mixwith.js

A simple, powerful and safe mixin library for ES6.

Overview

mixwith differs from other mixin approaches because it does not copy properties from one object to another. Instead, mixwith works with "subclass factories" which create a new class that extends a superclass with the mixin - this is called a mixin application.

Subclass factory style mixins take advantage of two awesome features of ES6 classes: class expressions, and expressions in the extends clause of a class declaration.

Quick Example

Define a Mixin:

let MyMixin = (superclass) => class extends superclass {
  // mixin methods here
};

Use a Mixin without mixwith:

class MyClass extends MyMixin(MySuperClass) {
  // class methods here, go ahead, use super!
}

Use a Mixin with mixwith:

class MyClass extends mix(MySuperClass).with(MyMixin, OtherMixin) {
  // class methods here, go ahead, use super!
}

mixwith preserves the object-oriented inheritance properties that classes provide, like method overriding and super calls, while letting you compose classes out of mixins without being constrained to a single inheritance hierarchy, and without monkey-patching or copying.

Advantages of subclass factories over typical JavaScript mixins

Method overriding that just works

Methods in subclasses can naturally override methods in the mixin or superclass, and mixins override methods in the superclass. This means that precedence is preserved - the order is: subclass -> mixin__1 -> ... -> mixin__N -> superclass.

super works

Subclasses and mixins can use super normally, as defined in standard Javascript, and without needing the mixin library to do special chaining of functions.

Mixins can have constructors

Since super() works, mixins can define constructors. Combined with ES6 rest arguments and the spread operator, mixins can have generic constructors that work with any super constructor by passing along all arguments.

Prototypes and instances are not mutated

Typical JavaScript mixins usually used to either mutate each instance as created, which can be bad for performance and maintainability, or modify a prototype, which means every object inheriting from that prototype gets the mixin. Subclass factories don't mutate objects, they define new classes to subclass, leaving the original superclass intact.

Usage

Defining Mixins

A mixin is simply a function that takes a superclass and returns a subclass of it, using ES6 class expressions:

let MyMixin = (superclass) => class extends superclass {

  constructor(args...) {
    // mixins should either 1) not define a constructor, 2) require a specific
    // constructor signature, or 3) pass along all arguments.
    super(...args);
  }

  foo() {
    console.log('foo from MyMixin');
    // this will call superclass.foo()
    super.foo();
  }

};

Mixins defined this way do not require any helpers to define or use. You can use this pattern without mixwith at all!

Using Mixins

Without mixwith, just invoke them inside a classes extends clause:

class MyClass extends MyMixin(MySuperClass) {
}

mixwith provides a helper that's a bit nicer when applying multiple mixins, and adds some features like mixin-deduplication and @@hasInstance support (@@hasInstance overloads instanceof, but isn't supported in any browsers yet).

class MyClass extends mix(MySuperClass).with(MyMixin) {
}

Classes that use mixins can define and override constructors and methods as usual.

class MyClass extends mix(MySuperClass).with(MyMixin) {

  constructor(a, b) {
    super(a, b); // calls MyMixin(a, b)
  }

  foo() {
    console.log('foo from MyClass');
    super.foo(); // calls MyMixin.foo()
  }

}