Undefined in JavaScript

Last Updated : 13 Mar, 2024

Undefined is a type of Data type in JavaScript. It is a primitive value undefined, when a variable is declared and not initialized or not assigned with any value. By default, the variable was stored with an Undefined value.

Undefined is a global read-only variable that represents the value Undefined. Undefined is a type of primitive type

Syntax:

You don’t explicitly create undefined. It’s automatically assigned to variables that lack a value:

let variable = undefined;
// or
let x;

Both no and variable contain an undefined value.

JavaScript Undefined Examples

1. undefined Value in Variables

When a variable is declared or not initialized but not assigned with any value then JavaScript automatically assigns it to an “undefined” value.

Example: In the below example, the variable name is declared but not assigned with any value, so it gets an Undefined value:


                    let newVar;
console.log(newVar)

Output

undefined

2. undefined Value in Functions

A method or statement also returns undefined. If a variable was assigned with a function that does not return any value, then the JavaScript assigns an undefined value to that variable.

Example: In the below example sayhi() function actually outputs and returns nothing. We assigned the sayhi function to the x variable, so we get an Undefined value in the x variable as the sayhi() function returns nothing.


                    function sayhi(name) {
    console.log(`hi ${name}`);
}
x = sayhi('hike');
console.log(`value in x= ${x}`);

Output

hi hike
value in x= undefined

3. undefined value in Object Properties

Accessing a property that doesn’t exist in an object returns undefined: Let’s understand with the below example code:

Example:


                    const person = { name: "Alice" };
console.log(person.age); // Output: undefined

Output

undefined

How to Check for `undefined value`

To check for undefined value , we can use the typeof to check if the value is undefined or not:


                    let myVariable;
if (typeof myVariable === "undefined") {
  console.log("myVariable is undefined");
} else {
  console.log("myVariable is defined");
}