Question

我正在尝试在自定义元素中使用继承。

那是我的页面

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8" />
  <title></title>
</head>

<body>
  <base-element></base-element>
  <language-drop-down></language-drop-down>

  <script type="module" src="./custom-elements/BaseHTMLElement.js"></script>
  <script type="module" src="./custom-elements/language-drop-down/js/LanguageDropDown.js"></script>
</body>

</html>

基本自定义元素

    export default class BaseHTMLElement extends HTMLElement {

    get currentDocument() { return document.currentScript.ownerDocument };

    constructor(params) {
        super();

        this.params = params;

        this.attachShadow({ mode: 'open' });
        debugger

        // Select the template and clone it. Finally attach the cloned node to the shadowDOM's root.
        // Current document needs to be defined to get DOM access to imported HTML
        if(this.params && this.params.templateName){
            this.template = this.currentDocument.querySelector(this.params.templateName);
            this.instance = this.template.content.cloneNode(true);

            this.shadowRoot.appendChild(this.instance);
        }else{
            let div = document.createElement("DIV");
            div.innerHTML = "element without template";
            this.shadowRoot.appendChild(div);
        }
    }
    connectedCallback() {
        this.loadLocalStrings();
    }
    childrenChangedCallback() {

    }
    disconnectedCallback() {

    }
    attributeChangedCallback(attrName, oldVal, newVal) {

    }

    loadLocalStrings() {

    }
   }

   window.customElements.define('base-element', BaseHTMLElement);

以及语言下拉自定义元素

import * as BaseHTMLElement from '../../BaseHTMLElement.js';

class LanguageDropDown extends BaseHTMLElement {

    constructor() {
        super({
            templateName: '#language-drop-down-template'
        });  
    }
    connectedCallback() {

        let dropdown = this.shadowRoot.querySelectorAll('.dropdown');        
        $(dropdown).dropdown();
    }
    childrenChangedCallback() {

    }
    disconnectedCallback() {

    }
    attributeChangedCallback(attrName, oldVal, newVal) {

    }
};

window.customElements.define('language-drop-down', LanguageDropDown);

及其模板部分

<template id="language-drop-down-template">
    <link href="./custom-elements/language-drop-down/css/languageDropDown.css" rel="stylesheet" />

    <div class="dropdown">
        <button class="btn dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
            Dropdown button
        </button>
        <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
            <a class="dropdown-item" href="#">Action</a>
            <a class="dropdown-item" href="#">Another action</a>
            <a class="dropdown-item" href="#">Something else here</a>
        </div>
    </div>
</template>

<script type="module" src="../BaseHTMLElement.mjs"></script>
<script type="module" src="./js/languageDropDown.mjs"></script>

现在，在chrome最新版本中，无需编译/使用polyfill就可以正常工作吗？我的问题是控制台中的此消息

未捕获的TypeError：类扩展值[object Module]不是构造函数或null

我不明白我哪里错了...有人有想法吗？

所有这些都是因为我想避免使用HTML导入，因为它已被弃用并将在M73中删除。

如果我使用

<script src="./custom-elements/BaseHTMLElement.js"></script>
<link rel="import" href="./custom-elements/language-drop-down/LanguageDropDown.html">

然后删除导出默认的BaseHTMLElement ... 从'../../BaseHTMLElement.js'中导入*作为BaseHTMLElement；

工作正常！

Answer 1

您的专线

import * as BaseHTMLElement from '../../BaseHTMLElement.js';

是错误的。该脚本将类导出为其默认导出。您要在此处将 all 导出作为模块名称空间对象导入-并在class extends子句中尝试像类一样使用它时，会得到“ [object Module] is not a constructor or null ”。

您应该使用

import {default as BaseHTMLElement} from '../../BaseHTMLElement.js';

或

import BaseHTMLElement from '../../BaseHTMLElement.js';

Answer 2

我发现使用

import BaseHTMLElement from '../../BaseHTMLElement.js';

使其有效

ES6自定义元素继承

2 个答案: