Understanding Own Properties of an Object in JavaScript

Summary: in this tutorial, you will learn about the own properties of an object in JavaScript.

In JavaScript, an object is a collection of properties, where each property is a key-value pair.

This example creates a new object called person using an object initializer:

const person = {
    firstName: 'John',
    lastName: 'Doe'
};
Code language: JavaScript (javascript)

The person object has two properties: firstName and lastName.

JavaScript uses prototypal inheritance. Therefore, a property of an object can be either own or inherited.

A property that is defined directly on an object is own while a property that the object receives from its prototype is inherited.

The following creates an object called employee that inherits from the person object:

const employee = Object.create(person, {
    job: {
        value: 'JS Developer',
        enumerable: true
    }
});
Code language: JavaScript (javascript)

The employee object has its own property job, and inherits firstName and lastName properties from its prototype person.

The hasOwnProperty() method returns true if a property is own. For example:

console.log(employee.hasOwnProperty('job')); // => true
console.log(employee.hasOwnProperty('firstName')); // => false
console.log(employee.hasOwnProperty('lastName')); // => false
console.log(employee.hasOwnProperty('ssn')); // => false
Code language: JavaScript (javascript)

Summary

A property that is directly defined on an object is an own property.
The obj.hasOwnProperty() method determines whether or not a property is own.

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