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How do I determine if a variable is undefined or null?

My code is as follows:

var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
  // DO SOMETHING
};

<div id="esd-names">
  <div id="name"></div>
</div>

But when I do this, the JavaScript interpreter halts execution.

How do I determine if a variable is undefined or null?

My code is as follows:

var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
  // DO SOMETHING
};

<div id="esd-names">
  <div id="name"></div>
</div>

But when I do this, the JavaScript interpreter halts execution.

• javascript
• jquery
• variables
• null
• undefined

Share Improve this question Follow edited Mar 16, 2023 at 17:13 Dave Mackey 4,4222121 gold badges8282 silver badges144144 bronze badges asked Apr 15, 2010 at 18:12 sadmicrowavesadmicrowave 40.9k3434 gold badges113113 silver badges184184 bronze badges 3

• Possible duplicate of How do you check for an empty string in JavaScript? – T.Todua Commented Dec 4, 2018 at 9:29
• Use the inbuilt Nullish coalescing operator (??) Ref; developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Dennis Nolan Commented Jul 30, 2020 at 1:19
• if(EmpName == 'undefined') will not test if EmpName is undefined, it will test the string "undefined", so get rid of those single quotes: if(EmpName == undefined) // because 'undefined'without the quotes is... undefined or the simpler if(!EmpName){ // DO SOMETHING }; – user7394313 Commented Oct 20, 2021 at 0:32

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33 Answers 33

Sorted by: Reset to default 1 2 Next 3566

You can use the qualities of the abstract equality operator to do this:

if (variable == null){
    // your code here.
}

Because null == undefined is true, the above code will catch both null and undefined.

The standard way to catch null and undefined simultaneously is this:

if (variable == null) {
     // do something 
}

--which is 100% equivalent to the more explicit but less concise:

if (variable === undefined || variable === null) {
     // do something 
}

When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.

---

Edit again

The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.

You should not have any references to undeclared variables in your code.

Combining the above answers, it seems the most complete answer would be:

if( typeof variable === 'undefined' || variable === null ){
    // Do stuff
}

This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.

if (variable == null) {
    // Do stuff, will only match null or undefined, this won't match false
}

if (typeof EmpName != 'undefined' && EmpName) {

will evaluate to true if value is not:

• null

• undefined

• NaN

• empty string ("")

• 0

• false

Probably the shortest way to do this is:

if(EmpName == null) { /* DO SOMETHING */ };

Here is proof:

function check(EmpName) {
  if(EmpName == null) { return true; };
  return false;
}

var log = (t,a) => console.log(`${t} -> ${check(a)}`);

log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");

// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined" 

And here are more details about == (source here)

BONUS: reason why === is more clear than == (look on agc answer)

jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.

So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:

if (!EmpName) { //do something }

Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.

---

I've come to write my own function for this. JavaScript is weird.

It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.

function empty(v) {
    let type = typeof v;
    if (type === 'undefined') {
        return true;
    }
    if (type === 'boolean') {
        return !v;
    }
    if (v === null) {
        return true;
    }
    if (v === undefined) {
        return true;
    }
    if (v instanceof Array) {
        if (v.length < 1) {
            return true;
        }
    } else if (type === 'string') {
        if (v.length < 1) {
            return true;
        }
        if (v === '0') {
            return true;
        }
    } else if (type === 'object') {
        if (Object.keys(v).length < 1) {
            return true;
        }
    } else if (type === 'number') {
        if (v === 0) {
            return true;
        }
    }
    return false;
}

TypeScript-compatible.

---

This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)

Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.

"0.0", NaN, " ", true are considered non-empty.

The shortest and easiest:

if(!EmpName ){
 // DO SOMETHING
}

this will evaluate true if EmpName is:

• null
• undefined
• NaN
• empty
• string ("")
• 0
• false

If the variable you want to check is a global, do

if (window.yourVarName) {
    // Your code here
}

This way to check will not throw an error even if the yourVarName variable doesn't exist.

Example: I want to know if my browser supports History API

if (window.history) {
    history.back();
}

How this works:

window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.

In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.

if (variable === undefined){
}

if (variable === null){
}

if (variable === ''){
}

Check all conditions:

if(variable === undefined || variable === null || variable === ''){
}

Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.

var s; // undefined
jQuery.isEmptyObject(s); // will return true;

s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;

// usage
if(jQuery.isEmptyObject(s)){
    alert('Either variable: s is undefined or its value is null');
}else{
     alert('variable: s has value ' + s);
}

s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;

You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).

if (x === null || x === undefined) {
 // Add your response code here, etc.
}

source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/

I've just had this problem i.e. checking if an object is null.
I simply use this:

if (object) {
    // Your code
}

For example:

if (document.getElementById("enterJob")) {
    document.getElementById("enterJob").className += ' current';
}

With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.

SO,

if(EmpName == null){  // if Variable EmpName null or undefined
  EmpName = 'some value';
};

Is equivalent to:

EmpName ??= 'some value';

jQuery check element not null:

var dvElement = $('#dvElement');

if (dvElement.length  > 0) {
    // Do something
}
else{
    // Else do something else
}

No one seems to have to posted this yet, so here we go:

a?.valueOf() === undefined works reliably for either null or undefined.

The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like ==

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