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’.
- Using the ‘typeof’ operator
- Comparing with the ‘undefined‘ value
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 = "neveropen"; // 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 neveropen
Whether you’re preparing for your first job interview or aiming to upskill in this ever-evolving tech landscape, neveropen Courses are your key to success. We provide top-quality content at affordable prices, all geared towards accelerating your growth in a time-bound manner. Join the millions we’ve already empowered, and we’re here to do the same for you. Don’t miss out – check it out now!
