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Mixin for Custom Elements (Web Components) to handle/sync properties and attributes.

What is this?

This class mixin adds functionality to your web components (custom elements v1) to help javascript properties and DOM element attributes be synchronised/reflected where configured. This allows automatic data-binding for both attributes and properties for a web component.

This is written with vanilla JavaScript. No external dependencies required.

No conflicting properties/methods are added to your class(es). The only properties/methods that are not part of the web component standards is the static get properties() method (can be named anything you like) used for configuring properties, and a non-enumerable Symbol property. This mixin makes use of the observedAttributes and attributeChangedCallback web component methods.

This JavaScript Module (ESM) exports a mixinPropertiesAttributes(base) method that will return a class which extends the provided base class.

This was designed to help your web components follow the best practices mentioned on web.dev.


Note: This project is written with new ECMAScript features such as ESM, arrow functions, object destructuring and etc. If you want to use this on older browsers, you may need to use a transpiler / bundling application, along with a polyfill.

Install via NPM, or github NPM package at @jashepp/ce-mixinprops

npm install ce-mixinprops --save

Or use via import with a CDN

import { mixinPropertiesAttributes, propTypes } from 'https://unpkg.com/ce-mixinprops'; // or https://cdn.jsdelivr.net/npm/ce-mixinprops

Or download the latest release, or git clone the repository on GitHub.


Simple display toggle (Javascript Module).

// Before running this example, install or specify the correct import paths
import { mixinPropertiesAttributes, propTypes } from 'ce-mixinprops/index.js'; // or https://unpkg.com/ce-mixinprops

// Define a class with the mixin on HTMLElement (or a class that extends it)
// * If you don't want the mixin within the class declaration, see below for alternate usage
export class exampleToggle extends mixinPropertiesAttributes(HTMLElement) {
	// Define the behaviour/options of properties
	static get properties() {
		return {
			show: {
				type: Boolean, // or propTypes.Boolean
				value: true
			hide: {
				type: Boolean,
				value: false
	// Toggle .style.display between none and previous value
	set show(show) {
		if(show) this.style.display = this._previousDisplay;
		else {
			this._previousDisplay = this.style.display || '';
			if(this._previousDisplay==='none') this._previousDisplay = '';
			this.style.display = 'none';
		this.hide = !show;
	set hide(hide) {
		this.show = !hide;
	constructor() {
		this._previousDisplay = this.style.display || '';

// Define a custom element name with the above class

HTML: (the show attribute will be set upon construction due to the default configured value above)

Hello, <example-toggle>Beautiful</example-toggle> World!

Rendered Result:

Hello, Beautiful World!

Freely remove the show or hide attributes in your browser developer tools and watch the property and rendered result change. Also change the show and/or hide properties on the element object itself via your developer tools console and watch the attribute and rendered result change.

Or see the example file for an interactive version.

If you don't want the mixin within the class declaration, it can also be applied afterwards instead:

import { mixinPropertiesAttributes, propTypes } from 'ce-mixinprops/index.js'; // or https://unpkg.com/ce-mixinprops

// Define a class without the mixin (which still extends HTMLElement)
class exampleToggle extends HTMLElement {
	// ...

// Define a custom element with the mixin applied after class declaration

However, to pass in a mixin's configuration with this method, provide it as the 3rd argument to mixinPropertiesAttributes.

More Examples

More examples are located within ./examples/ on the git repository on GitHub or locally if pulled. NPM version does not include the examples directory.

How To Use / API


Usage: mixinPropertiesAttributes(base,[propertiesName='properties'])

Simply import this module and call the mixinPropertiesAttributes method while extending HTMLElement (or any class that already extends it).

import { mixinPropertiesAttributes, propTypes } from 'ce-mixinprops/index.js'; // or https://unpkg.com/ce-mixinprops

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...

The first paramater of mixinPropertiesAttributes is the base class that you want to extend. If no argument is provided, it will default to extending HTMLElement.

The second paramater is an optional string which lets you specify the name of the properties static get method which is used for property & attribute configuration. This is configurable so you can have this mixin alongside other libraries.

Property/Attribute Configuration

Property definition example:

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...
	static get properties() {
		return {
			// Property/Attribute Name
			propName: {
				// All options are optional
				type: String, // or propTypes.String for newer behaviour
				value: 'World',
				reflectToAttribute: true,
				reflectFromAttribute: true,
				reflectFromProperty: true,
			// Additional configurations
	// ...

Properties are case sensitive, and attributes are not case sensitive (due to how attributes work). There cannot be duplicate properties with different cases. All options are optional.

Property Config Option Description (when true or specified) Default Value
type The type of property/attribute. Either use String, Number, Boolean (the actual object/keyword), any type on propTypes, or a string with a propType name. See Property Types. undefined
attribute Overwrite attribute name to differ from property name. See Alternate Attribute Name Property Name
value The default value of the property/attribute. See Default Value. undefined
reflectToAttribute true: Sync changes on the property to the attribute. See Data Reflection. Automatic
reflectToAttribute function: Transform value before being set as the attribute (type must not be valid). See Data Reflection. -
reflectFromAttribute true: Sync changes on the attribute to the property. See Data Reflection. Automatic
reflectFromAttribute function: Transform value after reading from the attribute (type must not be valid). See Data Reflection. -
reflectFromProperty true: Sync changes on from the property being set, to the property. See Data Reflection. Automatic
reflectFromProperty function: Transform value after reading from the attribute (type must not be valid). See Data Reflection. -
reflectToAttributeInConstructor Sets attribute in constructor when differs from default value. See Data Reflection. true if reflectToAttribute
readOnly Prevent the property from being modified. Attribute modifications will be ignored. See Data Reflection. false
delayChangeInConstructor Delay changes (set, observer, notify) in constructor. See Configuring Value Changes. true
observer A class method name (String) or an actual callback (Function) which is called upon change. See Configuring Value Changes. -
notify Emits an event on change with specified name (or propName-changed if not a string but truey). See Configuring Value Changes. false
overrideExisting This mixin checks the class and the classes the base extends to make sure the property does not already exist. This will ignore that check. false
order number: The sorting order in which the property gets setup on the class. -

Property Types

propTypes.Boolean: Value is converted with truey/falsy conversion via !!value. The property is true when the attribute exists, and false when the attribute does not exist (assuming the attribute is being reflected to/from the property).

propTypes.StringLegacy or String object/keyword: Null and Undefined are converted to an empty string '', and all other data types are converted by ''+value.

propTypes.NumberLegacy or Number object/keyword: Null and Undefined are converted to the number 0, and all other data types are converted by Number(value). Failed conversions may result with NaN.

propTypes.String: Can be either null or string. If the property is set to null or undefined, or if the attribute doesn't exist, it will be set as null. If the property is set as anything else, or if the attribute exists, it will be transformed via ''+value.

propTypes.Number: Can be either null or number. If the property is set to null, or if the attribute doesn't exist, it will be set as null. If the property is set as anything else, or if the attribute exists, it will be transformed via Number(value) to be either a number, or null (if it resulted in NaN).

Use the types specified on propTypes for future compatibility. The next major release might change String and Number objects/keywords to use the newer propTypes.String and propTypes.Number types.

type can be specified as a string with the name of the propType if you don't have access to propTypes at the time of class declaration.

You may also use your own type, if it has these 3 functions: toAttribute, fromAttribute, fromProperty. These are essentially the same as reflectToAttribute, reflectFromAttribute, and reflectFromProperty, respectively. See Data Reflection.

Alternate Attribute Name

If attribute is different than the property name, the property-attribute binding will use this value for the attribute name on the element. If the attribute already exists as a different property name (or attribute name if specified), an error will be thrown. It can be the name of an existing property if the property has an alternate attribute name.

Default Value

Upon construction (initialisation/upgrade of the element), if an attribute exists on the element, and it differs from the default value, it will cause a change. Except when reflectToAttribute is false (or fn results with null). If the attribute does not exist on the element, yet there is a default value, the attribute will be added to the element. Except for Boolean when false, when reflectToAttribute is false (or fn results with null), or when reflectToAttributeInConstructor is false.

The same applies if a property exists on the element upon construction. Where reflectFromProperty is used instead, and a property is set instead of an attribute.

If both an attribute and property exist on the element upon construction, see Upgrading Web Component.

reflectFromProperty will transform the value option before it's set as the property's default value. If reflectFromProperty is false, no default value will be set from the value option.

Data Reflection

For data transformation, see Data Transformation.

reflectToAttribute, reflectFromAttribute and reflectFromProperty default to true on a valid type. If they're set as false, it will simply disable the respective reflect method, so data transformation will not happen, and the attribute or property will not be set/changed.

If no type is specified, reflectFromProperty will default to true, so setting the property can be possible.

If readOnly is true, reflectFromAttribute and reflectFromProperty will default to false.

If readOnly is true, and the attribute's value or property's value is changed, the property will remain unchanged, so no changes will be emitted.

If reflectToAttributeInConstructor is false, the attribute will not be set during constructor. Handy for hidden attributes with default values.

Configuring Value Changes

The delayChangeInConstructor option, when true, delays all changes (set, observer, notify) during the constructor (and current call stack), with an event loop microtask, via Promise.resolve().then(). When false, the changes emit during the constructor as the change is made.

For the observer option, the Property Change Details Object will be the first argument.

All events emitted when the notify option is specified, will have event.detail set to the Property Change Details Object. The event will not bubble up the DOM.

It is recommended to use a set descriptor to listen for changes. See 'Watching For Changes' below.

Property Change Details Object

Property Description
element The custom element itself (this).
name The property/attribute name.
config The property/attribute configuration.
newValue The new value.
oldValue The old value.

Data Transformation

This mixin allows transform functions to determine what values get transformed to, when setting via an attribute, via a property, or when converting from property to attribute.

reflectToAttribute runs when an attribute is being set, and transforms the property value to an attribute value. When it results with null, the attribute will not exist or be removed.

reflectFromAttribute runs when fetching the value from the attribute or when the attribute is being set, and transforms the attribute value to a property value. When the attribute doesn't exist, or is removed, null is passed as the value argument.

reflectFromProperty runs when the property is being set, and transforms the setting property value to the actual property value. For example, when the default value is used, or when the property is set on the custom element.

If not specified, or when true, these options fallback to the type's toAttribute, fromAttribute and fromProperty transform functions, which behave the exact same way, respectively.

If any are specified as functions on the property/attribute config, they will override the type's transform functions. For example, you can use type: mixinTypes.Boolean, with reflectFromAttribute: (v)=>{ return v==null||v=='off'||v=='no'||v=='false' ? false : true; }, to allow specific attribute strings to be transformed differently. If all are specified as functions, then having type also set is redundant.

If any are specified with no type set, it will treat the other options as false. See Data Reflection for true/false reflect options.

The transform callbacks take one argument (the value), with the this keyword as the element.

Transform Example (with no type option):

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...
	static get properties() {
		return {
			propName: {
				value: 'test',
				// Remove prefixed _
				reflectFromAttribute: function(val){
					val = val===null ? '' : ''+val;
					return val.substr(0,1)==='_' ? val.substr(1) : val;
				// Add _ prefix
				reflectToAttribute: function(val){
					return '_'+val;
				// reflectFromProperty will simply pass through by default
			// Additional properties with configs
	// ...

A flag is set internally during reflectFromAttribute so a call stack overflow will not happen if you get the same property value within the transform callback. The get will instead result with the previous value or the default value (if no previous value) for that property.

Mixin Configuration

Upon construction, an optional options object can be passed to super() to configure this mixin.

Property Description (when specified)
protectedProperties An array of properties that are 'protected', meaning there can not be properties specified as such.
protectedAttributes An array of attributes that are 'protected', meaning there can not be attributes specified as such.
propertyStore An object which stores the values for the properties/attributes.
onPropertySet A callback which is called on any property/attribute change. The first paramater is the Property Change Details Object.
propertyDefaults An object of defaults for all properties/attributes (excluding overrideExisting option).
superArguments An array of arguments/paramaters passed to the base class constructor.

In addition to superArguments, the other arguments on super(), such as super(null,'one','two') (excluding first argument) will also passthrough to the extending class.

An example use case for onPropertySet, where you can do your own logic for your own property configure options:

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...
	static get properties() {
		return {
			propName: {
				type: Boolean,
				renderOnChange: true // own property to do logic elsewhere
				// observer: 'queueToRender' is also possible
			// Additional properties with configs
	// ...
	constructor() {
			onPropertySet: ({ element,name,config,newValue,oldValue })=>{
				// Queue changes to render if specified in property config
				if(config.renderOnChange) element.queueToRender();
	// ...
	queueToRender() {
		// Render code
	// ...

Existing Properties

An error will be thrown upon mixin construction if it detects duplicate properties, unless the overrideExisting option is specified.

Watching For Changes

Changes are emitted if the property or attribute (if reflectFromAttribute is true) has been changed. It can also emit shortly after construction if there is an attribute on the element that is different than the default property value.

The recommended way to listen for changes to a property is to have a set descriptor (setter) for that specific property.

The alternative is to use the observer or notify options on the property config (they may be overridden by extended classes), or the onPropertySet constructor option on the mixin config.

Setter Example:

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...
	set myPropertyName(newValue){
		console.log('myPropertyName has changed:',newValue);
		// your logic
	// ...

Changing the property (to a different value) within the set descriptor (setter) function may cause a call stack overflow, so be careful!

This feature only works if there is no get descriptor (getter) for that same property, unless the overrideExisting option is specified, which will remove the get descriptor.

There is no error handling or catching for functions/callbacks/events triggered by changes, so if an error occurs, it may effect the mixin's set code and further functions/callbacks/events (if multiple specified) will not emit.

Extending Classes

It is possible to have properties configured on a class (like the above examples), while also having a different class extend it with it's own configured properties. This mixin will search all parent constructors/prototypes for the static get properties method and combine all the properties & configurations (using Object.assign). All classes along the proto tree must have the same static get properties() method name, otherwise it will be ignored.

Properties & configurations on parent classes are overridden by classes which extend them. This means that if the parent class has observer or notify options set, they may be overridden.

However, a set descriptor (setter) will not be overridden and will fire upon property change for all classes in the proto tree where it is defined. They will fire in order from the top most parent class, down to the last extended class. It is done this way so parent logic is done first before extended logic.

When extending and you want to use observedAttributes or attributeChangedCallback, be sure to use super. For example:

class myCustomElement extends mixinPropertiesAttributes(HTMLElement) {
	// ...
	static get observedAttributes() {
		// Your logic
		let attributes = ['your','list','of','attributes','in','lowercase'];
		// Concat with super
		return (super.observedAttributes||[]).concat(attributes);
		// Call super method
		if(super.attributeChangedCallback) super.attributeChangedCallback(name,oldValue,newValue);
		// Your logic
	// ...

Upgrading Web Component

Web components / custom elements are designed to be lazy loaded. So attributes and/or properties can exist or be set before the web component is defined/upgraded (customElements.define), and can also be set after it has been defined.

When the element is first defined, this mixin will treat existing attributes and properties as changes if they differ from the default values for the configured properties. See Default Value.

If both a same-named property and attribute exists before the element is defined, this mixin will prioritise the attribute's value. This is due to attributeChangedCallback receiving attribute-change notifications after the element has been defined. So in this case, if you want to change the value when both exist, before and during definition, then use setAttribute, as it will queue further changes for attributeChangedCallback to handle.


Tests are located within ./tests/ on the git repository on GitHub or locally if pulled. NPM version does not include tests.

To get started with tests, enter local directory of this repository and run:

npm install --only=dev

This should install cypress and other dev dependancies needed to run the tests.

To run a UI interactive version of the tests using cypress, run:

npm run cypress+server

To run a CLI-only version of the tests, run:

npm run test

To continuously run tests while editing, run:

npm run watch:test


To submit a contribution, create issues or pull requests on the GitHub project.

Please be sure to run tests after any changes.

All help is appreciated. Even if it's just improvements to this readme or the tests.


MIT License

Copyright (c) 2023 Jason Sheppard @ https://github.com/Jashepp

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.



Github Repository: https://github.com/Jashepp/customElements-mixinPropertiesAttributes

NPM Package: https://npmjs.org/package/ce-mixinprops

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