How to check for “undefined” value in JavaScript ?

In JavaScript, 'undefined' is a primitive value that signifies a variable exists but hasn't been given a value yet. It can also indicate a function without a return statement or an object property that hasn't been defined.

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