How to check if property is undefined JavaScript

In this article we will learn how to check if property is undefined in JavaScript.

It is very common to check if property is undefined in JavaScript in JavaScript.

Problem

Now we have a problem which is that we want to check if property in object is undefined or not.

Imagine you have an object like this:

const object = {
    name: "John",
    age: 30,
    country: "USA"
};

And we want to check if age is undefined or not.

How to solve this problem?

The solution to this problem is easy, but first we have to make sure that the element already exists in the object, and then check whether it is undefined or not.

Solution:

As we said before, first we need to check if property is already exists in the object.

And we will use hasOwnProperty method to check if property is exists or not.

const object = {
    name: "John",
    age: 30,
    country: "USA"
};

console.log(object.hasOwnProperty('age'));

Output

true

Now we want to check if property is undefined or not.

const object = {
    name: "John",
    age: 30,
    country: "USA"
};

if (object.hasOwnProperty('age')) {
    // Check if property is `undefined`
    if (object['age'] === undefined) {
        console.log(true);
    } else {
        console.log(false);
    }
}

Output

false

The output was false because age not undefined.

Let's change age value to undefined and test our code.

const object = {
    name: "John",
    age: undefined,
    country: "USA"
};

if (object.hasOwnProperty('age')) {
    // Check if property is `undefined`
    if (object['age'] === undefined) {
        console.log(true);
    } else {
        console.log(false);
    }
}

Output

true

You may need to repeat the code more than once, so let's create a function that does that for us.

// Create a function
function isUndefined(object, key) {
    if (object.hasOwnProperty(key)) {
        if (object[key] === undefined) {
            return true;
        }
        return false;
    }
    return false;
}

// Object
const object = {
    name: "John",
    age: undefined,
    country: "USA"
};

// Tests
console.log(isUndefined(object, 'name'));
console.log(isUndefined(object, 'age'));
console.log(isUndefined(object, 'country'));

Output

false
true
false

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it). MDN

Thank you for reading

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