Javascript 如何在Javascript中检查空值？

How to check for empty value in Javascript?

提问by sqlmole

I am working on a method of retrieving an array of hidden inputs in my form like so

我正在研究一种以我的形式检索隐藏输入数组的方法

<input type="hidden" value="12:34:00" name="timetemp0">
<input type="hidden" value="14:45:00" name="timetemp1">
<input type="hidden" value="15:12:00" name="timetemp2">
<input type="hidden" value="16:42:12" name="timetemp3">
<input type="hidden" value="16:54:56" name="timetemp4">
<input type="hidden" value="17:03:10" name="timetemp5">

My javascript function retrieves these individually by using getElementsByName('timetemp'+i)

我的 javascript 函数使用 getElementsByName('timetemp'+i) 单独检索这些

    for (i ; i < counter[0].value; i++)
        {
//finds hidden element by using concatenation of base name plus counter

var timetemp = document.getElementsByName('timetemp'+i);

//if there is a value alert that value to user - this is just for testing purposes at the moment
//because there is only one of timetemp.i then it occupies position 0 in array
            if (timetemp[0].value == null)
            {
                alert ('No value');

            }
            else
            {
                alert (timetemp[0].value);

            }
}

So what should happen is it will alert the user of the value in that hidden input but if it comes accross an input with no value like this:

所以应该发生的是它会提醒用户该隐藏输入中的值，但如果它遇到一个没有值的输入，如下所示：

<input type="hidden" value="" name="timetemp16">

Then it will say "No value"

然后它会说“没有价值”

However th if function cannot seem to work with this:

但是，如果功能似乎无法与此一起使用：

I have tried:

我试过了：

1. (timetemp[0].value == null)
2. (timetemp[0].value === null)
3. (timetemp[0].value == undefined)
4. (timetemp[0].value == '')

1. (timetemp[0].value == null)
2. (timetemp[0].value === null)
3. (timetemp[0].value == undefined)
4. (timetemp[0].value == '')

It always seems to default to else clause.

它似乎总是默认为 else 子句。

Any ideas?

有任何想法吗？

回答by Richard Dalton

Comment as an answer:

评论作为答案：

if (timetime[0].value)

This works because any variable in JS can be evaluated as a boolean, so this will generally catch things that are empty, null, or undefined.

这是有效的，因为 JS 中的任何变量都可以作为布尔值进行评估，因此这通常会捕获空、空或未定义的内容。

回答by Jamter

In my opinion, using "if(value)" to judge a value whether is an empty value is not strict, because the result of "v?true:false" is false when the value of v is 0(0 is not an empty value). You can use this function:

在我看来，使用“if(value)”来判断一个值是否为空值并不严格，因为当v的值为0时，“v?true:false”的结果为false（0不是空的）价值）。您可以使用此功能：

const isEmptyValue = (value) => {
    if (value === '' || value === null || value === undefined) {
        return true
    } else {
        return false
    }
}

回答by FishBasketGordo

First, I would check what igets initialized to, to see if the elements returned by getElementsByNameare what you think they are. Maybe split the problem by trying it with a hard-coded name like timetemp0, without the concatenation. You can also run the code through a browser debugger (FireBug, Chrome Dev Tools, IE Dev Tools).

首先，我会检查什么i被初始化，看看返回的元素getElementsByName是否是你认为的那样。也许通过尝试使用硬编码名称（如timetemp0，而不是串联）来解决问题。您还可以通过浏览器调试器（FireBug、Chrome 开发工具、IE 开发工具）运行代码。

Also, for your if-condition, this should suffice:

此外，对于您的 if 条件，这应该足够了：

if (!timetemp[0].value) {
    // The value is empty.
}
else {
    // The value is not empty.
}

The empty string in Javascript is a falsey value, so the logical negation of that will get you into the if-block.

Javascript 中的空字符串是一个 falsey 值，因此逻辑否定将使您进入 if 块。

回答by Jason Gennaro

Your script seems incorrect in several places.

您的脚本在几个地方似乎不正确。

Try this

尝试这个

var timetemp = document.getElementsByTagName('input');

for (var i = 0; i < timetemp.length; i++){
    if (timetemp[i].value == ""){
        alert ('No value');
    }
    else{
        alert (timetemp[i].value);
    }
}

Example: http://jsfiddle.net/jasongennaro/FSzT2/

示例：http: //jsfiddle.net/jasongennaro/FSzT2/

Here's what I changed:

这是我改变的：

1. started by getting all the inputs via TagName. This makes an array
2. initialized iwith a varand then looped through the timetemparray using the timetemp.lengthproperty.
3. used timetemp[i]to reference each inputin the for statement

1. 通过获取所有inputs开始TagName。这使得一个数组
2. i用 a初始化，var然后timetemp使用timetemp.length属性循环遍历数组。
3. 用于timetemp[i]引用每个input在for statement

回答by James Hill

The counter in your for loop appears incorrect (or we're missing some code). Here's a working Fiddle.

for 循环中的计数器似乎不正确（或者我们缺少一些代码）。这是一个有效的 Fiddle。

This code:

这段代码：

//Where is counter[0] initialized?
for (i ; i < counter[0].value; i++)

Should be replaced with:

应替换为：

var timeTempCount = 5; //I'm not sure where you're getting this value so I set it.

for (var i = 0; i <= timeTempCount; i++)