Javascript 无法读取未定义的属性长度
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Cannot read property length of undefined
提问by Praveen
I am trying to simply check if I have an empty input text box but I get this error when I run this in Chrome:
我试图简单地检查我是否有一个空的输入文本框，但是当我在 Chrome 中运行它时出现此错误：
Uncaught TypeError: Cannot read property 'length' of undefined.
未捕获的类型错误：无法读取未定义的属性“长度”。
Here is how I go about doing it. I check for DOM readiness and then call the function:
这是我如何去做。我检查 DOM 准备情况，然后调用函数：
function walkmydog() {
//when the user starts entering
if(document.getElementById('WallSearch').value.length == 0) {
alert("nothing");
}
}
if (document.addEventListener) {
document.addEventListener("DOMContentLoaded", walkmydog, false);
}
采纳答案by GolezTrol
The id of the input seems is not WallSearch. Maybe you're confusing that nameand id. They are two different properties. nameis used to define the name by which the value is posted, while idis the unique identification of the element inside the DOM.
输入的 id 似乎不是WallSearch。也许你混淆了name和id。它们是两个不同的属性。name用于定义发布值的名称，而id是元素在 DOM 内部的唯一标识。
Other possibility is that you have two elements with the same id. The browser will pick any of these (probably the last, maybe the first) and return an element that doesn't support the valueproperty.
另一种可能性是您有两个具有相同 id 的元素。浏览器将选择其中任何一个（可能是最后一个，也可能是第一个）并返回一个不支持该value属性的元素。
回答by Lee
perhaps, you can first determine if the DOM does really exists,
或许，你可以先确定 DOM 是否真的存在，
function walkmydog() {
//when the user starts entering
var dom = document.getElementById('WallSearch');
if(dom == null){
alert('sorry, WallSearch DOM cannot be found');
return false;
}
if(dom.value.length == 0){
alert("nothing");
}
}
if (document.addEventListener){
document.addEventListener("DOMContentLoaded", walkmydog, false);
}
