Custom elements are part of Web Components, allowing new HTML tags to be created and given behavior and styles. It is implemented by inheriting the HTMLElement class, registering the class as a custom tag and using three steps in HTML; it also supports life cycle callback functions such as connectedCallback, disconnectedCallback, attributeChangedCallback, etc.; when defining, it is necessary to note that the tag name must contain hyphens, it is recommended to use Shadow DOM encapsulation style, use to define complex structures, and maintain the logic of the component to improve reusability.
Creating custom elements is not complicated, but it requires understanding some basic concepts and steps. HTML provides standard features of Web Components, including Custom Elements. Through it, you can create your own HTML tags and give them specific behaviors and styles.
What are custom elements?
Custom elements are part of Web Components, allowing you to define new HTML element tags, such as <my-button></my-button> or <user-card></user-card> . These elements can be used like native elements and can be reused across multiple projects.
In terms of browser support, modern mainstream browsers already support custom elements well, but it is best to confirm whether the target environment is compatible before use.
How to define a basic custom element
To create a custom element, there are three main steps:
- Write a class inherited from
HTMLElement - Use
customElements.define()to register this class as a custom tag - Use this new tag in HTML
For example, here is a simple custom button component:
class MyButton extends HTMLElement {
constructor() {
super();
const text = this.getAttribute('text') || 'Click me';
this.innerHTML = `<button>${text}</button>`;
}
}
customElements.define('my-button', MyButton);Use in HTML:
<my-button text="Submit"></my-button>
This way a button can be displayed on the page. While this example is simple, it shows the basic structure and how custom elements are used.
Custom element's life cycle callback function
Custom elements provide some life cycle hook functions to facilitate your operations at different stages:
-
connectedCallback: Called when an element is inserted into the document -
disconnectedCallback: Called when an element is removed from the document -
attributeChangedCallback(attrName, oldVal, newVal): Called when the attribute changes -
adoptedCallback: Called when the element is moved to a new document (less used)
If you want the component to respond to property changes, such as color or size adjustment, you can combine observedAttributes static method to listen for specific properties:
static get observedAttributes() {
return ['color'];
}
attributeChangedCallback(name, oldValue, newValue) {
if (name === 'color') {
this.style.color = newValue;
}
} This way, when you modify the color attribute of <my-button color="red"></my-button> , the text color will change.
Notes and best practices
Some details are easily overlooked when using custom elements:
- Custom tag names must contain hyphens (such as
my-button) to prevent conflicts with future HTML standards - Try to encapsulate styles in Shadow DOM to avoid global style pollution
- If you need complex template content, you can use the
<template></template>tag to define the structure in advance - Component logic is as modular as possible for maintenance and reuse
Basically that's it. Once you have mastered these key points, you can start trying to build your own reusable components.
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