<head>
<meta charset="utf-8">
<title>菜鸟教程(runoob.com)</title>
<script>
document.getElementById("myBtn").addEventListener("click", function(){
document.getElementById("demo").innerHTML = "Hello World";
});
</script>
</head>
<body>
<p>该实例使用 addEventListener() 方法来向按钮添加点击事件。</p>
<button id="myBtn">点我</button>
<p id="demo"></p>
</body>
</html>标签元素还没有创建就调用引用标签的脚本就会出现此问题,将脚本放置在脚本标签的后面就可以了
<head>
<meta charset="utf-8">
<title>菜鸟教程(runoob.com)</title>
</head>
<body>
<p>该实例使用 addEventListener() 方法来向按钮添加点击事件。</p>
<button id="myBtn">点我</button>
<p id="demo"></p>
<script>
document.getElementById("myBtn").addEventListener("click", function(){
document.getElementById("demo").innerHTML = "Hello World";
});
</script>
</body>
</html>