-->
    <!-- resolve any condition -->
    {{if foo}} ... {{/if}}

    <!-- unless directive - opposite of if -->
    {{unless fez.list.length}}
      <p>No items to display</p>
    {{/unless}}

    <!-- runs in node scope, you can use for loop -->
    {{each fez.list as name, index}} ... {{/each}}
    {{for name, index in fez.list}} ... {{/for}}

    <!-- Block definitions -->
    {{block image}}
      <img src={{ props.src}} />
    {{/block}}
    {{block:image}} <!-- Use the header block -->
    {{block:image}} <!-- Use the header block -->

    {{raw data}} <!-- unescape HTML -->
    {{json data}} <!-- JSON dump in PRE.json tag -->

    <!-- fez-this will link DOM node to object property (inspired by Svelte) -->
    <!-- linkes to -> this.listRoot -->
    <ul fez-this="listRoot">

    <!-- when node is added to dom fez-use will call object function by name, and pass current node -->
    <!-- this.animate(node) -->
    <li fez-use="animate">

    <!-- fez-bind for two-way data binding on form elements -->
    <input type="text" fez-bind="state.username" />

    <!--
      fez-class for adding classes with optional delay.
      class will be added to SPAN element, 100ms after dom mount (to trigger animations)
    -->
    <span fez-class="active:100">Delayed class</span>

    <!-- preserve state by key, not affected by state changes-->>
    <p fez-keep="key">...</p>

    <!-- memoize DOM content by key (component-scoped) -->
    <!-- stores DOM on first render, restores on subsequent renders with same key -->
    <div fez-memoize="unique-key">expensive content</div>

    <!-- :attribute for evaluated attributes (converts to JSON) -->
    <div :data-config="state.config"></div>
  </div>
</xmp>

how to call custom FEZ node from the outside, anywhere in HTML

Inside init(), you have pointer to this. Pass it anywhere you need, even store in window.

Example: Dialog controller

<ui-dialog id="main-dialog"></ui-dialog>

Fez('ui-dialog', class {
  init() {
    // makes dialog globally available
    window.Dialog = this
  }

  close() {
    ...
  }
})

// close dialog window, from anywhere
Dialog.close()

// you can load via Fez + node selector
Fez('#main-dialog').close()

Fez.fetch API

Fez includes a built-in fetch wrapper with automatic JSON parsing and session-based caching:

Basic Usage

// GET request with promise
const data = await Fez.fetch('https://api.example.com/data')

// GET request with callback, does not create promise
Fez.fetch('https://api.example.com/data', (data) => {
  console.log(data)
})

// POST request
const result = await Fez.fetch('POST', 'https://api.example.com/data', { key: 'value' })

Features

• Automatic JSON parsing: Response is automatically parsed if Content-Type is application/json
• Session caching: All requests are cached in memory until page refresh
• Flexible parameter order: Method can be omitted (defaults to GET), callback can be last parameter
• Error handling: When using callbacks, errors are passed to Fez.onError with kind 'fetch'
• Logging: Enable with Fez.LOG = true to see cache hits and live fetches

Custom Error Handler

// Override default error handler
Fez.onError = (kind, error) => {
  if (kind === 'fetch') {
    console.error('Fetch failed:', error)
    // Show user-friendly error message
  }
}

Default Components

Fez includes several built-in components available when you include defaults.js:

fez-component

Dynamically includes a Fez component by name:

<fez-component name="some-node" :props="fez.props"></fez-component>

fez-include

Loads remote HTML content via URL:

<fez-include src="./demo/fez/ui-slider.html"></fez-include>

fez-inline

Creates inline components with reactive state:

<fez-inline :state="{count: 0}">
  <button onclick="fez.state.count += 1">+</button>
  {{ state.count }} * {{ state.count }} = {{ state.count * state.count }}
</fez-inline>

fez-memoize

Memoizes DOM content by key (global scope):

<!-- First render: stores the content -->
<fez-memoize key="unique-key">
  <expensive-component></expensive-component>
</fez-memoize>

<!-- Subsequent renders: restores stored content instantly -->
<fez-memoize key="unique-key">
  <!-- Content here is ignored, stored version is used -->
</fez-memoize>

Global State Management

Fez includes a built-in global state manager that automatically tracks component subscriptions. It automatically tracks which components use which state variables and only updates exactly what's needed.

How it Works

• Components access global state via this.globalState proxy
• Reading a value by key automatically subscribes the component to changes to that key.
• Setting a value notifies all subscribed components to that key.
• Components are automatically cleaned up when disconnected

Basic Usage

class Counter extends FezBase {
  increment() {
    // Setting global state - all listeners will be notified
    this.globalState.count = (this.globalState.count || 0) + 1
  }

  render() {
    // Reading global state - automatically subscribes this component
    return `<button onclick="fez.increment()">
      Count: ${this.globalState.count || 0}
    </button>`
  }
}

External Access

// Set global state from outside components
Fez.state.set('count', 10)

// Get global state value
const count = Fez.state.get('count')

// Iterate over all components listening to a key
Fez.state.forEach('count', (component) => {
  console.log(`${component.fezName} is listening to count`)
})

Optional Change Handler

Components can define an onGlobalStateChange method for custom handling:

class MyComponent extends FezBase {
  onGlobalStateChange(key, value) {
    console.log(`Global state "${key}" changed to:`, value)
    // Custom logic instead of automatic render
    if (key === 'theme') {
      this.updateTheme(value)
    }
  }

  render() {
    // Still subscribes by reading the value
    return `<div class="${this.globalState.theme || 'light'}">...</div>`
  }
}

Real Example: Shared Counter State

// Multiple counter components sharing max count
class Counter extends FezBase {
  init(props) {
    this.state.count = parseInt(props.start || 0)
  }

  beforeRender() {
    // All counters share and update the global max
    this.globalState.maxCount ||= 0

    // Find max across all counter instances
    let max = 0
    Fez.state.forEach('maxCount', fez => {
      if (fez.state?.count > max) {
        max = fez.state.count
      }
    })

    this.globalState.maxCount = max
  }

  render() {
    return `
      <button onclick="fez.state.count++">+</button>
      <span>Count: ${this.state.count}</span>
      <span>(Global max: ${this.globalState.maxCount})</span>
    `
  }
}