How to check if a property is undefined in JavaScript Object

Learn the proper way 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');
}

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 defined');
} else{
    console.log('age property is undefined');
}