JavaScript - Check if an Object property is undefined

In this tutorial, we are going to learn how to check if an object property is undefined in JavaScript.

Consider we have the following object.

const obj = {
    name: "king",
    place: "denmark"
}

If we log for a property that is not defined in the above object, we will see an undefined value.

console.log(obj.age); // undefined

Using typeof operator

We can use the typeof operator to check if an object property is undefined.

The typeof operator returns the string representation of a given object property type.

if (typeof obj.age === 'undefined'){
   console.log('age property is undefined');
}else{
   console.log('age property is not undefined');
}

Using in operator

Similary, we can also use the in operator to check the object property existence.

The in operator returns true if a property is present in the object, else it returns false.

const obj = {
    name: "king",
    place: "denmark"
}

if ('age' in obj){
    console.log('age property is not undefined');
} else{
    console.log('age property is undefined');
}