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, ...

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.

<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

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 ...

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.

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.

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 ...

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 ...

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'.

Learn about the !==undefined comparison operator in JavaScript and how it is used to check if a variable is not undefined.

❮ 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....

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 ...

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) { /* ...

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); }

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).

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.

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);