How to check for an undefined or null variable in

问题:

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?

回答1:

You have to differentiate between two cases:

1. Undefined variables , like foo. You\'ll get an error if you access an undefined variable in any context other than typeof.

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

   So, for variables, the typeof check is a must. On the other side, it\'s only rarely needed - you usually know if your variables are defined or not.

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
   

回答2:

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;

回答3:

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

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

回答4:

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.

回答5:

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.

回答6:

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.

回答7:

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)

回答8:

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. e.g.

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

回答9:

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.

回答10:

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

回答11:

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

回答12:

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.

回答13:

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 :)

回答14:

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

Working example at:

http://www.thesstech.com/tryme?filename=nullandundefined

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.\");
    }   
}

回答15:

You must define a function of this form:

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