How to check for "undefined" value in JavaScript ?

Last Updated : 28 May, 2024

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: