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Undefined in JavaScript

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  • Last Updated : 11 Jan, 2023
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Undefined is a type of Data type in JavaScript. 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


var variable = undefined;
// or
var no; 

Both no and variable contain an undefined value.

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:


var newVar;



2. Undefined Value using 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. In the above 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}`);


hi hike
value in x=undefined
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