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:

JavaScript

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.

JavaScript

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:

JavaScript

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:

JavaScript

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

Output

myVariable is undefined

Practical Use Cases of undefined values

Handle optional function parameters.
Check if an object property exists before accessing it.
Detect uninitialized variables during debugging.

Remember, understanding undefined is essential for writing reliable and bug-free JavaScript code!

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Undefined in JavaScript

JavaScript Undefined Examples

1. undefined Value in Variables

2. undefined Value in Functions

3. undefined value in Object Properties

How to Check for undefined value

Practical Use Cases of undefined values

Similar Reads

How to Check for `undefined value`