如何修复“未捕获的类型错误:无法调用 null 的方法‘appendChild’" [英] How to fix "Uncaught TypeError: Cannot call method 'appendChild' of null"

问题描述

我正在尝试从 HTML 文本区域获取文本，并在按下提交"按钮时调用 create() 方法.该方法试图使用来自文本区域的消息，并将其发布到它自己的带有类的 p 标签，并在它自己的 p 标签和它自己的类中发布一个日期戳.

I am attempting to grab text from a HTML text area, and call the create() method when a 'Submit' button is pressed. The method is trying to use the message from the text area, and post that to its own p tag with class, and post a date stamp in its own p tag, and its own class.

这些都将在 div 的评论"中.我得到的错误(在 Chrome 中使用开发者工具)是

These will both be in the div 'comments'. The error I am getting (using developer tools in Chrome), is

未捕获的类型错误:无法调用 null 的方法appendChild".

Uncaught TypeError: Cannot call method 'appendChild' of null.

这是针对cmt.appendChild(divTag);"的.我对 Javascript 很陌生，这只是我提高技能的练习.非常感谢所有帮助！

This is aimed at "cmt.appendChild(divTag);". I am very new to Javascript, and this is just practise for me to increase my skills. All help is greatly appreciated!

var cmt = document.getElementById('comments');

function create() {

    var username = 'User',
        message = document.getElementById("textBox").value,
        divTag = document.createElement('div'),
        p1 = document.createElement('p'),
        p2 = document.createElement('p');

    divTag.className = 'comment';

    p1.className = 'date';
    p1.innerHTML = new Date();
    divTag.appendChild(p1);

    p2.className = 'message';
    p2.innerHTML = username + ': ' +message;
    divTag.appendChild(p2);

    cmt.appendChild(divTag);
}

推荐答案

您收到的错误表明您的页面上没有 ID 为comments"的元素.document.getElementById 将返回null 当没有找到具有该 ID 的元素时，因此 cmd.appendChild(divTag) 将作为 null.appendChild(divTag) 执行.

The error you are getting suggests that there is no element with the ID "comments" on your page. document.getElementById will return null when no element with such an ID is found, and thus cmd.appendChild(divTag) will be executed as null.appendChild(divTag).

如果您确定该元素存在，您可能正在执行分配 cmt 变量的 JavaScript，在浏览器创建该元素之前.为防止这种情况发生，标准做法是将包含您的外部 JavaScript 的 <script> 标记放在结束 </body> 标记.

If you are certain that the element exists, you may be executing your JavaScript that assigns the cmt variable before that element is created by the browser. To prevent that, standard practice is to place the <script> tag which includes your external JavaScript just before the closing </body> tag.

如果由于某种原因无法移动脚本标记，请尝试运行使用 $(document).ready() (jQuery) 或等效.

If you can't move your script tag for some reason, try running the code that assigns the variable with $(document).ready() (jQuery) or equivalent.

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