Mixins in JavaScript

Published August 9, 2020

Mixins are those functions that cannot be used independently but when used with any class provide useful functionalities.

Consider this class User,

// class User
class User {
  constructor(name) {
    this.name = name;
  }

  getName() {
    return this.name;
  }
}

Here we have a class User with the property name and a method that returns the property name.

Let's now make an object with methods sayGreeting() and sayGoodBye() which greets a user by name and says goodbye to a user respectively.

const greetAndBye = {
  // greeting method
  sayGreeting() {
    return `Hello, ${this.name}`;
  },

  // goodbye method
  sayGoodBye() {
    return `Goodbye, ${this.name}`;
  },
};

As you can see, if we try to call the methods independently the methods are not useful at all.

Let's now copy all these methods to the prototype chain of the User class using the Object.assign() static method.

// greetAndBye Mixin
const greetAndBye = {
  // greeting method
  sayGreeting() {
    return `Hello, ${this.name}`;
  },

  // goodbye method
  sayGoodBye() {
    return `Goodbye, ${this.name}`;
  },
};

// class User
class User {
  constructor(name) {
    this.name = name;
  }

  getName() {
    return this.name;
  }
}

// copy greetAndBye object methods
// to prototype of User class
Object.assign(User.prototype, greetAndBye);

// make an object
const John = new User("John");

// invoke mixin methods
console.log(John.sayGreeting());
console.log(John.sayGoodBye());

Now you can call the mixin methods we copied to the prototype chain of User class.

  • The greetAndBye object here is called a mixin.
  • We are not doing any inheritance but copying of all the methods to the prototype chain of the User class.
  • Mixins can be attached to more than one class.

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