標籤命名必定要加「-」,浪費了半個小時。。。java
class MxmBox extends HTMLElement { constructor() { super(); console.log(this); let but=document.createElement('button'); but.innerText='第一個按鈕'; this._shadowDom = this.attachShadow({ mode: "open" }); this._shadowDom.appendChild(but); } } customElements.define("mx-m", MxmBox);