如何检查 JavaScript 中的未定义或 null 变量？

Question

We are frequently using the following code pattern in our JavaScript code

if (typeof(some_variable) != 'undefined' && some_variable != null)
{
    // Do something with some_variable
}

Is there a less verbose way of checking that has the same effect?

According to some forums and literature saying simply the following should have the same effect.

if (some_variable)
{
    // Do something with some_variable
}

Unfortunately, Firebug evaluates such a statement as error on runtime when some_variable is undefined, whereas the first one is just fine for it. Is this only an (unwanted) behavior of Firebug or is there really some difference between those two ways?

Answer 1

I think the most efficient way to test for "value is null or undefined " is

if ( some_variable == null ){
  // some_variable is either null or undefined
}

So these two lines are equivalent:

if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}

Note 1

As mentioned in the question, the short variant requires that some_variable has been declared, otherwise a ReferenceError will be thrown. However in many use cases you can assume that this is safe:

check for optional arguments:

function(foo){
    if( foo == null ) {...}

check for properties on an existing object

if(my_obj.foo == null) {...}

On the other hand typeof can deal with undeclared global variables (simply returns undefined ). Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained.

Note 2

This - even shorter - variant is not equivalent:

if ( !some_variable ) {
  // some_variable is either null, undefined, 0, NaN, false, or an empty string
}

so

if ( some_variable ) {
  // we don't get here if some_variable is null, undefined, 0, NaN, false, or ""
}

Note 3

In general it is recommended to use === instead of == . The proposed solution is an exception to this rule. The JSHint syntax checker even provides the eqnull option for this reason.

From the jQuery style guide :

Strict equality checks (===) should be used in favor of ==. The only exception is when checking for undefined and null by way of null.

// Check for both undefined and null values, for some important reason. 
undefOrNull == null;

EDIT 2021-03:

Nowadays most browsers support the Nullish coalescing operator ( ?? ) and the Logical nullish assignment (??=) , which allows a more concise way to assign a default value if a variable is null or undefined, for example:

if (a.speed == null) {
  // Set default if null or undefined
  a.speed = 42;
}

can be written as any of these forms

a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;

Answer 2

You have to differentiate between cases:

1. Variables can be undefined or undeclared . You'll get an error if you access an undeclared variable in any context other than typeof .

if(typeof someUndeclaredVar == whatever) // works
if(someUndeclaredVar) // throws error

A variable that has been declared but not initialized is undefined .

let foo;
if (foo) //evaluates to false because foo === undefined

2. Undefined properties , like someExistingObj.someUndefProperty . An undefined property doesn't yield an error and simply returns undefined , which, when converted to a boolean, evaluates to false . So, if you don't care about 0 and false , using if(obj.undefProp) is ok. There's a common idiom based on this fact:

    value = obj.prop || defaultValue

   which means "if obj has the property prop , assign it to value , otherwise assign the default value defautValue ".

   Some people consider this behavior confusing, arguing that it leads to hard-to-find errors and recommend using the in operator instead

   value = ('prop' in obj) ? obj.prop : defaultValue

Answer 3

Checking null with normal equality will also return true for undefined.

if (window.variable == null) alert('variable is null or undefined');

Answer 4

This is the only case in which == and != should be used:

if (val == null) console.log('val is null or undefined')

if (val != null) console.log('val is neither null nor undefined')

For any other comparisons, the strict comparators ( === and !== ) should be used.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
• https://2ality.com/2011/12/strict-equality-exemptions.html

Answer 5

In newer JavaScript standards like ES5 and ES6 you can just say

> Boolean(0) //false
> Boolean(null)  //false
> Boolean(undefined) //false

all return false, which is similar to Python's check of empty variables. So if you want to write conditional logic around a variable, just say

if (Boolean(myvar)){
   // Do something
}

here "null" or "empty string" or "undefined" will be handled efficiently.

Answer 6

If you try and reference an undeclared variable, an error will be thrown in all JavaScript implementations.

Properties of objects aren't subject to the same conditions. If an object property hasn't been defined, an error won't be thrown if you try and access it. So in this situation you could shorten:

 if (typeof(myObj.some_property) != "undefined" && myObj.some_property != null)

to

if (myObj.some_property != null)

With this in mind, and the fact that global variables are accessible as properties of the global object ( window in the case of a browser), you can use the following for global variables:

if (window.some_variable != null) {
    // Do something with some_variable
}

In local scopes, it always useful to make sure variables are declared at the top of your code block, this will save on recurring uses of typeof .

Answer 7

Firstly you have to be very clear about what you test. JavaScript has all sorts of implicit conversions to trip you up, and two different types of equality comparator: == and === .

A function, test(val) that tests for null or undefined should have the following characteristics:

 test(null)         => true
 test(undefined)    => true
 test(0)            => false
 test(1)            => false
 test(true)         => false
 test(false)        => false
 test('s')          => false
 test([])           => false

Let's see which of the ideas here actually pass our test.

These work:

val == null
val === null || val === undefined
typeof(val) == 'undefined' || val == null
typeof(val) === 'undefined' || val === null

These do not work:

typeof(val) === 'undefined'
!!val

I created a jsperf entry to compare the correctness and performance of these approaches. Results are inconclusive for the time being as there haven't been enough runs across different browsers/platforms. Please take a minute to run the test on your computer!

At present, it seems that the simple val == null test gives the best performance. It's also pretty much the shortest. The test may be negated to val != null if you want the complement.

Answer 8

这是使用 Array includes()方法的另一种方法：

[undefined, null].includes(value)

Answer 9

Since there is no single complete and correct answer, I will try to summarize:

In general, the expression:

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

cannot be simplified, because the variable might be undeclared so omitting the typeof(variable) != "undefined" would result in ReferenceError. But, you can simplify the expression according to the context :

If the variable is global , you can simplify to:

if (window.variable != null)

If it is local , you can probably avoid situations when this variable is undeclared, and also simplify to:

if (variable != null)

If it is object property , you don't have to worry about ReferenceError:

if (obj.property != null)

Answer 10

This is an example of a very rare occasion where it is recommended to use == instead of === . Expression somevar == null will return true for undefined and null , but false for everything else (an error if variable is undeclared).

Using the != will flip the result, as expected.

Modern editors will not warn for using == or != operator with null , as this is almost always the desired behavior.

Most common comparisions:

undeffinedVar == null     // true
obj.undefinedProp == null // true
null == null              // true
0 == null                 // false
'0' == null               // false
'' == null                // false

Try it yourself:

let undefinedVar;
console.table([
    { test : undefinedVar,     result: undefinedVar     == null },
    { test : {}.undefinedProp, result: {}.undefinedProp == null },
    { test : null,             result: null             == null },
    { test : false,            result: false            == null },
    { test : 0,                result: 0                == null },
    { test : '',               result: ''               == null },
    { test : '0',              result: '0'              == null },
]);

Answer 11

You can just check if the variable has a value or not. Meaning,

if( myVariable ) {
//mayVariable is not :
//null
//undefined
//NaN
//empty string ("")
//0
//false

}

If you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. eg

if( typeof myVariable !== 'undefined' ) {
    // myVariable will get resolved and it is defined
}

Answer 12

与您所拥有的类似，您可以执行类似的操作

if (some_variable === undefined || some_variable === null) { do stuff }

Answer 13

whatever yyy is undefined or null, it will return true

if (typeof yyy == 'undefined' || !yyy) {
    console.log('yes');
} else {
    console.log('no');
}

yes

if (!(typeof yyy == 'undefined' || !yyy)) {
    console.log('yes');
} else {
    console.log('no');
}

no

Answer 14

Open the Developer tools in your browser and just try the code shown in the below image.

Answer 15

If the purpose of the if statement is to check for null or undefined values before assigning a value to a variable, you can make use of the Nullish Coalescing Operator . According to the data from caniuse , it should be supported by around 85% of the browsers(as of January 2021). An example of the operator is shown below:

const a = some_variable ?? '';

This will ensure that the variable will be assigned to an empty string (or any other default value) if some_variable is null or undefined .

This operator is most suited for your use case, as it does not return the default value for other types of falsy value such as 0 and '' .

Answer 16

As mentioned in one of the answers, you can be in luck if you are talking about a variable that has a global scope. As you might know, the variables that you define globally tend to get added to the windows object. You can take advantage of this fact so lets say you are accessing a variable called bleh, just use the double inverted operator (!!)

!!window['bleh'];

This would return a false while bleh has not been declared AND assigned a value.

Answer 17

In order to understand, Let's analyze what will be the value return by the Javascript Engine when converting undefined , null and ''(An empty string also). You can directly check the same on your developer console.

You can see all are converting to false , means All these three are assuming 'lack of existence' by javascript. So you no need to explicitly check all the three in your code like below.

if (a === undefined || a === null || a==='') {
    console.log("Nothing");
} else {
    console.log("Something");
}

Also I want to point out one more thing.

What will be the result of Boolean(0)?

Of course false. This will create a bug in your code when 0 is a valid value in your expected result. So please make sure you check for this when you write the code.

Answer 18

I have done this using this method

save the id in some variable

var someVariable = document.getElementById("someId");

then use if condition

if(someVariable === ""){
 //logic
} else if(someVariable !== ""){
 //logic
}

Answer 19

In ES5 or ES6 if you need check it several times you cand do:

 const excluded = [null, undefined, '']; if (!exluded.includes(varToCheck) { // it will bee not null, not undefined and not void string }

Answer 20

This is also a nice (but verbose) way of doing it:

if(someObject.someMember ?? null === null) {
  // bladiebla
}

It's very clear what's happening and hard to misunderstand. And that can be VERY important! :-)

This uses the ?? operator ( https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator ). If the value of someObject.someMember is null or undefined , the ?? operator kicks in and will make the value null .

TBH, I like the explicitness of this thing, but I usualle prefer someObject.someMember == null , it's more readable and skilled JS developers probably know what's going on here.

Answer 21

 let varToCheck = ""; //U have to define variable firstly ,or it throw error const excluded = [null, undefined, ""]; if (!excluded.includes(varToCheck)) { // it will bee not null, not undefined and not void string console.log("pass"); } else { console.log("fail"); }

for example I copy vladernn's answer to test, u can just click button "Copy snippets to answer" to test too .

Answer 22

Testing nullity ( if (value == null) ) or non-nullity ( if (value != null) ) is less verbose than testing the definition status of a variable.

Moreover, testing if (value) (or if( obj.property) ) to ensure the existence of your variable (or object property) fails if it is defined with a boolean false value. Caveat emptor :)

Answer 23

使用 Ramda，您可以简单地执行R.isNil(yourValue) Lodash 和其他帮助库具有相同的功能。

Answer 24

Best way to compare undefined or null or 0 with ES5 and ES6 standards

 if ((Boolean(some_variable_1) && Boolean(some_variable_2)) === false) {
    // do something
    }

Answer 25

You can make use of lodash library.

_.isNil(value) gives true for both null and undefined

Test on - https://bazinga.tools/lodash

Answer 26

You must define a function of this form:

validate = function(some_variable){
    return(typeof(some_variable) != 'undefined' && some_variable != null)
}

Answer 27

Both values can be easily distinguished by using the strict comparison operator.

Sample Code:

function compare(){
    var a = null; //variable assigned null value
    var b;  // undefined
    if (a === b){
        document.write("a and b have same datatype.");
    }
    else{
        document.write("a and b have different datatype.");
    }   
}