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1.0.0 • Public • Published


Mara is a MIT licensed, pick-n-mix style base for Custom Elements built around providing mixins with extensions and common behaviors for web components.

Creating elements

The base of Mara is a set of extensions for Custom Elements to make it easier to build both simple and advanced elements.

Mara provides HTMLCustomElement which can be used as a base for elements:

import { HTMLCustomElement, define } from 'mara';
define('test-element', class extends HTMLCustomElement {
  createdCallback() {
    // This callback is invoked when the element is constructed
    // Make sure to always call the super-method of callbacks

Note: To maximize compatibility with different polyfills, use createdCallback instead of constructor during element creation.

HTMLCustomElement also provides support for mixins via the static method HTMLCustomElement.with(mixin1, mixin2, ...).


A bit of extra care is needed to support attributeChangedCallback. When specifying attributes to observe either in an element or a mixin care is needed to fetch the attributes of the super class and to call the callback of the super class:

class extends HTMLCustomElement.with(Mixin1, Mixin2) {
  static get observedAttributes() {
    // Be sure to combine own observed attributes with those of the super
    return [ 'attribute1', ...super.observedAttributes ];
  attributeChangedCallback(name, oldValue, newValue) {
    // Call super to make sure super class can handle its attributes
    switch(name) {
      case 'attribute1':
        // Handle own attribute

Core mixins

Initial rendering

The Custom Elements specification enforces things such as not changing attributes during element construction and most of the initialization of an element should be done in connectedCallback. But connectedCallback can be called several times and in many cases you only want to do something the first time an element is connected to the DOM.

The mixin InitialRender provides a callback that will only be called once:

import { HTMLCustomElement, InitialRender, define } from 'mara';
define('test-element', class extends HTMLCustomElement.with(InitialRender) {
  initialRenderCallback() {
    // This is called when we are first connected to the DOM
    // Make sure to always call the super-method of callbacks

Children Ready

If an element needs access to its children you can use the ChildrenReady mixin to get a callback when the children are available.

import { HTMLCustomElement, ChildrenReady, define } from 'mara';
define('test-element', class extends HTMLCustomElement.with(ChildrenReady) {
  childrenReadyCallback() {
    // This is called when the children of the element are available
    // Make sure to always call the super-method of callbacks

Help wanted: This implementation of this mixin could use an overhaul. If you have ideas or fixes, feel free to open an issue or a pull request.

Templates via Shadow DOM

A component can be enhanced with a shadow root initalized from a template via the Template mixin:

import { HTMLCustomElement, Template, define } from 'mara';
define('test-element', class extends HTMLCustomElement.with(Template) {
  static get templateHTML() {
    return `<h1>Template HTML goes here</h1>
  createdCallback() {
    // this.shadowRoot is initialized from the static template

Shadow DOM

If a component just needs access to a shadow root but no special template handling the ShadowDOM mixin will create a shadow root that can be used:

import { HTMLCustomElement, ShadowDOM, define } from 'mara';
define('test-element', class extends HTMLCustomElement.with(ShadowDOM) {
  createdCallback() {
    // this.shadowRoot is available here


Polyfills are provided for the web site or web app that is consuming the elements, in the case your target environment needs it.

To import the standard polyfills, such as for DOM4 you can use:

import 'mara/polyfill/standard';

Imported polyfill: DOM4.

To include a polyfill just for Custom Elements use:

import 'mara/polyfill/ce';

Imported polyfill: Custom Elements (v1)

For Custom Elements with support for Shadow DOM use:

import 'mara/polyfill/webcomponents';

Imported polyfill: Custom Elements (v1), ShadyDOM, ShadyCSS

Creating mixins

Mixins contain limited functionality that can be composed together to help with implementation of a new custom element.

import { Mixin } from 'mara';
export let CustomMixin = Mixin(superclass => class extends superclass {
  // Callbacks and functions go here

Mixins themselves can be composed with other mixins:

import { Mixin, ChildrenReady } from 'mara';
export let CustomMixin = Mixin(superclass => class extends superclass.with(ChildrenReady) {
  childrenReadyCallback() {
    // Some nifty code for the mixin

Basic behaviors

Mara contains a set of behaviors for creating certain components that need special attention to keyboard navigation, focus management and accessibility. These behaviors are provided as mixins and base classes.

Managing disabled state

These behaviors manage the disabled state of a component and optionally its children. When a component is disabled it is made inert both from pointer use and from keyboard use.

import { DisableBehavior /* or DisableSubtreeBehavior */ } from 'mara/disabled';
define('test-element', class extends HTMLCustomElement.with(DisableBehavior) {

The attribute disabled on the HTML element will reflect the disabled state, so <element-name disabled> will disable an element. An element can also be controlled via the disabled property:

element.disabled = true;

Use DisableBehavior if the element does not contain anything focusable or clickable. DisableSubtreeBehavior can be used to disable and maintain the disabled state of both the element and its children.

Focusable element

FocusableBehavior in mara/focus can be used to make an element focusable via keyboard and clicks:

import { FocusableBehavior } from 'mara/focus';
define('test-element', class extends HTMLCustomElement.with(FocusableBehavior) {

Focus locking

When implementing certain components, such as dialogs, it can be useful to lock the focus to a certain component. A locked focus means that keyboard navigation via Tab and Shift-Tab will cycle through focusable children of the element.

import { FocusLocking } from 'mara/focus';
define('test-element', class extends HTMLCustomElement.with(FocusLocking) {

The property focusLocked is made available on the element and can be used to control the focus lock:

element.focusLocked = true;

A special version of this behavior is SubtreeFocus which adds extensions for also focusing the first focusable child in the element.

import { SubtreeFocus } from 'mara/focus';
define('test-element', class extends HTMLCustomElement.with(SubtreeFocus) {

Two functions are available to control the focus:

  • element.grabFocus() will lock the focus and move focus within the element
  • element.releaseFocus() will unlock the focus and move focus the element that was focused when grabFocus was called.

Component behaviors

Component behaviors are a step up from the basic behaviors and provide mixins that make elements behave as common components.


The button behavior provides a button with correct keyboard control, ARIA roles and support for disabling it. The button will emit a click event when it is invoked both via pointer and keyboard.

// Using as a mixin
import { ButtonBehavior } from 'mara/buttons';
class extends HTMLCustomElement.with(ButtonBehavior) {}
// Using as a class
import { Button } from 'mara/buttons';
class extends Button {}

Submit Button

SubmitButton is a class/mixin that can be used to create a custom submit button. This behavior creates a button that submits the form it is a child of. To integrate properly with forms this behavior manages a hidden button, this is needed to make forms submittable via keyboard as with a regular <button>.

import { SubmitButton } from 'mara/buttons';
define('custom-submit', class extends SubmitButton {
  submitCallback() {
    // Called when button is invoked, can be used to set custom input values
    // Call super to actually submit form

Modal Dialog

A modal dialog is a dialog that is displayed over other content and only allows interaction with its children.

import { ModalDialog } from 'mara/dialogs';
define('custom-dialog', class extends ModalDialog {
  dialogOpenCallback() {
    // Call super to allow dialog to open
    // Do some extra stuff for dialog here
  dialogCloseCallback() {
    // Do some extra close stuff here
    // Continue to close dialog
  dialogDefaultCloseCallback() {
    // Call super to allow the dialog to be closed via default action

The attribute open on the HTML element will reflect if the dialog is open or not, so <custom-dialog open> indicates a dialog that is open. Setting the property open will also open the dialog: element.open = true.

The functions show and close can also be used to show and close the dialog.


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