javascript位置错误导致Uncaught TypeError: Cannot read property 'elements' of null
2018-01-21 20:50
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在学习js的表单脚本的时候，写了一个取表单字段的demo：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script>
var form=document.getElementById("myForm");
var field=form.elements[0];
alert(field);
</script>
</head>
<body>
<form method="post" id="myForm">
<ul>
<li><input type="radio" name="color" value="red">Red</li>
<li><input type="radio" name="color" value="green">Green</li>
<li><input type="radio" name="color" value="blue">Blue</li>
</ul>
</form>
</body>
</html>
发现浏览器报错如下：Uncaught TypeError: Cannot read property 'elements' of null，发现原来是js代码的位置放错了，JS运行的时候页面还没有加载完成，所以JS代码找不到页面元素，就会抛出这个问题。将js代码放在body最后就ok了。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script>
var form=document.getElementById("myForm");
var field=form.elements[0];
alert(field);
</script>
</head>
<body>
<form method="post" id="myForm">
<ul>
<li><input type="radio" name="color" value="red">Red</li>
<li><input type="radio" name="color" value="green">Green</li>
<li><input type="radio" name="color" value="blue">Blue</li>
</ul>
</form>
</body>
</html>
发现浏览器报错如下：Uncaught TypeError: Cannot read property 'elements' of null，发现原来是js代码的位置放错了，JS运行的时候页面还没有加载完成，所以JS代码找不到页面元素，就会抛出这个问题。将js代码放在body最后就ok了。
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