mixwith.ts

A fork of Justin Fagnani’s excellent mixwith.js to add TypeScript compatibility and design time completions.

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.

my-mixin.ts:

const MyMixin = (s: Constructable) => class extends s {
    // mixin methods here
}

my-class.ts

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

Changes from mixwith.js

API & Functional changes

The builder MixinBuilder and its supporting method mix() were modified:

• MixinBuilder.with() accepts up to six mixins rather than an array. This change was made to enable design time completions. The source code can be modified if more mixins are needed.
• MixinBuilder.with() applies instanceOf support for mixins automatically

Types

To resolve Mixins at design time, two types were defined.

export type Constructable<T = {}> = new (...args: any[]) => T;
export type mixin<C extends Constructable, T> = (args: C) => T 

• Constructable<T> defines a constructor function (can be called with new) that creates instances of objects of type T.
• mixin defines a function that takes a Constructable<T> and returns a new type T.

TypeScript will infer both T and C implicitly. However, if you want to refer to a super’s properties from a mixin without red squigglies, reference it.

//open ended mixin.  Use to add new functionality to any type
const openMix = (s: Constructable) => class extends s...

//closed mixin.  Use to apply to a specific interface or class
const closedMix = (s: Constructable<MyInterface>) => class extends s... 

mixwith.ts

The subclass factory pattern does not require a library. This library makes working with the pattern more powerful and easier to use.

• Determine if an object or class has had a particular mixin applied to it.
• Cache mixin applications so that a mixin repeatedly applied to the same superclass reuses its resulting subclass.
• De-duplicate mixin application so that including a mixin multiple times in a class hierarchy only applies it once to the prototype type chain.
• Add instanceof support to mixin functions. That is, the following test passes

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

const foo = new MyClass();

assert(foo instanceof MySuperClass); //true
assert(foo instanceof MyMixin); //true

Advantages of subclass factories over typical JavaScript mixins

Subclass factory style mixins preserve 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.

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

Using Mixins

Classes use mixins in their extends clause. Classes that use mixins can define and override constructors and methods as usual. In conflicts, the right most Mixin wins.

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()
  }

}

A Full Example

In this example, I model a 2D war game that has common functionality across units (shooters, spawners, etc.).

// deno-lint-ignore-file no-unused-vars
import type { Constructable } from "./mod.ts";
import {mix } from "./mod.ts";
interface Position {
    xPos: number;
    yPos: number;
}

class AirForce implements Position {
    xPos = 0;
    yPos = 0;
}

class GroundForce implements Position {
    xPos = 0;
    yPos = 0;
}

const bomber = <c extends Constructable>(s : c) => class extends s {
    bomb() { }
}

const shooter = <c extends Constructable>(s : c)=> class extends s {
    shoot() { }
}

//Notice that this takes a 'Position' type since 
//it needs to refer back to its super class (which implements Position)
const spawner = <c extends Constructable<Position>>(s : c) => class extends s {
    spawn() {
        //spawn logic for things that spawn
        this.yPos = Math.random() * 100;
        this.xPos = Math.random() * 100;
    }
}

//airplanes spawn, shoot, and bomb
class Airplane extends mix(AirForce).with(spawner, shooter, bomber) { }
//helicopters spawn and shoot
class Helicopter extends mix(AirForce).with(spawner, shooter) { }
//tanks spawn and shoot
class Tank extends mix(GroundForce).with(spawner, shooter) { }
//fortifications don't spawn in, but do shoot
class fortification extends mix(GroundForce).with(shooter) { }

// You can also skip the final class definition, but intent is less clear
const myJetFighter = new (mix(AirForce).with(spawner, shooter))

Recommended Usage

1. Use the mix utility builder and it’s supporting with method. The mix and with combination apply all of the library’s goodness (caching, deduplication, instanceOf behavior).

2. Define mixins using <c extends Constructable>(s : c)> instead of (s : Constructable). The latter will complain if used in a mixin before referencing a Constructable<MyInterface>

References

• https://www.typescriptlang.org/docs/handbook/mixins.html
• https://justinfagnani.com/2015/12/21/real-mixins-with-javascript-classes/
• https://mariusschulz.com/blog/mixin-classes-in-typescript

Full API Documentation

Classes

MixinBuilder

• MixinBuilder: Class MixinBuilder

MixinBuilder helper class (returned by mix()).

Type parameters

Defined in

mixwith.ts:237

Method

Method: with()

with<A, B, C, D, E, F>(
  a, 
  b?, 
  c?, 
  d?, 
  e?, 
  f?): Base & A & B & C & D & E & F

Applies a chain of mixins to a base class. The method supports up to six mixins. The mixins are applied in reverse sequence (e.g. the right most mixin is applied first, etc.)

Type parameters

Parameters

Returns

Base & A & B & C & D & E & F

• A new class constructor that includes the functionalities of all mixins and the base class.

BareMixin

▸ BareMixin<C, T>(mixin): mixin

A basic mixin decorator that applies the mixin using the apply function so that it can be used with isApplicationOf, hasMixin, and other mixin decorator functions.

Type parameters

Parameters

Returns

mixin

• A new mixin function.

Defined in

mixwith.ts:196

Cached

▸ Cached<C, T>(mixin): mixin

Decorates mixin so that it caches its applications. When applied multiple times to the same superclass, mixin will only create one subclass, memoize it and return it for each application.

Note: If mixin somehow stores properties its classes constructor (static properties), or on its classes prototype, it will be shared across all applications of mixin to a super class. It’s reccomended that mixin only access instance state.

Type parameters

Parameters

Returns

mixin

• Returns the cached mixin application.

Defined in

mixwith.ts:125

DeDupe

▸ DeDupe<C, T>(mixin): mixin

Decorates mixin so that it only applies if it’s not already on the prototype chain.

Type parameters

Parameters

Returns

mixin

• A deduplicated mixin that extends the original superclass if needed.

Defined in

mixwith.ts:154

HasInstance

▸ HasInstance<T>(mixin): T

Adds a Symbol.hasInstance implementation to the provided mixin object to enable the use of the instanceof operator with instances of classes that include the mixin.

Type parameters

Parameters

Returns

T

• The mixin object with the Symbol.hasInstance implementation.

Defined in

mixwith.ts:174

Mixin

▸ Mixin<C, T>(mixin): mixin

Decorates a mixin function to add deduplication, application caching, and instanceof support.

Type parameters

Parameters

Returns

mixin

• A new mixin function.

Defined in

mixwith.ts:206

apply

▸ apply<C, T>(superclass, mixin): T

Applies mixin to superclass.

apply stores a reference from the mixin application to the unwrapped mixin to make isApplicationOf and hasMixin work.

This function is useful for mixin wrappers that want to automatically enable hasMixin support.

Type parameters

Parameters

Returns

T

• A new class with the mixin’s behavior applied.

Defined in

mixwith.ts:28

hasMixin

▸ hasMixin<T>(o, mixin): boolean

Checks if the provided mixin has been applied to the given prototype object.

Type parameters

Parameters

Returns

boolean

• Returns true if the mixin has been applied, otherwise false.

Defined in

mixwith.ts:59

isApplicationOf

▸ isApplicationOf<T>(proto, mixin): boolean

Returns true iff proto is a prototype created by the application of mixin to a superclass.

isApplicationOf works by checking that proto has a reference to mixin as created by apply.

Type parameters

Parameters

Returns

boolean

whether proto is a prototype created by the application of mixin to a superclass

Defined in

mixwith.ts:47

mix

▸ mix<C>(superclass?): MixinBuilder

A fluent interface to apply a list of mixins to a superclass.

class X extends mix(Object).with(A, B, C) {}

The mixins are applied in order to the superclass, so the prototype chain will be: X->C’->B’->A’->Object.

This is purely a convenience function. The above example is equivalent to:

C = Mixin(C)
B = Mixin(B)
A = Mixin(A)
class X extends C(B(A(Object))) {}

Function

Type parameters

Parameters

Returns

MixinBuilder

• A builder object to apply mixins to the superclass.

Defined in

mixwith.ts:231

unwrap

▸ unwrap<T>(wrapper): T

Unwraps the function wrapper to return the original function wrapped by one or more calls to wrap. Returns wrapper if it’s not a wrapped function.

Type parameters

Parameters

Returns

T

• Returns the original mixin if available, otherwise the wrapper itself.

Defined in

mixwith.ts:106

wrap

▸ wrap<C, T>(mixin, wrapper): mixin

Sets up the function mixin to be wrapped by the function wrapper, while allowing properties on mixin to be available via wrapper, and allowing wrapper to be unwrapped to get to the original function.

wrap does two things:

1. Sets the prototype of mixin to wrapper so that properties set on mixin inherited by wrapper.
2. Sets a special property on mixin that points back to mixin so that it can be retreived from wrapper

Function

Type parameters

Parameters

Returns

mixin

• Returns the wrapper mixin.

Defined in

mixwith.ts:89