JavaScript中的HTMLCollection

本文作者: jsweibo

本文链接: https://jsweibo.github.io/2020/06/15/JavaScript%E4%B8%AD%E7%9A%84HTMLCollection/

摘要

本文主要讲述了：

什么是HTMLCollection
作用
实例属性
原型方法

正文

什么是`HTMLCollection`

一个类数组对象，原型为Object.prototype

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      // true
      console.log(
        Object.getPrototypeOf(HTMLCollection.prototype) === Object.prototype
      );
    </script>
  </body>
</html>

作用

表示 HTML 元素集合，作为document.getElementsByClassName()、document.getElementsByTagName()、ParentNode.children、document.forms、document.scripts、document.links等的返回值类型

示例：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <script>
      console.log(document.getElementsByClassName('foo'));
      console.log(document.getElementsByTagName('p'));
      console.log(document.getElementsByTagName('p')[0].children);
      console.log(document.forms);
      console.log(document.scripts);
      console.log(document.links);
    </script>
  </body>
</html>

注意：HTMLCollection是动态的，当 DOM 发生变更的时候，它也会自动更新

示例：动态的HTMLCollection

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <script>
      const tempHTMLCollection = document.getElementsByClassName('foo');
      console.log(tempHTMLCollection.length);
      window.addEventListener('click', function () {
        const foo = document.createElement('p');
        foo.textContent = 'hello, jsweibo';
        foo.className = 'foo';
        document.body.appendChild(foo);
        console.log(tempHTMLCollection.length);
      });
    </script>
  </body>
</html>

实例属性

length

实例中 HTML 元素的个数

注意：该属性为可枚举属性，这意味着你不应该使用for...in遍历HTMLCollection实例的属性名称

示例：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      console.log(Object.getOwnPropertyDescriptors(HTMLCollection.prototype));
      for (let key in HTMLCollection.prototype) {
        console.log(key);
      }
    </script>
  </body>
</html>

如果需要遍历HTMLCollection实例中的 HTML 元素，可以使用Array.prototype.forEach或for...of

示例：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <script>
      const tempHTMLCollection = document.getElementsByClassName('foo');
      Array.prototype.forEach.call(tempHTMLCollection, function (value) {
        console.log(value);
      });
      for (let value of tempHTMLCollection) {
        console.log(value);
      }
    </script>
  </body>
</html>

原型方法

item

用于获取HTMLCollection实例中指定下标的元素，可以使用[]替代

示例：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <script>
      const tempHTMLCollection = document.getElementsByClassName('foo');
      console.log(tempHTMLCollection.item(0));
    </script>
  </body>
</html>

注意：该属性为可枚举属性，这意味着你不应该使用for...in遍历HTMLCollection实例的属性名称

namedItem

用于获取HTMLCollection实例中指定name或id的元素

示例：获取name=test的元素

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <p class="foo" name="test">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <script>
      const tempHTMLCollection = document.getElementsByClassName('foo');
      console.log(tempHTMLCollection.namedItem('test'));
    </script>
  </body>
</html>

示例：获取id=test的元素

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <p class="foo">hello, jsweibo</p>
    <p class="foo" id="test">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <p class="foo">hello, jsweibo</p>
    <script>
      const tempHTMLCollection = document.getElementsByClassName('foo');
      console.log(tempHTMLCollection.namedItem('test'));
    </script>
  </body>
</html>