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In a JavaScript, the correct way to check if an object property is undefined is to use the typeofoperator.

The typeof operator returns a string indicating the type of the unevaluated operand.

For example:

console.log(typeof 42);
// expected output: "number"

console.log(typeof 'blubber');
// expected output: "string"

console.log(typeof true);
// expected output: "boolean"

console.log(typeof declaredButUndefinedVariable);
// expected output: "undefined";

If the value is not defined, typeof returns the ‘undefined’ string.

How to check if a JavaScript object property is undefined?

Let's create a user object with the following properties in it:

// using Object Literals
var user = {
    emailId : 'ramesh@gmail.com',
    age : 29,
    getFullName : function (){
        return user.firstName + " " + user.lastName;
    }
};

Note that firstName and lastName properties not added to the above user object.

Now, let's create a isUndefined() method and test firstName and lastName properties:

function isUndefined(user) {
    if(typeof user.firstName === 'undefined'){
        console.log("User first name is undefined");
    }

    if(typeof user.lastName === 'undefined'){
        console.log("User last name is undefined");
    }
}

Let's call the above function:

isUndefined(user);

Output:

User first name is undefined
User last name is undefined

Complete code

// using Object Literals
var user = {
    emailId : 'ramesh@gmail.com',
    age : 29,
    getFullName : function (){
        return user.firstName + " " + user.lastName;
    }
};

function isUndefined(user) {
    if(typeof user.firstName === 'undefined'){
        console.log("User first name is undefined");
    }

    if(typeof user.lastName === 'undefined'){
        console.log("User last name is undefined");
    }
}

isUndefined(user);

Output:

User first name is undefined
User last name is undefined