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JavaScript错误

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我刚开始并进行代码培训，第一个任务是构建一个Web应用程序。 为了获得良好的结果，代码必须没有错误地运行，不幸的是我的代码没有错误地运行：

main.js：11未捕获的TypeError：无法读取null的属性'addEventListener'     在main.js：11 nextBtn.addEventListener（'click'，nextSlide）;

当我对此进行注释时，下一个错误返回： main.js：12未捕获的TypeError：无法读取null的属性'addEventListener'     在main.js：12

当我对此进行注释时，下一个错误返回： main.js：27未捕获的TypeError：无法读取null的属性“ getcontext”     在main.js：27

这些错误是在我从引导程序中获取标准模板时发生的。

将寻求任何帮助。

我也想使用jquery进行表单验证，但是Visuals studio代码无法识别$

此致

Bas 

enter code here     https://jsfiddle.net/Basso/vkrogpf6/3/

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1,shrink-to-fit=no">
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
    <link href="CSS/style.css" rel="stylesheet"/>
    <title>Eindopdracht WebOntwikkeling</title>
  </head>
  <body>

      <nav class="navbar navbar-expand-lg navbar-light bg-light">

          <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
          </button>

          <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
              <li class="nav-item active">
                <a class="nav-link" href="index.html">Home <span class="sr-only">(current)</span></a>
              </li>
              <li class="nav-item">
                <a class="nav-link" href="Form.html">Formulier</a>
              </li>
              <li class="nav-item">
                  <a class="nav-link" href="audio_video.html">Audio Video</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="Afbeeldingen.html">Afbeeldingen</a>
                  </li>
                  <li class="nav-item">
                      <a class="nav-link" href="Blog.html">Blog</a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link" href="Krant.html">Nieuws</a>
                      </li>
            </ul>
          </div>
        </nav>
 <canvas class="myCanvas1"></canvas> 

<img id="myImage" src="../Eindopdracht/Images/bcg-3.jpg" alt="palmboom" />

<svg id="mySvg" height="210" width="400">
  <path d="M150 0 L75 200 L225 200 Z" />
</svg>


    <script src="JS/main.js"></script>
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnqq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmvgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
  </body>
</html>

    const nextBtn = document.querySelector('.nextBtn');
const prevBtn = document.querySelector('.prevBtn');


nextBtn.addEventListener('click',nextSlide);
prevBtn.addEventListener('click',prevSlide);


function nextSlide(){

    document.body.style.backgroundImage = "url('Images/bcg-3.jpg')";

}

function prevSlide(){
    document.body.style.backgroundImage = "url('Images/bcg-4.jpg')";
}


var canvas = document.querySelector("myCanvas1");
var ctx = canvas.getcontext("2d");
var img = document.getElementById ("myImage");
ctx.drawImage(img,canvas.width,canvas.height);



enter code here 

<!doctype html>
<html lang="en">
  <head>
     <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,shrink-to-fit=no">
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
    <link href="CSS/style.css" rel="stylesheet"/>
    <title>Eindopdracht WebOntwikkeling</title>
  </head>
  <body class="page1">

      <nav class="navbar navbar-expand-lg navbar-light bg-light">

          <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
          </button>

          <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
              <li class="nav-item active">
                <a class="nav-link" href="index.html">Home <span class="sr-only">(current)</span></a>
              </li>
              <li class="nav-item">
                <a class="nav-link" href="Form.html">Formulier</a>
              </li>
              <li class="nav-item">
                  <a class="nav-link" href="audio_video.html">Audio Video</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="Afbeeldingen.html">Afbeeldingen</a>
                  </li>
                  <li class="nav-item">
                      <a class="nav-link" href="Blog.html">Blog</a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link" href="Krant.html">Nieuws</a>
                      </li>
            </ul>
          </div>
        </nav>




  <button type="button" class="btn prevBtn">PrevBtn</button>
  <button type="button" class="btn nextBtn">NextBtn</button>


    <script src="JS/main.js"></script>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
    <script src="https://cdn.jsdelivr.net/npm/jquery-validation@1.19.1/dist/jquery.validate.js"></script> 
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnqq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmvgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>

  </body>
</html>
s5705219 回答：JavaScript错误

您没有nextBtn类的按钮...您将在prevBtn上遇到相同的错误。

并且由于您试图将事件监听器添加到不存在的html元素中，因此会出现这些错误。

将按钮添加到html中，或将当前按钮类更改为nextBtn，一切都会好起来的。

,

看起来nextBtn没有函数“ addEventListener”。确保“ addEventListener”是有效函数。其他错误。

javascriptnulltypeerror
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