Dynamic multiple inheritance for JavaScript and TypeScript. Without mixins.

Polytype is a library that adds support for dynamic multiple inheritance to JavaScript and TypeScript with a simple syntax. “Dynamic” means that changes to base classes at runtime are reflected immediately in all derived classes just as programmers would expect when working with single prototype inheritance.

Polytype runs in Node.js, Deno and in current versions of all major browsers.

• Contents
• Features
• Setup Instructions
  • In Node.js
  • In Deno
  • In the browser
• Usage
  • Inheriting from multiple base classes
  • Using methods and properties from multiple base classes
  • Static methods and properties
  • Invoking multiple base constructors
  • instanceof
  • in
  • isPrototypeOf
  • Finding the base classes
  • Dispatching invocations to multiple base classes
  • Dynamic base class changes
• TypeScript support
• Caveats
  • this in base constructors
  • Base classes with private instance members
  • for...in iterations
  • Member resolution order
  • Ambiguous protected instance members
• Compatibility

• Python style multiple inheritance
• Works in Node.js, Deno and in all new browsers
• Full TypeScript support
• Zero dependencies
• Access to all inherited base class features
  • constructors
  • methods, getters and setters, class fields
  • value properties on base classes and base instance prototypes
• in, instanceof and isPrototypeOf integration

Polytytpe is available in two flavors: a module build (comprising CommonJS and ECMAScript modules) with exported definitions and a script build where all definitions are accessible through global objects. Apart from this, both builds provide the same features and are available in the standard package.

If you are using Node.js, you can install Polytype with npm.

npm install polytype

Then you can import it in your code like any module.

const { classes } = require("polytype"); // CommonJS syntax

or

import { classes } from "polytype"; // ECMAScript module syntax

Alternatively, you can import the script build at the start of your application and access Polytype definitions through global objects.

require("polytype/global"); // CommonJS syntax

or

import "polytype/global"; // ECMAScript module syntax

You can import the module or script build of Polytype from a CDN of your choice, e.g.

import { classes } from "https://esm.sh/polytype"; // Module build

or

import "https://esm.sh/polytype/global"; // Script build

In an HTML‐based application, the script build of Polytype can be simply embedded. Just download polytype.min.js and include it in your HTML file.

<script src="polytype.min.js"></script>

Alternatively, you can hotlink the script from the latest release package using a CDN of your choice.

<script src="https://cdn.jsdelivr.net/npm/polytype@0.17.0/lib/polytype.min.js"></script>

If your browser application already uses ECMAScript modules, you can also import the module build (“.mjs”) in contexts where Polytype specific definitions like classes are required. This has the advantage to avoid possible naming conflicts on global objects.

import { classes } from "https://cdn.jsdelivr.net/npm/polytype@0.17.0/lib/polytype.min.mjs";

For example, declare a derived class ColoredCircle that inherits from both base classes Circle and ColoredObject.

class Circle { constructor(centerX, centerY, radius = 1) { this.moveTo(centerX, centerY); this.radius = radius; } get diameter() { return this.radius * 2; } set diameter(diameter) { this.radius = diameter / 2; } moveTo(centerX, centerY) { this.centerX = centerX; this.centerY = centerY; } reset() { this.moveTo(0, 0); this.radius = 1; } toString() { return `circle with center (${this.centerX}, ${this.centerY}) and radius ${this.radius}`; } } class ColoredObject { constructor(color) { this.color = color; } static areSameColor(obj1, obj2) { return obj1.color === obj2.color; } paint() { console.log(`painting in ${this.color}`); } reset() { this.color = "white"; } toString() { return `${this.color} object`; } } class ColoredCircle extends classes(Circle, ColoredObject) // Base classes as comma‐separated params { // Add methods here. }

const c = new ColoredCircle(); c.moveTo(42, 31); c.radius = 2; c.color = "red"; console.log(c.centerX, c.centerY); // 42, 31 console.log(c.diameter); // 4 c.paint(); // "painting in red"

As usual, the keyword super invokes a base class method or property accessor when used inside a derived class.

class ColoredCircle extends classes(Circle, ColoredObject) { paint() { super.paint(); // Using method paint from some base class } }

If different base classes include a member with the same name, the syntax

super.class(DirectBaseClass).member

can be used to make the member access unambiguous.

class ColoredCircle extends classes(Circle, ColoredObject) { toString() { // Using method toString from base class Circle const circleString = super.class(Circle).toString(); return `${circleString} in ${this.color}`; } }

More generally, super.class(DirectBaseClass)[propertyKey] can be used to reference a (possibly inherited) property of a particular direct base class in the body of a derived class.

Note: In TypeScript, the syntax described here cannot be used to access protected instance members, so it is currently not possible to disambiguate between protected instance members having the same name, the same index or the same symbol in different base classes.

Static methods and property accessors are inherited, too.

ColoredCircle.areSameColor(c1, c2)

same as

ColoredObject.areSameColor(c1, c2)

In the constructor of a derived class, use arrays to group together parameters to be passed to the constructors of each direct base class.

class ColoredCircle extends classes(Circle, ColoredObject) { constructor(centerX, centerY, radius, color) { super ( [centerX, centerY, radius], // Circle constructor params [color] // ColoredObject constructor params ); } }

If you prefer to keep parameter lists associated to their base classes explicitly without relying on order, there is an alternative syntax.

class GreenCircle extends classes(Circle, ColoredObject) { constructor(centerX, centerY, radius) { super ( { super: ColoredObject, arguments: ["green"] }, { super: Circle, arguments: [centerX, centerY, radius] } ); } }

There is no need to specify an array of parameters for each direct base constructor. If the parameter arrays are omitted, the base constructors will still be invoked without parameters.

class WhiteUnitCircle extends classes(Circle, ColoredObject) { constructor() { super(); // Base constructors invoked without parameters this.centerX = 0; this.centerY = 0; // The radius has been already set to 1 by the Circle constructor. this.color = "white"; } }

The instanceof operator works just as it should.

const c = new ColoredCircle(); console.log(c instanceof Circle); // true console.log(c instanceof ColoredObject); // true console.log(c instanceof ColoredCircle); // true console.log(c instanceof Object); // true console.log(c instanceof Array); // false

In pure JavaScript, the expression

B.prototype instanceof A

determines if A is a base class of class B.

Polytype takes care that this test still works well with multiple inheritance.

console.log(ColoredCircle.prototype instanceof Circle); // true console.log(ColoredCircle.prototype instanceof ColoredObject); // true console.log(ColoredCircle.prototype instanceof ColoredCircle); // false console.log(ColoredCircle.prototype instanceof Object); // true console.log(Circle.prototype instanceof ColoredObject); // false

The in operator determines whether a property is in an object or in its prototype chain. In the case of multiple inheritance, the prototype “chain” looks more like a directed graph, yet the function of the in operator is the same.

const c = new ColoredCircle(); console.log("moveTo" in c); // true console.log("paint" in c); // true

console.log("areSameColor" in ColoredCircle); // true console.log("areSameColor" in Circle); // false console.log("areSameColor" in ColoredObject); // true

isPrototypeOf works fine, too.

const c = new ColoredCircle(); console.log(Circle.prototype.isPrototypeOf(c)); // true console.log(ColoredObject.prototype.isPrototypeOf(c)); // true console.log(ColoredCircle.prototype.isPrototypeOf(c)); // true console.log(Object.prototype.isPrototypeOf(c)); // true console.log(Array.prototype.isPrototypeOf(c)); // false

console.log(Circle.isPrototypeOf(ColoredCircle)); // true console.log(ColoredObject.isPrototypeOf(ColoredCircle)); // true console.log(ColoredCircle.isPrototypeOf(ColoredCircle)); // false console.log(Object.isPrototypeOf(ColoredCircle)); // false console.log(Function.prototype.isPrototypeOf(ColoredCircle)); // true

In single inheritance JavaScript, the direct base class of a derived class is obtained with Object.getPrototypeOf.

const DirectBaseClass = Object.getPrototypeOf(DerivedClass);

If a class has no explicit extends clause, Object.getPrototypeOf returns Function.prototype, the ancestor of all classes.

Of course this method cannot work with multiple inheritance, since there is no way to return multiple classes without packing them in some kind of structure. For this and other use cases, Polytype exports the function getPrototypeListOf, which can be used to get an array of direct base classes given a derived class.

const { getPrototypeListOf } = require("polytype"); // Or some other kind of import. function getBaseNames(derivedClass) { return getPrototypeListOf(derivedClass).map(({ name }) => name); } console.log(getBaseNames(ColoredCircle)); // ["Circle", "ColoredObject"] console.log(getBaseNames(Int8Array)); // ["TypedArray"] console.log(getBaseNames(Circle)); // [""] i.e. [Function.prototype.name]

When the the script build of Polytype is used, no functions will be exported. Instead, getPrototypeListOf will be defined globally as Object.getPrototypeListOf.

Sometimes it is useful to have a method or setter invocation dispatched to all direct base classes rather than just to one of them. Common examples are event handlers and Angular lifecycle hooks implemented in multiple base classes.

Polytype has no dedicated syntax for this use case: simply override the method or setter in the derived class and invoke the base implementations from there.

class ColoredCircle extends classes(Circle, ColoredObject) { reset() { super.class(Circle).reset(); super.class(ColoredObject).reset(); } }

This can also be done with an iteration instead of referencing the base classes one by one.

class ColoredCircle extends classes(Circle, ColoredObject) { reset() { for (const baseClass of getPrototypeListOf(ColoredCircle)) baseClass.reset(); } }

If a property in a base class is added, removed or modified at runtime, the changes are immediately reflected in all derived classes.

const c = new ColoredCircle(); Circle.prototype.sayHello = () => console.log("Hello!"); c.sayHello(); // "Hello!"

Polytype has built‐in TypeScript support: you can take advantage of type checking while working with multiple inheritance without installing any additional packages. If you are using an IDE that supports TypeScript code completion like Visual Studio Code, you will get multiple inheritance sensitive suggestions as you type. A TypeScript version of the ColoredCircle sample code above can be found in ColoredCircle.ts in the example folder.

Neither JavaScript nor TypeScript offer native support for multiple inheritance of any kind. Polytype strives to make up for this deficiency, but some important limitations remain.

In single inheritance, the value of this inside a constructor and in the constructors of all ancestor classes is the same object that the new operator returns. Not so in Polytype multiple inheritance, where the value of this inside base constructors is a special object called a substitute. Substitutes are necessary to make base constructors run independently from each other, each one with its own fresh instance. Only after all base constructors of a derived class have run, the properties of the substitues are merged into one object.

let aThis, bThis, cThis; class A { constructor() { aThis = this; } } class B { constructor() { bThis = this; } } class C extends classes(A, B) { constructor() { super(); cThis = this; } } const obj = new C(); console.log(aThis === obj); // Prints false. console.log(bThis === obj); // Prints false. console.log(aThis === bThis); // Prints false. console.log(cThis === obj); // Prints true.

After the base constructors have run, the substitutes become detached, meaning that they no longer reflect changes to the real instance and vice versa. This may cause problems with classes that store or bind the value of this in the constructor to use it later.

For example, the following code that attaches a click handler to an HTML button will not work as one might expect, because this in the event handler refers to a substitute that is unaware of the name property later assigned to the instance of the derived class.

class A { constructor() { button.onclick = () => alert(this.name); } } class B { } class C extends classes(A, B) { } const c = new C(); c.name = 'Test'; button.click(); // Alerts "undefined" rather than "Test".

While Polytype takes some actions to mitigate the effect of detached substitutes, like retargeting bound methods if necessary, classes that access the value of this in the constructor in order to use it later are generally not safe to subclass.

Polytype classes do not inherit private members declared in their base classes. For this reason, extending base classes with private instance members will not work well and likely result in TypeErrors at runtime.

When only single inheritance is used, a for...in iteration over a class constructor enumerates not only names of enumerable properties defined on the constructor object itself, but also names of enumerable properties defined on all base constructors in its prototype chain. Enumerable properties on class constructors can be defined with a static field, or assigned dynamically.

class FooBarClass { static foo = "foo"; } FooBarClass.bar = "bar"; class BazClass extends FooBarClass { static baz = "baz"; } for (const name in BazClass) console.log(name); // Prints "baz", "foo" and "bar".

As it happens, this behavior no longer holds with Polytype multiple inheritance. The effect is that names of static fields and other enumerable properties defined on a base constructor are not enumerated by for...in statements when the inheritance line crosses a class listed in some extends classes(...) clause.

class BazClass extends classes(FooBarClass) { static baz = "baz"; } for (const name in BazClass) console.log(name); // Prints just "baz".

For this reason, and because generally better alternatives exist, iterating over Polytype classes and their derived classes with for...in is not recommended.

Multiple base classes may expose members with the same name, the same index or the same symbol. When this happens, any unqualified access to one of those members will have to determine the implementation to be used. The approach taken by Polytype is to pick the implementation found in the first direct base class that contains the (possibly inherited) member.

class A { } class B { whoAmI() { console.log("B"); } } class C { whoAmI() { console.log("C"); } } class ABC extends classes(A, B, C) { } const abc = new ABC(); abc.whoAmI(); // Prints "B".

This is similar to the depth‐first search algorithm of old‐style classes in Python 2, but it is different from the behavior of several other programming languages that support multiple inheritance, and it may not match your expectations if you come from a C++ background.

When a derived class inherits from multiple base classes, it is possible for inherited members in different base classes to share the same property key, i.e. the same name, the same index or the same symbol. For these cases, Polytype provides the syntax super.class(DirectBaseClass)[propertyKey] to specify the direct base class containing the member to be accessed. This works all the time in JavaScript and works in TypeScript for any public or static member, but results in a compiler error when applied to a protected instance member.

class RecordLeft { protected id: number; } class RecordRight { protected id: string; } class Record extends classes(RecordLeft, RecordRight) { public printRightId(): void { // error TS2446: Property 'id' is protected… console.log(super.class(RecordRight).id.padStart(10, ' ')); } }

As a type‐safe workaround, use an intermediate class to expose the inherited member using a different name without making it public.

class RecordRightProxy extends RecordRight { protected get rightId(): string { return super.id; } } class Record extends classes(RecordLeft, RecordRightProxy) { public printRightId(): void { console.log(super.rightId.padStart(10, ' ')); // OK } }

Polytype was successfully tested in the following browsers/JavaScript engines.

  Chrome 80+
  Safari 14+
  Edge 80+
  Firefox 74+
  Opera 67+
  Node.js 16+
  Deno 1.24+

The minimum supported TypeScript version is 4.7.

Bundlers and other tools that process uncompressed Polytype source files are required to parse ECMAScript 2020 or higher syntax.