🔸custom element

browser ⟩ web components ⟩ implement ⟩ custom elements

associate a JavaScript class with an HTML tag name.

⭐️ 重點

define new custom elements with customElements.define().

// define new custom element
customElements.define(
  'web-component',                    // ⭐️ HTML tag name, must have "-"❗️ 
  class extends HTMLElement { ... }   // ⭐️ must extend `HTMLElement`❗️ 
);

custom elements render as inline elements by default.

🔴 主題

• lifecycle methods

• getter/setter for attributes - make element's attributes available as properties.

• Element Types

• Element Info

• Observing Attributes

• Custom Element Upgrade

• Element-defined Content

📘 手冊

• Window ⟩ .customElements (CustomElementRegistry)

• CustomElementRegistry ⟩ .define()

• ​​📘 Using custom elements ⭐️

• ​​📘 HTMLElement

• Google ⟩ Custom Elements ⭐️⭐️⭐️

• HTML Spec ⟩ valid custom element name

📗 參考

• JavaScript: The Definitive Guide (15.6 Web Components)

• can I use custom elements?

💈範例

💾 replit: <inline-circle>

📁 HTML

    <p> 
        The document has one marble: 
        <inline-circle size="3em" color="red"></inline-circle>. <br>
            
        The HTML parser instantiates two more marbles:
        <inline-circle size="2em" color="blue"></inline-circle>
        <inline-circle color="green"></inline-circle>.

        <hr>
            
        How many marbles does the document contain now? 
    </p>

📁 JS

// define custom element: 
customElements.define("inline-circle", class extends HTMLElement {

    // 🔸 when <inline-circle> inserted into document.
    connectedCallback() {        
        
        // styles for circles    
        // ⭐️ demo only: don't do this in real apps.    
        this.style.display = "inline-block";    
        this.style.borderRadius = "50%";     
        this.style.border = "solid black 1px";    
        this.style.transform = "translateY(10%)";         
        
        // default size    
        if (!this.size) this.size = "0.8em";
    }
    
    // 🔸 observed attributesstatic 
    get observedAttributes() { return ["size", "color"]; }
    
    // 🔸 when one of the attributes listed above changes.
    attributeChangedCallback(name, oldValue, newValue) {    
        switch (name) {                    
            case "size":            
                this.style.width = newValue;             
                this.style.height = newValue;             
                break;        
            case "color":            
                this.style.backgroundColor = newValue;             
                break;    
        }
    }
    
    // 🔸 getter/setter for element's attributes.
    get size() { return this.getAttribute("size"); } 
    set size(size) { this.setAttribute("size", size); } 
    get color() { return this.getAttribute("color"); } 
    set color(color) { this.setAttribute("color", color); }
});

Valid Custom Element Name

• 開頭：[a-z] ⭐️

• 接著：[0-9], [a-z], "-", ".", "_" (optional)

• 中間一定要有："-" ⭐️

• 接著：[0-9], [a-z], "-", ".", "_" (optional)

<vstack-> 或 <i-vstack> ❓(這兩個都是合法的名字)

PotentialCustomElementName ::= 
    [a-z] (PCENChar)* '-' (PCENChar)*
    
PCENChar ::=
    "-" | "." | [0-9] | "_" | [a-z] | #xB7 | 
    [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x37D] | [#x37F-#x1FFF] | 
    [#x200C-#x200D] | [#x203F-#x2040] | [#x2070-#x218F] | 
    [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | 
    [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]

Register New Tag Name

We can create our tags by using the CustomElementRegistry object (window.customElements).

• custom element names must contain a hyphen❗ 📘 customElements.define()
• Custom elements cannot be self-closing because HTML only allows a few elements to be self-closing. Always write a closing tag (<custom-element></custom-element>) 📘 Google ⟩ Custom Elements

// ⭐️ custom tag <my-component>
// ⭐️ custom element names must contain a "hyphen".
customElements.define('my-component', MyComponent);

Supporting Class

To create a custom element, we need to tell the browser several details about it: how to show it, what to do when the element is added or removed to page, etc.

class MyComponent extends HTMLElement {

  constructor() {
    super();
    // ...
  }

  // triggered when added to the document
  connectedCallback() {}

  // triggered when removed from the document
  disconnectedCallback() {}

  // attribute names to monitor for changes
  static get observedAttributes() {
    return [ /* ... */ ];
  }

  // triggered when one of attributes listed above is modified.
  // doesn’t trigger for unlisted attributes (for performance reasons).
  attributeChangedCallback(name, oldValue, newValue) {}

  // called when moved to a new document
  // happens in document.adoptNode, very rarely used
  adoptedCallback() {}

  // other methods and properties ...
}