Javascript 无法读取未定义的属性“值”
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Cannot read property 'value' of undefined
提问by Sir
I have a simple form like this:
我有一个像这样的简单表格：
<form method="post" id="login">
Username: <input type="text" value="" name="usr" />
Password: <input type="password" value="" name="pw"/>
<input type="submit" id="log" name="log" value="Login" style="width:250px;"/><br/>
</form>
But i keep getting cannot read property for my javascript with this line:
但是我一直无法使用以下行读取我的 javascript 的属性：
var usr = login.usr.value;
var pw = login.pw.value;
What is the reason i get this error ?
我收到此错误的原因是什么？
回答by Mehdi Karamosly
Try to give your form a name and change your code like below :
尝试为您的表单命名并更改您的代码，如下所示：
<form method="post" id="login" name="login">
Username: <input type="text" value="" name="usr" />
Password: <input type="password" value="" name="pw"/>
<input type="submit" id="log" name="log" value="Login" style="width:250px;"/><br/>
</form>
and the in your javascript :
并且在您的 javascript 中：
var usr = document.login.usr.value;
You can check here (jsfiddle link).
回答by Diodeus - James MacFarlane
<input type="text" value="" name="usr" />does not magically become the variable login.usr, you need to define the value based on the DOM.
<input type="text" value="" name="usr" />不会神奇地变成变量login.usr，需要根据DOM定义值。
var usr = document.login.usr.value;
回答by KingKongFrog
Try using getElementsByName:
尝试使用getElementsByName：
The getElementsByName()method accesses all elements with the specified name.
该getElementsByName()方法访问具有指定名称的所有元素。
var usr = document.getElementsByName('usr')[0].value;
var pw = document.getElementsByName('pw')[0].value;
回答by mplungjan
Method 1:
方法一：
<form method="post" id="login">
Username: <input type="text" value="" name="usr" />
var form = document.getElementById("login"); //DOM access
var usr = form.elements["usr"]; // Forms access
Method 2 - not valid xhtml:
方法 2 - 无效的 xhtml：
<form method="post" name="login">
Username: <input type="text" value="" name="usr" />
var form = document.login; // Forms access
var usr = form.elements["usr"]; // Forms access
Method 3:
方法三：
<form method="post">
Username: <input type="text" value="" name="usr" />
var usr = document.getElementsByName("usr")[0]; // first field on page named this
Method 4: - preferred these days since ID MUST be unique - less useful if there are more forms with usr on the page that needs same validation
方法 4： - 这些天首选，因为 ID 必须是唯一的 - 如果页面上有更多带有 usr 的表单需要相同的验证，则不太有用
<form method="post">
Username: <input type="text" value="" id="usr" name="usr" />
var usr = document.getElementById("usr");
回答by KingKongFrog
Works in every browser:
适用于所有浏览器：
document.forms["login"].elements["usr"].value;
document.forms["login"].elements["pw"].value;
