Undefined in JavaScript (original) (raw)
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)
`
**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
`
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!