Javascript check if a key exists in an object

A JavaScript object is a special data structure that contains data as a key-value pair.

The object key is also known as the object property, while the value is called property value.

There are two ways you can check if a key exists in an object:

1. Use the in operator
2. Use the hasOwnProperty() method

Both solutions work in a similar way. They will check for the existence of a key or property in your object.

Here’s the code snippet to perform the check in case you’re in a hurry:

"key" in object;

object.hasOwnProperty("key");

Let’s see an example of how to use these solutions next.

1. Check if a key exists using the in operator

The in operator checks if a specified key exists in an object. You need to specify the key as the left operand, and the object as the right operand. The key should be specified as a string.

The operator returns true when the key exists, otherwise it returns false.

The following example checks if an object named user has a property named age and address:

let user = {
  name: "John",
  age: 28,
};

console.log("age" in user); // true
console.log("address" in user); // false

Here, you can see that the age key exists, so the operator returns true.

2. Check if a key exists using the hasOwnProperty() method

The hasOwnProperty() method is a built-in method of the Object data type that allows you to check if a specific property exists in an object.

Below you can see how to use the method to check if a property named ‘age’ and ‘address’ exists in the user object:

let user = {
  name: "John",
  age: 28,
};

console.log(user.hasOwnProperty("age")); // true
console.log(user.hasOwnProperty("address")); // false

When using the hasOwnProperty() method, you need to specify the property name or key you want to check as a string type argument to the method.

The method returns true if the key exists, or false if not.

The hasOwnProperty() method ignores built-in properties provided by the Object data type, while the in operator doesn’t.

You can check for the hasOwnProperty method itself to verify this difference:

let user = {
  name: "John",
  age: 28,
};

console.log("hasOwnProperty" in user); // true
console.log(user.hasOwnProperty("hasOwnProperty")); // false

console.log("toString" in user); // true
console.log(user.hasOwnProperty("toString")); // false

Here, you can see that built-in methods like hasOwnProperty and toString are ignored when using the hasOwnProperty() method, while the in operator takes them.

As a developer, you’re most likely going to check for a key that you added to the object, so it’s not going to be a problem whichever solution you use

Conclusion

In this article, you’ve learned how to check if an object has a key using the two standard solutions: the hasOwnProperty() method and the in operator.

I hope this tutorial is useful. Thanks for reading and Happy coding!