效果描述：点击按钮，切换图片
1.png
2.png
源代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>切换图片</title>
</head>
<body>
<button id="btn">点我</button>
<br>
<img id="mv" width="400" height="300" src="images/a.jpg" alt="">
<script>
//1.获取元素
var btn = document.getElementById('btn');
var mv = document.getElementById('mv');
//2.给元素注册事件
var flag = 1;
btn.onclick = function () {
if (flag === 1) {
mv.src = 'images/b.jpg';
flag = 2;
} else if (flag === 2) {
mv.src = 'images/a.jpg';
flag = 1;
}
}
</script>
</body>
</html>
过程报错：Uncaught TypeError: document.getElementsById is not a function
at 01%E5%88%87%E6%8D%A2%E5%9B%BE%E7%89%87.html:15
原因：细节错误getElementById，不是getElementsById
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