如何检查JavaScript中的空值?

How can I check for null values in JavaScript? I wrote the code below but it didn't work.

如何检查JavaScript中的空值?我写了下面的代码，但是它不起作用。

if (pass == null || cpass == null || email == null || cemail == null || user == null) {      

    alert("fill all columns");
    return false;  

}   

And how can I find errors in my JavaScript programs?

如何在JavaScript程序中找到错误?

15 个解决方案

#1

Javascript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:

Javascript在检查“空”值方面非常灵活。我猜你实际上是在寻找空字符串，在这种情况下，这个更简单的代码可以工作:

if(!pass || !cpass || !email || !cemail || !user){

Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN

哪个将检查空字符串(“”)、null、undefined、false和数字0和NaN

Please note that if you are specifically checking for numbers it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf)

请注意，如果您正在特别地检查数字，那么在这种方法中忽略0是一个常见的错误，而对于返回-1的函数，num !== 0是首选的(或者num != -1或~num (hacky代码也检查-1))，例如indexOf)

#2

To check for null SPECIFICALLY you would use this:

要检查空值，您可以使用以下方法:

if(variable === null && typeof variable === "object")

...or more simply:

…或者更简单:

if(variable === null)

This test will ONLY pass for null and will not pass for "", undefined, false, 0, or NaN.

这个测试只会通过null，不会通过“”、未定义、false、0或NaN。

The rest of this is in response to inorganik's comment, Yes, you can check each one individually.

剩下的部分是针对inorgank的评论，是的，您可以逐个检查。

You need to implement use of the absolutely equals: === and typeof to be absolutely sure with your checks.

您需要实现对equals: === =和typeof的使用，以确保您的检查绝对可靠。

I've created a JSFiddle here to show all of the individual tests working

我在这里创建了一个JSFiddle，以显示所有的测试工作。

Here is all of the output of the tests:

以下是所有测试的输出:

Null Test:

if(variable === null && typeof variable === "object")

- variable = ""; (false) typeof variable = string

- variable = null; (true) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



Empty String Test:

if(variable === "" && typeof variable === "string")

- variable = ""; (true) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number




Undefined Test:

if(variable === undefined && typeof variable === "undefined")

- variable = ""; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (true) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



False Test:

if(variable === false && typeof variable === "boolean")

- variable = ""; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (true) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (false) typeof variable = number



Zero Test:

if(variable === 0 && typeof variable === "number")

- variable = ""; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (true) typeof variable = number

- variable = NaN; (false) typeof variable = number



NaN Test:

if(!parseFloat(variable) && variable != 0 && typeof variable === "number")

- variable = ""; (false) typeof variable = string

- variable = null; (false) typeof variable = object

- variable = undefined; (false) typeof variable = undefined

- variable = false; (false) typeof variable = boolean

- variable = 0; (false) typeof variable = number

- variable = NaN; (true) typeof variable = number

As you can see, it's a little more difficult to test against NaN;

如您所见，对NaN进行测试有点困难;

#3

just replace the == with === in all places.

在所有地方替换=== ==。

== is a loose or abstract equality comparison

=是一个松散的或抽象的相等比较

=== is a strict equality comparison

===是一个严格的平等比较。

See the MDN article on Equality comparisons and sameness for more detail.

更多细节请参见MDN关于平等比较和同一性的文章。

#4

Strict equality operator:-

严格平等接线员:-

We can check null by ===

我们可以用===检查null

if ( value === null ){

}

Just by using if

通过使用如果

if( value ) {

}

will evaluate to true if value is not:

如果值不是:

• null
• 零
• undefined
• 未定义的
• NaN
• 南
• empty string ("")
• 空字符串(“”)
• false
• 假
• 0
• 0

#5

Firstly, you have a return statement without a function body. Chances are that that will throw an error.

首先，您有一个没有函数体的返回语句。可能会抛出一个错误。

A cleaner way to do your check would be to simply use the ! operator:

一个更干净的方式做你的支票将是简单地使用!接线员:

if (!pass || !cpass || !email || !cemail || !user) {

    alert("fill all columns");

}

#6

you can use try catch finally

最后你可以用try catch。

 try {
     document.getElementById("mydiv").innerHTML = 'Success' //assuming "mydiv" is undefined
 } catch (e) {

     if (e.name.toString() == "TypeError") //evals to true in this case
     //do something

 } finally {}   

you can also throw your own errors. See this.

你也可以抛出你自己的错误。看到这个。

#7

to check for undefined and null in javascript you need just to write the following :

要检查javascript中未定义和null，您只需编写以下代码:

if (!var) {
        console.log("var IS null or undefined");
} else {
        console.log("var is NOT null or undefined");
}

#8

This is a comment on WebWanderer's solution regarding checking for NaN (I don't have enough rep yet to leave a formal comment). The solution reads as

这是对WebWanderer关于检查NaN的解决方案的评论(我还没有足够的代表留下正式的评论)。解决方案如

if(!parseInt(variable) && variable != 0 && typeof variable === "number")

but this will fail for rational numbers which would round to 0, such as variable = 0.1. A better test would be:

但对于接近0的有理数，比如变量= 0。1，这将失败。一个更好的测试应该是:

if(isNaN(variable) && typeof variable === "number")

#9

In JavaScript, no string is equal to null.

在JavaScript中，没有字符串等于null。

Maybe you expected pass == null to be true when pass is an empty string because you're aware that the loose equality operator == performs certain kinds of type coercion.

当pass是空字符串时，您可能希望pass == null为真，因为您知道loose equality运算符==执行某些类型强制。

For example, this expression is true:

例如，这个表达是正确的:

'' == 0

In contrast, the strict equality operator === says that this is false:

与此相反，严格等式运算符=== =则表示这是错误的:

'' === 0

Given that '' and 0 are loosely equal, you might reasonably conjecture that '' and null are loosely equal. However, they are not.

考虑到“0和0是大致相等的，您可以合理地推测”和null是大致相等的。然而,他们不是。

This expression is false:

这个表达式是错误的:

'' == null

The result of comparing any string to null is false. Therefore, pass == null and all your other tests are always false, and the user never gets the alert.

将任何字符串与null进行比较的结果都是错误的。因此，pass == null，所有其他测试都是假的，用户永远不会得到警告。

To fix your code, compare each value to the empty string:

要修改代码，请将每个值与空字符串进行比较:

pass === ''

If you're certain that pass is a string, pass == '' will also work because only an empty string is loosely equal to the empty string. On the other hand, some experts say that it's a good practice to always use strict equality in JavaScript unless you specifically want to do the type coercion that the loose equality operator performs.

如果您确定pass是一个字符串，那么pass == "也会起作用，因为只有空字符串松散地等于空字符串。另一方面，一些专家说，最好在JavaScript中始终使用严格的等式，除非您特别希望执行宽松等式操作符执行的类型强制。

If you want to know what pairs of values are loosely equal, see the table "Sameness comparisons" in the Mozilla article on this topic.

如果您想知道哪些值对是松散相等的，请参阅Mozilla关于这个主题的文章中的“相同比较”表。

#10

Improvement over the accepted answer by explicitly checking for null but with a simplified syntax:

通过显式地检查null而使用简化的语法改进已接受的答案:

if ([pass, cpass, email, cemail, user].every(x=>x!==null)) {
    // your code here ...
}

// Test
let pass=1, cpass=1, email=1, cemail=1, user=1; // just to test

if ([pass, cpass, email, cemail, user].every(x=>x!==null)) {
    // your code here ...
    console.log ("Yayy! None of them are null");
} else {
    console.log ("Oops! At-lease one of them is null");
}

#11

underscore: _.isNull

强调:_.isNull

_.isNull(null);
=> true
_.isNull(undefined);
=> false

jQuery: jQuery.type

jQuery:jQuery.type

jQuery.type( null ) === "null"

#12

This will not work in case of Boolean values coming from DB for ex:

如果DB的布尔值为例:

 value = false

 if(!value) {
   // it will change all false values to not available
   return "not available"
 }

#13

Try this:

试试这个:

if (!variable && typeof variable === "object") {
    // variable is null
}

#14

You can also use this reusable is-nil open source component which determines whether val reference is null or undefined and returns a boolean.

您还可以使用这个可重用的is-nil开放源码组件，该组件确定val引用是null还是无定义的，并返回一个布尔值。

Examples:

例子:

isNil(null) // => true
isNil('') // => true

It will return false even if given an empty string for example.

即使给定一个空字符串，它也会返回false。

#15

Please view carefully before downvote.

请在投票前仔细查看。

AFAIK in JAVASCRIPT when a variable is declared but has not assigned value, its type is undefined. so we can check variable even if it would be an object holding some instance in place of value.

在JAVASCRIPT中，当一个变量被声明但没有赋值时，它的类型是未定义的。所以我们可以检查变量，即使它是一个对象，它持有一些实例来代替值。

create a helper method for checking nullity that returns true and use it in your API.

创建一个helper方法，检查返回true的nullity，并在API中使用它。

helper function to check if variable is empty:

辅助函数检查变量是否为空:

function isEmpty(item){
    if(item){
        return false;
    }else{
        return true;
    }
}

try-catch exceptional API call:

try - catch特殊API调用:

try {

    var pass, cpass, email, cemail, user; // only declared but contains nothing.

    // parametrs checking
    if(isEmpty(pass) || isEmpty(cpass) || isEmpty(email) || isEmpty(cemail) || isEmpty(user)){
        console.log("One or More of these parameter contains no vlaue. [pass] and-or [cpass] and-or [email] and-or [cemail] and-or [user]");
    }else{
        // do stuff
    }

} catch (e) {
    if (e instanceof ReferenceError) {
        console.log(e.message); // debugging purpose
        return true;
    } else {
        console.log(e.message); // debugging purpose
        return true;
    }
}

some test cases:

一些测试用例:

var item = ""; // isEmpty? true
var item = " "; // isEmpty? false
var item; // isEmpty? true
var item = 0; // isEmpty? true
var item = 1; // isEmpty? false
var item = "AAAAA"; // isEmpty? false
var item = NaN; // isEmpty? true
var item = null; // isEmpty? true
var item = undefined; // isEmpty? true

console.log("isEmpty? "+isEmpty(item));