为什么我总是得到“无法调用 null 的方法'addEventListener'”

Why do I keep getting "Cannot call method 'addEventListener' of null"?

本文关键字：null 方法 无法调用 addEventListener 调用 为什么      更新时间：2023-09-26

我在webdev方面非常缺乏经验，但在这里我正在制作一个网站。我使用 Fabric.js 和来自 stackoverflow 的答案的脚本，以便用户可以将图像从一个div 拖到画布中。当我测试它时，我总是收到错误"无法调用 null 的方法'addEventListener'"。我想这意味着脚本尝试加载的任何内容都没有加载，因此将其保留为空。控制台说它来自我的JS文件中的第77行（canvasContainer.addEventListener（'dragenter'...，靠近底部）。我尝试将导入脚本代码向下移动（在正文下方），以便它在所有其他内容之后加载，但仍然不起作用。有什么想法吗？

javascript来自natchiketa。

.HTML：

<!doctype html>
<html lang="en";>
<head>
    <meta charset="utf-8" />
    <title>CustomCase</title>
    <link rel="stylesheet" href="SkapaDesign.css">
    <script src="Jquery.js"></script>
    <script src="Modernizr.js"></script>
    <script src="Fabric.js"></script>
    <script src="Canvas.js"></script>
</head>
<body>  
    <div id="Wrapper">  
        <section id="Body">
                <div id="CanvasDiv">
                    <canvas id="Canvas" width="270px" height="519px"></canvas>
                </div>
                <div id="TextureView">
                    <div id="TextureViewInside">
                        <ul class="products">
                            <img draggable="true" src="FärgadePapper.png">
                            <img draggable="true" src="Hajar.png">
                            <img draggable="true" src="Labyrint.png">
                            <img draggable="true" src="Martini.png">
                        </ul>
                    </div>
                </div>  
        </section>
    </div>
</body>
</html>

.CSS：

#Body{
    height: 675px;
}
#Canvas, #TextureView{
    float: left;
}
#LeftColumn{
    float: left;
    width: 355px;
    margin: 20px 30px 0px 0px;
}
#RightColumn{
    float: left;
    width: 600px;
    margin-top: 20px;
}
#CanvasDiv{
    width: 270px;
    height: 519px;
    margin-left: 15px;
}
#Canvas{
    overflow: hidden;
}
#TextureView{
    width: 470px;
    height: 559px;
    overflow: auto;
}
#TextureViewInside{
    padding: 10px;
}
ul.products img{
    width: 97px;
    display: inline-block;
    vertical-align: top;
    padding: 4px;
}

JavaScript：

var canvas = new fabric.Canvas('Canvas');
function handleDragStart(e) {
    [].forEach.call(images, function (img) {
        img.classList.remove('img_dragging');
    });
    this.classList.add('img_dragging');
}
function handleDragOver(e) {
    if (e.preventDefault) {
        e.preventDefault(); 
    }
    e.dataTransfer.dropEffect = 'copy';
    return false;
}
function handleDragEnter(e) {
    this.classList.add('over');
}
function handleDragLeave(e) {
    this.classList.remove('over');
}
function handleDrop(e) {
    if (e.stopPropagation) {
        e.stopPropagation();
    }
    var img = document.querySelector('#images img.img_dragging');
    console.log('event: ', e);
    var newImage = new fabric.Image(img, {
        width: img.width,
        height: img.height,
        left: e.layerX,
        top: e.layerY
    });
    canvas.add(newImage);
    return false;
}
function handleDragEnd(e) {
    [].forEach.call(images, function (img) {
        img.classList.remove('img_dragging');
    });
}
if (Modernizr.draganddrop) {
    var images = document.querySelectorAll('#images img');
    [].forEach.call(images, function (img) {
        img.addEventListener('dragstart', handleDragStart, false);
        img.addEventListener('dragend', handleDragEnd, false);
    });
    var canvasContainer = document.getElementById('CanvasDiv');
    canvasContainer.addEventListener('dragenter', handleDragEnter, false);
    canvasContainer.addEventListener('dragover', handleDragOver, false);
    canvasContainer.addEventListener('dragleave', handleDragLeave, false);
    canvasContainer.addEventListener('drop', handleDrop, false);
} else {
    alert("This browser doesn't support the HTML5 Drag and Drop API.");
}

看起来你只需要将部分代码包装在一个 document.ready 处理程序中，就像这样......

$(function() {
    if (Modernizr.draganddrop) {
        var images = document.querySelectorAll('#images img');
        [].forEach.call(images, function (img) {
            img.addEventListener('dragstart', handleDragStart, false);
            img.addEventListener('dragend', handleDragEnd, false);
        });
        var canvasContainer = document.getElementById('CanvasDiv');
        canvasContainer.addEventListener('dragenter', handleDragEnter, false);
        canvasContainer.addEventListener('dragover', handleDragOver, false);
        canvasContainer.addEventListener('dragleave', handleDragLeave, false);
        canvasContainer.addEventListener('drop', handleDrop, false);
    } else {
        alert("This browser doesn't support the HTML5 Drag and Drop API.");
    }
});

document.getElementById("CanvasDiv")不会返回元素，因为当时未加载文档，但接下来的几行预计那里会有一些东西。 将其包装在 document.ready 事件处理程序中应该可以解决这个问题，因为 DOM 将在您尝试引用它时加载。

在

加载文档之前无法获取元素...

   window.addEventListener('loaded', function(){
       if (Modernizr.draganddrop) {
           var images = document.querySelectorAll('#images img');
           [].forEach.call(images, function (img) {
               img.addEventListener('dragstart', handleDragStart, false);
               img.addEventListener('dragend', handleDragEnd, false);
           });
           var canvasContainer = document.getElementById('CanvasDiv');
           canvasContainer.addEventListener('dragenter', handleDragEnter, false);
           canvasContainer.addEventListener('dragover', handleDragOver, false);
           canvasContainer.addEventListener('dragleave', handleDragLeave, false);
           canvasContainer.addEventListener('drop', handleDrop, false);
       } else {
           alert("This browser doesn't support the HTML5 Drag and Drop API.");
       }
     });
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