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How to check for “undefined” value in JavaScript ?

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In JavaScript, undefined’ is a primitive value that represents a variable that has been declared but has not been assigned a value, a function that has no return statement, or an object property that does not exist.

There are a few ways to check for ‘undefined’

Approach 1: Using the ‘typeof’ operator:

Syntax:

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");
}

                    

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

Output:

myVariable is undefined

Approach 2: Comparing with the ‘undefined’ value:

Syntax:

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");
}

                    

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

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


Last Updated : 14 Mar, 2023
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