javascript 使用 if 语句检查是否为 NaN
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using an if statement to check if NaN
提问by user2084813
I'm getting a numeric value from a form. Then I check to see if it's NaN. If it is a number I want to set that value to a variable. The problem is that when I enter a valid number I still get an alert and the number isn't passed to the variable "date". How should I modify my statement so that when it is a valid number I can assign it to the variable date?
我从表单中获取数值。然后我检查它是否是 NaN。如果它是一个数字，我想将该值设置为一个变量。问题是，当我输入一个有效数字时，我仍然收到警报，并且该数字没有传递给变量“日期”。我应该如何修改我的语句，以便当它是一个有效数字时，我可以将它分配给变量日期？
var adate = document.getElementById("dueDate").value;
if ( adate == NaN || " ") {
alert("Please enter a due date");
return;
}
else {
var date = (new Date()).setDate(adate);
}
processDate(date);
回答by Bryan Herbst
Use Javascript's isNaN()function.
使用 Javascript 的isNaN()函数。
Checking equality with NaN is always false, as per IEEE's standards. Stephen Canon, a member of the IEEE-754 committee that decided this, has an excellent answer explaining this here.
根据 IEEE 的标准，使用 NaN 检查相等性总是错误的。决定这一点的 IEEE-754 委员会成员斯蒂芬·佳能 (Stephen Canon) 有一个很好的答案，在这里解释了这一点。
回答by benekastah
As strange as it seems, NaN !== NaN.
尽管看起来很奇怪，NaN !== NaN.
if (adate !== adate || adate !== " ") {
//...
}
The isNaNfunction would work in a lot of cases. There is a good case to be made that it is broken, though.
该isNaN功能在很多情况下都可以使用。不过，有一个很好的理由表明它已损坏。
One nice way of getting around this is:
解决这个问题的一种好方法是：
MyNamespace.isNaN = function (x) {
return x !== x;
}
回答by lonesomeday
You have two problems here. The result is that the conditional will always pass. This is what it does:
你在这里有两个问题。结果是条件总是会通过。这是它的作用：
adate == NaN // first, test if adate == NaN (this always returns false)
|| // if the first test fails (i.e. always), carry on checking
" " // test if the string " " is truthy (this always returns true)
The ||does two separate checks. It does nottest to see if adateis "either NaNor " "", which seems to be what you expect.
在||做两个独立的检查。它不是测试，看看是否adate是“要么NaN或" "”，这似乎是你所期望的。
Your code might as well say
你的代码也可能会说
if ( true ) {
You would be able to sort this out, however, if you tried two comparisons:
但是，如果您尝试两个比较，您将能够解决这个问题：
if ( (adate == NaN) || (adate === " ")) {
As other people have said, however, this doesn't work, because NaN !== NaN. So the solution is to use isNaN:
然而，正如其他人所说，这不起作用，因为NaN !== NaN. 所以解决方案是使用isNaN：
if (isNaN(adate) || (adate === " ")) {
回答by Sandun
you could Use if( isNaN(adate))
你可以使用 if( isNaN(adate))
good luck
祝你好运
