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Mar 26, 2021 · There are two kinds of custom elements: Autonomous custom elements – “all-new” elements, extending the abstract HTMLElement class. Customized built-in elements – extending built-in elements, like a customized button, based on HTMLButtonElement etc. First we’ll cover autonomous elements, and then move to customized built-in ones.
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- Overview
- Types of custom element
- Implementing a custom element
- Registering a custom element
- Using a custom element
- Responding to attribute changes
- Examples
One of the key features of web components is the ability to create custom elements: that is, HTML elements whose behavior is defined by the web developer, that extend the set of elements available in the browser.
This article introduces custom elements, and walks through some examples.
There are two types of custom element:
•Customized built-in elements inherit from standard HTML elements such as HTMLImageElement or HTMLParagraphElement. Their implementation customizes the behavior of the standard element.
A custom element is implemented as a class which extends HTMLElement (in the case of autonomous elements) or the interface you want to customize (in the case of customized built-in elements).
Here's the implementation of a minimal custom element that customizes the element:
Here's the implementation of a minimal autonomous custom element:
In the class constructor, you can set up initial state and default values, register event listeners and perhaps create a shadow root. At this point, you should not inspect the element's attributes or children, or add new attributes or children. See Requirements for custom element constructors and reactions for the complete set of requirements.
To make a custom element available in a page, call the define() method of Window.customElements.
The define() method takes the following arguments:
name
The name of the element. This must start with a lowercase letter, contain a hyphen, and satisfy certain other rules listed in the specification's definition of a valid name.
constructor
The custom element's constructor function.
Once you've defined and registered a custom element, you can use it in your code.
To use a customized built-in element, use the built-in element but with the custom name as the value of the is attribute:
Like built-in elements, custom elements can use HTML attributes to configure the element's behavior. To use attributes effectively, an element has to be able to respond to changes in an attribute's value. To do this, a custom element needs to add the following members to the class that implements the custom element:
•A static property named observedAttributes. This must be an array containing the names of all attributes for which the element needs change notifications.
•An implementation of the attributeChangedCallback() lifecycle callback.
The attributeChangedCallback() callback is then called whenever an attribute whose name is listed in the element's observedAttributes property is added, modified, removed, or replaced.
The callback is passed three arguments:
•The name of the attribute which changed.
An autonomous custom element
First, we'll look at an autonomous custom element. The custom element takes an image icon and a text string as attributes, and embeds the icon into the page. When the icon is focused, it displays the text in a pop up information box to provide further in-context information. •See the example running live •See the source code
Referencing external styles
In the above example we apply styles to the shadow DOM using a
Customized built-in elements
Now let's have a look at a customized built in element example. This example extends the built-in element to support expanding and collapsing the list items. •See the example running live •See the source code
Code sample
class PopUpInfo extends HTMLElement {constructor() {super();...}...Sep 12, 2023 · A new object with the specified prototype object and properties. Exceptions. TypeError. Thrown if proto is neither null nor an Object. Examples. Classical inheritance with Object.create () Below is an example of how to use Object.create() to achieve classical inheritance. This is for a single inheritance, which is all that JavaScript supports. js.
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