How to use mixins to extend the functionality of classes?

In general, a class can extend only one class at a time to extend its behavior by inheriting the member functions and variables of the class. But in TypeScript, we are allowed to use the mixins that are used to extend the functionality of a class without extending them. It helps us to write reusable code that can be used to extend the functionality of other classes as well.

We can use the mixins to extend the functionality of the classes by assigning them to the class using the assign() method. The assign() method is a method of Object constructor that takes the class with its prototype and the name of the mixin as parameters to extend the functionality of the specified class.

Syntax:

Object.assign(class.prototype, mixinName);

Example: The below example will illustrate the use of the assign method to extend the functionality of the class by adding a method using mixin.

Javascript

// Defining Mixin
let testingMixin = {
    greet(){
      console.log(
`This method is defined inside the
 testing mixin and invoked
 using testing class object.`
)
    }
}

// Defining Class
class testingClass{
  name: string;
  constructor(name: string){
    this.name = name;
  }
  logMessage(){
    console.log(`Hey ${this.name},
     Welcome to GeeksforGeeks!`);
  }
}

// Assigning the mixin to the class
Object.assign(testingClass.prototype,
     testingMixin);

// Invoking the mixin using
// the class instance
const testingObj = new
testingClass("Emrit");
testingObj.logMessage();
testingObj.greet();

Output:

Hey Emrit, Welcome to GeeksforGeeks!
This method is defined inside the testing mixin and invoked using class object.

Example 2: The below example will assign the mixin with two methods and uses the member variables of class inside them.

Javascript

// Defining Mixin
let testingMixin = {
    greet() {
        console.log(`Hey ${this.name},
         Welcome to GeeksforGeeks!`)
    }
    getDetails() {
        console.log(`You are working as a
        ${this.designation} at ${this.company}.`)
    }
}

// Defining Class
class testingClass {
    name: string;
    designation: string;
    company: string;
    constructor(name: string,
        designation: string,
        company: string) {
        this.name = name;
        this.designation = designation;
        this.company = company;
    }
    logMessage() {
        console.log(
            `This method is defined inside
             the testing class as
              member function.`
        );
    }
}

// Assigning mixin to the class
Object.assign(testingClass.prototype,
    testingMixin);

// Invoking mixin methods using the class object
const testingObj = new testingClass("Emrit",
    "Software Developer", "Google");
testingObj.logMessage();
testingObj.greet();
testingObj.getDetails();

Output:

This method is defined inside the testing class as member function.
Hey Emrit, Welcome to GeeksforGeeks!
You are working as a Software Developer at Google.