Javascript 未捕获的类型错误：无法读取未定义的属性“值”
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Uncaught TypeError: Cannot read property 'value' of undefined
提问by John
I have some JavaScript code that gives this error
我有一些 JavaScript 代码会出现此错误
Uncaught TypeError: Cannot read property 'value' of undefined
Code
代码
var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if( __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }
if( __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }
What does this error mean?
这个错误是什么意思？
采纳答案by OsQu
Seems like one of your values, with a property key of 'value' is undefined. Test that i1, i2and __iare defined before executing the if statements:
似乎是您的值之一，属性键为 'value' 未定义。测试i1,i2和__i在执行 if 语句之前定义：
var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if(i1 && i2 && __i.user && __i.pass)
{
if( __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }
if( __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }
}
回答by nfechner
Either document.getElementById('i1'), document.getElementById('i2'), or document.getElementsByName("username")[0]is returning no element. Check, that all elements exist.
要么document.getElementById('i1')，document.getElementById('i2')或者document.getElementsByName("username")[0]是没有返回元素。检查，所有元素都存在。
回答by Joe L.
Try this, It always works, and you will get NO TypeError:
试试这个，它总是有效，你不会得到任何类型错误：
try{
var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if( __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }
if( __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }
}catch(e){
if(e){
// If fails, Do something else
}
}
回答by Abhijit
First, you should make sure that document.getElementsByName("username")[0] actually returns an object and not "undefined". You can simply check like
首先，您应该确保 document.getElementsByName("username")[0] 实际上返回一个对象而不是“未定义”。你可以简单地检查一下
if (typeof document.getElementsByName("username")[0] != 'undefined')
Similarly for the other element password.
其他元素密码也类似。
回答by Festus Tamakloe
The posts here help me a lot on my way to find a solution for the Uncaught TypeError: Cannot read property 'value' of undefinedissue.
此处的帖子在我为Uncaught TypeError: Cannot read property 'value' of undefinedissue寻找解决方案的过程中帮了我很多忙。
There are already here many answers which are correct, but what we don't have here is the combination for 2 answers that i think resolve this issue completely.
这里已经有很多正确的答案，但我们这里没有的是我认为完全解决这个问题的 2 个答案的组合。
function myFunction(field, data){
if (typeof document.getElementsByName("+field+")[0] != 'undefined'){
document.getElementsByName("+field+")[0].value=data;
}
}
The difference is that you make a check(if a property is defined or not) and if the check is true then you can try to assign it a value.
不同之处在于您进行检查（是否定义了属性），如果检查为真，那么您可以尝试为其分配一个值。
回答by Mahmoud Mostafa
You can just create a function to check if the variable exists, else will return a default value :
您可以创建一个函数来检查变量是否存在，否则将返回一个默认值：
function isSet(element, defaultVal){
if(typeof element != 'undefined'){
return element;
}
console.log('one missing element');
return defaultVal;
}
And use it in a variable check:
并在变量检查中使用它：
var variable = isSet(variable, 'Default value');
回答by Kamil Kie?czewski
You code looks like automatically generated from other code - you should check that html elements with id=i1and i2and name=usernameand passwordexists before processing them.
您的代码看起来像是从其他代码自动生成的 -在处理它们之前，您应该检查带有id=i1andi2和name=usernameand 的html 元素是否password存在。
