How to check if a key exists in a JavaScript object

The Problem

You want to check if a key exists in a JavaScript object. How do you do this?

The Solution

There are multiple methods you can use to check if a property, which can also be called a key, exists in an object. The method you choose to use depends on whether you want to check for inherited properties and whether the value of the property could be undefined.

Using the hasOwnProperty() Method

The hasOwnProperty() method takes a property name as an argument and returns a boolean indicating whether the object has the property:

const item = { name: "Macbook Pro", price: 20000, year: 2021 };
console.log(item.hasOwnProperty("price")); // true

Using the in Operator

The in operator returns a boolean indicating if the specified property is in the object:

const item = { name: "Macbook Pro", price: 20000, year: 2021 };
console.log("price" in item); // true

Unlike the hasOwnProperty() method, the in operator also checks whether the property exists on the object’s prototype chain, which contains properties inherited by all newly created objects such as the toString() method. However, if you specifically want to check for a property of an object instance that was added and not inherited, the hasOwnProperty() method is a better choice than the in operator.

Using Property Accessors

Property accessors use dot notation or bracket notation to give access to an object’s properties. You can use property accessors with the strict inequality operator (!==) to determine if a key exists in an object:

console.log(item.price !== undefined); // true
console.log(item["price"] !== undefined); // true

If the property you’re checking for may have a value of undefined, using property accessors is not recommended. This method may lead you to conclude that the property does not exist when it does exist but has a value of undefined. In these situations, it is preferable to use the hasOwnProperty() method or the in operator.