javascript 检查一个值是否为空？
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Check if a value is null?
提问by Wilson
In javascript, I want to have an if else statement that will change an elements value(a text field in an html form) to zero if nothing is typed in. I had the following:
在 javascript 中，我想要一个 if else 语句，如果没有输入任何内容，它会将元素值（html 表单中的文本字段）更改为零。我有以下内容：
if(document.getElementById('dollarinput').value == null){
var dollars = 0;
}
else{
var dollars = parseInt(document.getElementById('dollarinput').value);
}
Something isn't working correctly in the if condition, but I'm not sure what I did wrong.
在 if 条件下有些东西不能正常工作，但我不确定我做错了什么。
回答by mattytommo
You need to move the variable declaration above the ifand check for empty quotes, like so:
您需要将变量声明移到 上方if并检查空引号，如下所示：
var dollars;
if(document.getElementById('dollarinput').value == ''){
dollars = 0;
}
else{
dollars = parseInt(document.getElementById('dollarinput').value);
}
回答by Jeff Hines
var dollars = 0;
var dollarinput = document.getElementById('dollarinput');
if( ! dollarinput.length )
{
dollars = parseInt(dollarinput.value);
}
回答by bfavaretto
Check if it's an empty string instead:
检查它是否为空字符串：
if(document.getElementById('dollarinput').value == ''){
var dollars = 0;
} else{
var dollars = parseInt(document.getElementById('dollarinput').value, 10);
}
回答by gdoron is supporting Monica
- Value of an input can never be
null! if there is no value, it will be an empty string. alert('' == null);?? // false
- 输入的值永远不可能是
null! 如果没有值，它将是一个空字符串。 alert('' == null);?? // false
Fixed code:
固定代码：
if(document.getElementById('dollarinput').value){
var dollars = 0;
}
else{
var dollars = parseInt(document.getElementById('dollarinput').value);
}
