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How to Check empty/undefined/null String in JavaScript?

How to check for "undefined" value in JavaScript ?

Last Updated : 28 May, 2024

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In JavaScript, undefined is a primitive value that represents the absence of a value or the uninitialized state of a variable. It's typically used to denote the absence of a meaningful value, such as when a variable has been declared but not assigned a value. It can also indicate the absence of a return value in a function or the absence of a defined property in an object. Understanding undefined is crucial for writing robust and error-free JavaScript code.

There are a few ways to check for 'undefined'.

Using the 'typeof' operator
Comparing with the 'undefined' value

1. Using the 'typeof' operator:

Here, 'typeof' operator returns the string 'undefined' if the variable has not been assigned a value.

JavaScript

// Declare a variable let myVariable;  // Condition to check variable is defined or not if (typeof myVariable === "undefined") {     console.log("myVariable is undefined"); } else {     console.log("myVariable is defined"); }

Output

myVariable is undefined

Comparing with the 'undefined' value

Here, the '===' operator checks if the value of the variable is exactly equal to 'undefined'.

JavaScript

// Declare a variable let myVariable;  // Condition to check variable is defined or not if (myVariable === undefined) {     console.log("myVariable is undefined"); } else {     console.log("myVariable is defined"); }

Output

myVariable is undefined

Example 1:

JavaScript

// Using the 'typeof' operator:  // Declare a variable let fruit;  // Condition for check variable is defined or not if (typeof fruit === "undefined") {   console.log("fruit is undefined"); } else {   console.log("fruit is defined"); }

Output

fruit is undefined

Example 2:

JavaScript

// Using the 'typeof' operator:  // Declare a variable and assign a value // it will return fruit is defined let fruit = "apple";  // Condition for check variable is defined or not if (typeof fruit === "undefined") {   console.log("fruit is undefined"); } else {   console.log("fruit is defined"); }

Output

fruit is defined

Example 3:

JavaScript

// Comparing with the 'undefined' value:  // Declare a variable let profile;  // Condition for check variable is defined or not if (profile === undefined) {   console.log("profile is undefined"); } else {   console.log("profile is defined"); }

Output

profile is undefined

Example 4:

JavaScript

// Comparing with the 'undefined' value:  // Declare a variable and assign value let profile = "geeksforgeeks";  // Condition for check variable is defined or not if (profile === undefined) {   console.log("profile is undefined"); } else {   console.log("profile is defined as", profile); }

Output

profile is defined as geeksforgeeks

How to Check empty/undefined/null String in JavaScript?

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Article Tags :

JavaScript
Web Technologies

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