检查是否存在一个等于0的var

I have a piece of code that tests for the existence of a variable, using an if statement like the example below. I need to do one thing if the var is set, a different thing if its not. In a certain test case, the var needed to be set to 0, but that is the same is having an unset var in JS, apparently:

我有一段代码来测试变量的存在性，使用如下面的示例所示的if语句。如果设置了var，我需要做一件事，如果没有，我需要做另一件事。在某个测试用例中，var需要设置为0，但显然，在JS中有一个未设置的var:

var toMatch;
toMatch = 0;
if (!toMatch) {
    document.write("no");
} else {
    document.write(toMatch);
}

// html is "no"

jsFiddle

So my problem is, how do I test for a var if its value is legitimately zero. I should point out, that in my function possible values to be passed are 0-40+.

我的问题是，如果var的值合理地为零，我该如何测试它呢?我应该指出，在我的函数中，可能要传递的值是0-40+。

In the past I've used a work around like setting the initial value to a number high enough that it is not likely to be passed to the function but that seems hackey to me. Is there a better way to do it?

在过去，我使用过这样的工作，比如将初始值设置为足够高的值，这样它就不太可能被传递给函数，但对我来说，这似乎是hackey。有更好的方法吗?

4 个解决方案

#1

var toMatch;
toMatch = 0;
if (toMatch === 0) { // or !== if you're checking for not zero
    document.write("no");
} else {
    document.write(toMatch);
}

toMatch === 0 will check for zero.

番茄=== 0将检查是否为0。

toMatch === undefined will check for undefined

toMatch === == undefined将检查是否为undefined

the triple equals are strict comparison operators for this sort of scenario. See this blessed question: Difference between == and === in JavaScript

对于这种情况，三重等号是严格的比较运算符。请参见JavaScript中==和===之间的差异

#2

You can see if a name is undefined with:

您可以看到名称是否未定义为:

if (typeof bad_name === "undefined") {

#3

Instead of

而不是

if (toMatch)

use

使用

if (toMatch == null)

#4

Javascript is a bit funny when it comes to values and boolean checks. I suggest reading about Truthy and Falsey in Javascript.

当涉及到值和布尔值检查时，Javascript有点滑稽。我建议用Javascript阅读Truthy和Falsey。

You should use the identity inequality operator !==:

您应该使用identity不等式操作符!=:

var toMatch;
toMatch = 0;
if (toMatch !== 0) {
    document.write("no");
} else {
    document.write(toMatch);
}

It is also worth understanding the differences between == and ===.

==和==之间的差异也值得理解。

#1

var toMatch;
toMatch = 0;
if (toMatch === 0) { // or !== if you're checking for not zero
    document.write("no");
} else {
    document.write(toMatch);
}