🔸<template>

browser ⟩ web components ⟩ implement ⟩ HTML templates

⭐️ 重點

<template> tags (and their children)：

• never rendered by web browser.

• only useful on web pages that use JavaScript.

• template.content is a DocumentFragment.

• can be cloned by template.content.cloneNode().

• when cloning/inserting template.content into document, the fragment itself will not be inserted, but its children will.

Use a <template> element to clone DOM, instead of setting shadowRoot.innerHTML. 👉 Custom Elements ⟩ Element-defined Content

📗 參考

• <template> element - js.info

📘 手冊

• 📘 Using templates and slots

  • Using templates with web components

• <template>

• Node ⟩ .cloneNode()

• 📘 DocumentFragment

• HTMLTemplateElement

  • .content (DocumentFragment)

👥 相關

• Custom Events ⟩ Element-defined Content

• DOM hierarchy

• content of <template> is a DocumentFragment.

HTML templates examples

1️⃣

💾 replit.com

• can put <style> and <script> into <template> as well.

<!-- styles and scripts in template -->
<template>

  <style>
    p { font-weight: bold; }
  </style>
  
  <script>
    alert("Hello");
  </script>
  
</template>

2️⃣

• content becomes live (styles apply, scripts run etc) when inserted into document.

<!-- template -->
<template id="tmpl">
  <script>
    console.log("Hello");
  </script>
  <div class="message">Hello, world!</div>
</template>

<!-- script -->
<script>

  let div = document.createElement('div');

  // ⭐️ clone template content
  //    tmpl            : HTMLTemplateElement
  //    tmpl.content    : DocumentFragment
  //    node.cloneNode(): Node (return type)
  div.append(tmpl.content.cloneNode(true));

  // ⭐️ inserted into document
  // ⭐️ now script (from <template>) runs
  document.body.append(div);    
  
</script>

3️⃣

• we can use <template> to populate shadow root.

📁 script.js 👉 replit

div.onclick = function() {

    // attach shadow root to <div> (shadow host)
    let root = div.attachShadow({mode: 'open'});

    // clone template content (<style>, <p>)
    let clonedContent = tmpl.content.cloneNode(true);

    // append to shadow root
    // ⭐️ 注意：light DOM (text node: "Click me") 被 shadow DOM 取代了❗️
    root.append(clonedContent);

    let p = root.$('#message');
    p.innerHTML = "Hello from the shadows❗️";
}; 

📁 index.html

<!-- ⭐️ template -->
<template id="tmpl">
	<style>
		p {
			font-weight: bold;
                margin: 0 auto;
		}
	</style>
	<p id="message"></p>
</template>

<!-- shadow host -->
<div id="div">Click me❗️</div>

<!-- scripts -->
<script src="helpers.js"></script>
<script src="script.js"></script>

👉 helpers.js

HTML

<!-- web component -->
<my-component>
  a "none" error message
</my-component>

<my-component kind="warning">
  this is a warning
</my-component>

<my-component kind="error">
  this is an error message
</my-component>

JS

// ⭐️ 1. custom web component
class MyComponent extends HTMLElement {

    // constructor
    constructor() {

        // tell HTMLElement to initialize itself.
        super();

    }

    // choose which template to use
    connectedCallback() {

        // create <div> elements
        this.templates = document.createElement('div');
        this.container = document.createElement('div');

        // ⭐️ add a shadow DOM to component
        //    to activate/deactivate templates
        this.root = this.attachShadow({mode: 'open'});
        this.root.appendChild(this.templates); 
        this.root.appendChild(this.container);

        // fill in templates
        this.templates.innerHTML = MyComponent.template(); 
        
        // choose which template to use
        const kind = this.getAttribute(`kind`) || `none`;
        const template = this.templates.querySelector(`template.${kind}-type`); 
        
        if (template) {
            const clone = template.content.cloneNode(true);
            this.container.innerHTML = '';
            this.container.appendChild(clone); 
        }

    }

    // template HTML
    static template() {

        // 3 templates
        // - template.warning-type
        // - template.error-type
        // - template.none-type
        return ` 
            <template class="warning-type">
                <style> 
                    .warning { 
                        background-color: yellow; 
                        padding: 15px; 
                        color: red; 
                    } 
                </style>

                <div class="warning">
                    <slot>Error component<slot> 
                </div> 
            </template>

            <template class="error-type">
                <style>
                    .error { 
                        background-color: red; 
                        padding: 15px; 
                        color: yellow; 
                    }
                </style>

                <div class="error">
                    <slot>Error component<slot> 
                </div> 
            </template>

            <template class="none-type">
                <style> 
                    .none { 
                        background-color: gray; 
                        padding: 15px; 
                        color: #eee; 
                    } 
                </style>

                <div class="none">
                    <slot>Error component<slot> 
                </div> 
            </template>
        `;
    }
} 

// ⭐️ 2. custom tag <my-component>
// ⭐️⭐️ custom element names must contain a "hyphen".
customElements.define('my-component', MyComponent);

codepen