var lastname = "Hi";
if(typeof lastname !== "undefined")
{
alert("Hi. Variable is defined.");
}
Reddit
reddit.com › r/javascript › good way to check for variable being not null and not undefined.
r/javascript on Reddit: Good way to check for variable being not null and not undefined.
October 20, 2016 -
Hi, all, I often do stuff like this in my code, to check for a variable being not null and not undefined.
// check if value is not null and not undefined
if (value) {
...
}However, I'm now thinking this can leads to bugs, because of 0, "", false and NaN also being falsy.
What is a better way to check a variable is not null and not undefined? I could use this I think, wondering if there is something shorter than this:
if (typeof value !== 'undefined' || value !== null) {
...
} Top answer 1 of 5
29
There are some style guides that basically say you always should use ===, but you can use == in order to check for null or undefined at the same time. So you could do the following: if (value != null) { // This will run if `value` is not `null` and not `undefined`. }
2 of 5
18
fyi your if statement doesn't match your description of it: What is a better way to check a variable is not null and not undefined? what you have now checks if it's not null or not undefined, which will always pass the check and enter the body of the if statement. you want something like: if (typeof value !== 'undefined' && value !== null) { ... } But honestly, the majority of the time if (value) { ... } is exactly what you want, and is well understood and idiomatic
Videos
05:53
How to Check for Undefined or Null in JavaScript (2025) - YouTube
03:28
How to check for undefined variable in Javascript tutorial | How ...
05:39
Null vs Undefined - which one should you use in JS? - YouTube
01:01
undefined is not null in javascript - YouTube
Best Way to Check 'Null', 'Undefined' or 'Empty' in JavaScript ...
08:03
Javascript "undefined" - the best way to check this for every ...
freeCodeCamp
freecodecamp.org › news › javascript-check-if-undefined-how-to-test-for-undefined-in-js
JavaScript Check if Undefined – How to Test for Undefined in JS
November 7, 2024 - An undefined variable or anything without a value will always return "undefined" in JavaScript. This is not the same as null, despite the fact that both imply an empty state. You'll typically assign a value to a variable after you declare it, ...
MDN Web Docs
developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined
undefined - JavaScript | MDN
Note: The strict equality operator (as opposed to the standard equality operator) must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. This is because null is not equivalent to undefined. See Equality comparison and sameness for details. ... One reason to use typeof is that it does not throw an error if the variable has not been declared.
BrowserStack
browserstack.com › home › guide › how to check if a variable is undefined in javascript
How to Check if a Variable is Undefined in JavaScript | BrowserStack
February 18, 2025 - In JavaScript, undefined is the default value for variables that have been declared but not initialized. On the other hand, null is an intentional assignment that explicitly indicates the absence of a value. ... This article covers different ways to check if a variable is undefined, helping you write code that handles these situations smoothly.
Tutorial Republic
tutorialrepublic.com › faq › how-to-determine-if-variable-is-undefined-or-null-in-javascript.php
How to Determine If Variable is Undefined or NULL in JavaScript
<script> var firstName; var lastName = null; // Try to get non existing DOM element var comment = document.getElementById('comment'); console.log(firstName); // Print: undefined console.log(lastName); // Print: null console.log(comment); // Print: null console.log(typeof firstName); // Print: undefined console.log(typeof lastName); // Print: object console.log(typeof comment); // Print: object console.log(null == undefined) // Print: true console.log(null === undefined) // Print: false /* Since null == undefined is true, the following statements will catch both null and undefined */ if(firstNa
ui.dev
ui.dev › check-for-undefined-javascript
How to check for undefined in JavaScript
The way I recommend to check for undefined in JavaScript is using the strict equality operator, ===, and comparing it to the primitive undefined. ... Checking for `undefined`` this way will work in every use case except for one, if the variable ...
Stack Abuse
stackabuse.com › javascript-check-if-variable-is-a-undefined-or-null
JavaScript: Check if Variable is undefined or null
March 29, 2023 - 0, "" and [] are evaluated as false as they denote the lack of data, even though they're not actually equal to a boolean. That being said - since the loose equality operator treats null and undefined as the same - you can use it as the shorthand version of checking for both: // Undefined variable let a; if (a == null) { console.log('Null or undefined value!'); } else { console.log(a); }
Codedamn
codedamn.com › news › javascript
How to check if value is undefined or null in JavaScript
June 8, 2023 - If you want to check if a value is specifically undefined or null without type coercion, you can use the identity operator (===). The identity operator checks for both value and type equality, which means it does not perform type coercion.
Mastering JS
masteringjs.io › tutorials › fundamentals › undefined-check
How to Check if a JavaScript Variable is Undefined - Mastering JS
If you want to check if x is strictly equal to undefined regardless of whether is has been declared or not, you should use typeof x === 'undefined'.
GeeksforGeeks
geeksforgeeks.org › javascript › how-to-check-for-undefined-value-in-javascript
How to check for "undefined" value in JavaScript ? - GeeksforGeeks
July 23, 2025 - Here, the '===' operator checks if the value of the variable is exactly equal to 'undefined'. ... // Declare a variable let myVariable; // Condition to check variable is defined or not if (myVariable === undefined) { console.log("myVariable ...
SheCodes
shecodes.io › athena › 81408-what-does-undefined-mean-in-javascript
[JavaScript] - What does !==undefined mean in JavaScript? - | SheCodes
Learn about the !==undefined comparison operator in JavaScript and how it is used to check if a variable is not undefined.
The Valley of Code
thevalleyofcode.com › how-to-check-undefined-property-javascript
How to check if a JavaScript object property is undefined
In a JavaScript program, the correct way to check if an object property is undefined is to use the typeof operator. typeof returns a string that tells the type of the operand. It is used without parentheses, passing it any value you want to ...
Sentry
sentry.io › sentry answers › javascript › how can i check for "undefined" in javascript?
How can I Check for "undefined" in JavaScript? | Sentry
You can use the strict equality operator (===) to check if a value is undefined: ... An interesting thing to note is that undefined is not a reserved word in JavaScript. A reserved word is a keyword that can’t be used as an identifier for ...
W3Schools
w3schools.com › jsref › jsref_undefined.asp
JavaScript undefined Property
❮ Previous JavaScript Global ... Try it Yourself » · More examples below. The undefined property indicates that a variable has not been assigned a value, or not declared at all....
Programiz
programiz.com › javascript › examples › check-undefined-null
JavaScript Program To Check If A Variable Is undefined or null
To understand this example, you should have the knowledge of the following JavaScript programming topics: ... // program to check if a variable is undefined or null function checkVariable(variable) { if(variable == null) { console.log('The variable is undefined or null'); } else { console.log('The variable is neither undefined nor null'); } } let newVariable; checkVariable(5); checkVariable('hello'); checkVariable(null); checkVariable(newVariable);
Top answer 1 of 16
3579
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.
2 of 16
1403
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.
Medium
medium.com › deno-the-complete-reference › five-ways-to-check-for-undefined-in-javascript-b5568090df77
Five ways to check for undefined in JavaScript | Tech Tonic
March 10, 2024 - The typeof operator returns the data type of variable. Use strict equality to compare the result with the string "undefined". This can be useful in cases where you need to handle other data types that might evaluate to false in an if statement (e.g., 0, empty string).
W3docs
w3docs.com › javascript
How to Check if the Variable is Undefined
If you check by value, you will get that variable is assigned a value or not. In the case of undefined, the assigned variable don’t have any value but the variable exists. Checking the type is done with the typeof operator. In the case of variable === “undefined”, the type of variable is undefined.
Index.dev
index.dev › blog › check-undefined-variable-javascript
How to Check if a Variable is Undefined in JavaScript
It works consistently across all JavaScript environments · It's particularly valuable for checking global variables or properties that might not exist · However, there are some important considerations and best practices: For declared variables, using a direct comparison with undefined is more precise and slightly more performant: let x; // Preferred for known variables if (x === undefined) { /* ...