Understanding Own Properties of an Object in JavaScript
In JavaScript, an object is a collection of properties, where each property 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.