How to Extend an Object in JavaScript ?

The extends keyword can be used to extend the objects as well as classes in JavaScript. It is usually used to create a class which is child of another class.

Syntax:

class childclass extends parentclass {...}
class parentclass extends in-built object {...}

Below example depicts how a child class uses properties of parent class using the keyword extends and by creating objects of the child class. In example 1, we see that class Profile has two attributes name and age. Now we will see that class Student acquires both attributes of class Profile using the keyword extends with an added attribute languages and then all attributes are displayed.

Example 1: In this example, we use the extends keyword.

• Program:

  <script>
  
      // Declaring class
      class Profile {
  
          // Constructor of profile class
          constructor(name, age) {
              this.name = name;
              this.age = age;
          }
  
          getName() {
  
              // Method to return name
              return this.name;
          }
  
          getAge() {
  
              // Method to return age
              return this.age;
          }
  
          getClass() {
              return this;
          }
      }
  
  // Class Student extends class Profile
  class Student extends Profile {
  
      // Each data of class Profile can be
      // accessed from student class.
      constructor(name, age, languages) {
  
          // Acquiring of attributes of parent class
          super(name, age);
          this.lang = [...languages];
      }
  
      // Method to display all attributes
      getDetails() {
          console.log("Name : " + this.name);
          console.log("Age : " + this.age);
          console.log("Languages : " + this.lang);
      }
  }
  
  // Creating object of child class with passing of values
  var student1 = new Student("Ankit Dholakia", 24,
              ['Java', 'Python', 'PHP', 'JavaScript']);
  student1.getDetails();
  </script>
• Output:

  Name : Ankit Dholakia
  Age : 24
  Languages : Java,Python,PHP,JavaScript
  

Example 2: In this example, we will see how spread syntax can be used to extend two objects into a third object and display the containing attributes of both objects.

• Program:

  <script>
  // Creating first object
  var obj1 = {
       name: 'Ankit',
       age: 20
  };
  
  // Creating second object
  var obj2 = {
      marks: 50
  };
  
  // Using spread syntax to extend
  // both objects into one
  var object = {
      ...obj1,
      ...obj2
  };
  console.log(object);
  </script>
• Output:

  { name: 'Ankit', age: 20, marks: 50 }