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A simple module for building and extending classes, with mixins and more.


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class-plus is a simple class builder utility, which adds support for specifying class member variables as well as mix-ins. It does this by providing a custom API that augments an ES2015 class with custom features.


Native JavaScript classes (introduced in ECMAScript 2015) are a welcome addition to the language, but two things bother me about them:

  1. You cannot declare properties inside the class (you have to do it in the constructor, which I do not like).
    • Yes, there is a TC39 proposal for this, but it's not yet available to normal users.
  2. Classes can only inherit from one single parent class.

I wrote the class-plus module to provide these features to ES2015 classes using a simple API, while offering some additional niceties. The full set of features are:

  • Provide an easy API to generate classes with additions.
  • Support for adding properties right above the class definition (as close to inside it as we can get).
  • Support for multiple mix-ins, will merge both properties and methods from multiple classes.
  • Support for adding static properties.
  • Optional easy way to mix-in the Node.js EventEmitter class.
  • Optional hook system (async event emitters).
  • Optional automatic conversion of callback methods to async ones.
  • No dependencies


Use npm to install the module:

npm install class-plus

Then use require() to load it in your code:

const Class = require('class-plus');

Then call Class() to create classes. The function takes two arguments: an object with properties for the class, and the class definition itself (in native ES2015 format). Example:

const Animal = Class({ // class member variables nickname: '', color: '' }, class Animal { // class methods constructor(new_name, new_color) { this.nickname = new_name; this.color = new_color; } getInfo() { return("Nickname: " + this.nickname + "\nColor: " + this.color); } });

This defines a class called Animal, with two member variables, nickname and color, a constructor and a getInfo() method which returns the nickname and color. Usage of this class is exactly what you would expect:

var dog = new Animal('Spot', 'Green'); console.log( dog.getInfo() );

Of course, you can also access the class member variables, as all members are public.

dog.nickname = 'Skippy'; dog.color = 'Blue'; console.log( dog.getInfo() );

Creating Subclasses

To create a subclass that inherits from a base class, use the built-in extends keyword:

const Bear = Class({ // define a new member variable wants: 'Honey' }, class Bear extends Animal { // and a new method roar() { console.log("Roar! Give me " + this.wants + "!"); } });

This defines a Bear class which inherits from the base Animal class, including its constructor. What we did is extend the base class by introducing a new member variable wants, and a new method roar(). Everything else from the base class will be present in subclass instances.

var grizzly = new Bear('Fred', 'Brown'); console.log( grizzly.getInfo() ); grizzly.wants = 'blood'; grizzly.roar();

Calling Superclass methods

You can also explicitly invoke a superclass method using the built-in super keyword:

const Bear = Class({ // define a new member variable wants: 'Honey' }, class Bear extends Animal { // and a new method roar() { console.log("Roar! Give me " + this.wants + "!"); } // override base class method getInfo() { // first, get info from base class var info = super.getInfo(); // append bear info and return combined info info += "\nWants: " + this.wants; return info; } });

So here we are overriding the base class getInfo() method, but the first thing we do is call the superclass method of the same name. This is done using the super keyword, which points to the parent class.

Static Members

While the native built-in class system supports static methods it doesn't support non-function static members.

Using class-plus you can define static class members (variables or methods) by using the __static property. These members do not become part of class instances, but instead live inside the class reference object, and must be accessed that way too. Example class definition:

const Beer = Class({ // static members __static: { types: ['Lager', 'Ale', 'Stout', 'Barleywine'] }, // class member variables name: '', type: '' }, class Beer { // standard class syntax for methods // class constructor constructor(new_name, new_type) { this.name = new_name; if (Beer.types.indexOf(new_type) == -1) throw("Type not known: " + new_type); this.type = new_type; } getInfo() { return("Name: " + this.name + "\nType: " + this.type); } });

Here we define a Beer class which has a static member defined in the __static property. Anything placed there will not be propagated to class instances, and must be accessed using the class reference variable instead (e.g. Beer in the above example). As you can see in the constructor, we are checking the new type against the types array which is declared static, so we are getting to the list by using the syntax: Beer.types rather than this.types.

If you were to change Beer.types later on, then all classes would see the changes instantly. The content is effectively shared.


While achieving mix-ins (essentially multiple inheritance) is technically possible using ES2015 classes, the syntax leaves much to be desired.

Using class-plus you can simply merge in one or more "mix-in" classes using the __mixins property. This will import all the variables, methods and static members from the specified classes, excluding constructors. Example:

const Liquid = Class({ flavor: "sweet" }, class Liquid {}); const Glass = Class({ size: 8 }, class Glass {}); const Soda = Class({ __mixins: [ Liquid, Glass ] }, class Soda { drink() { console.log("Yum, " + this.size + " oz of " + this.flavor + " drink!"); } });

In the above example we are importing all the variables and methods of the Liquid and Glass classes into our Soda class. Then, they are accessible using the normal this keyword, as if they were defined in the class.

I often use mix-ins to spread my larger classes across multiple source files, like this:

const Soda = Class({ __mixins: [ require('./liquid.js'), require('./glass.js') ] }, ... );

Note that mix-in properties and methods will only be imported if they aren't already defined in your class. Meaning, they will not clobber any existing class members.

If the mix-in classes you are importing have their own parent classes, those should be separately listed in the __mixins array. Meaning, the prototype chain of the mix-ins is not automatically imported -- only the top-level methods and properties on the specified class are merged in. You'll need to specify parent classes if you want those merged in as well.

Event Emitters

I find myself frequently inheriting from Node's EventEmitter in my classes, so I added a shortcut for it in class-plus. Simply include a property named __events and set it to true, and your class will magically become an EventEmitter. Example:

const Party = Class({ __events: true }, class Party { start() { console.log("Let's get this party started!"); this.emit('dance'); } }); var birthday = new Party(); birthday.on('dance', function() { console("I'm dancing!"); } ); birthday.start();

Setting the __events property is equivalent to including Node's EventEmitter class in your mix-ins array:

const Party = Class({ __mixins: [ require('events').EventEmitter ] }, ... );

So __events is really just a shortcut for that.


Taking event listeners one step further, class-plus introduces an optional "hook" system for use in your classes, where custom events can be hooked, and you can run asynchronous operations in each listener. If multiple listeners are registered on a given hook, they are all fired in sequence. If any listener returns an error, the sequence is aborted, and the error passed to the original caller.

Your listeners can be either callback based, or native async functions. To enable hooks in your class, simply include a __hooks property and set it to true. Example:

const Party = Class({ __hooks: true }, class Party { start() { console.log("The party has finally started."); } }); var birthday = new Party(); birthday.registerHook( 'prestart', function(item, callback) { // delay the party by 100ms setTimeout( function() { callback(); }, 100 ); } ); birthday.fireHook( 'prestart', "Get ready!", function(err) { // all prestart hooks completed, let's go // this will run about 100ms later birthday.start(); });

The idea here is similar with events, where one or more listeners can be registered, but in this case the hooks fire in an asynchronous manner, each with a callback to advance to the next listener, or to complete the hook sequence. In fact, the whole system can be used with native async functions (in Node 8+). Example:

const Party = Class({ __hooks: true }, class Party { start() { console.log("The party has finally started."); } }); (async function() { var birthday = new Party(); birthday.registerHook( 'prestart', async function(item) { // do something async here await myfunc(); } ); await birthday.fireHook( 'prestart', "Get ready!"); // all async prestart hooks completed, let's go birthday.start(); })();

Async/Await Conversion

Node.js version 8 introduced native support for the async/await pattern. If your class has callback-based methods that you want to auto-convert into async/await, simply declare a __asyncify property, and set it to true:

var Sleeper = Class({ __asyncify: true }, class Sleeper { sleep(milliseconds, callback) { // sleep for N milliseconds, then fire callback setTimeout( function() { callback(); }, milliseconds ); } });

This will automatically detect all your callback-based methods in the class, and then convert them to async using Node's util.promisify(), making them instantly ready for async/await. Example usage:

var snooze = new Sleeper(); async function main() { await snooze.sleep( 1000 ); // waits for 1 second here console.log("This happened 1 second later!"); }; main();

The automatic detection mechanism looks inside your method signatures for argument lists ending with the name callback (or cb). If you use any other variable name for the callback argument, this will skip over it. Also, class-plus will take care not asyncify a function that is already async.

If you only want some of your methods to be asyncified, set the __asyncify property to an array containing all the method names. Example:

{ // only asyncify some methods __asyncify: ["sleep"] }

Alternatively, you can set __asyncify to a regular expression, to match against all of your class method names. If the pattern matches, the function will be converted. Example:

{ // only asyncify some methods __asyncify: /^(sleep|someOtherFunction)$/ }

Note that in order for your methods to be async-compatible, they must accept a callback as the final argument, and that callback must be called using the standard Node.js convention (i.e. (err) or (err, result)). The error must be the first argument sent to the callback (or false/undefined on success), and a result, if any, must be the second argument.

Async Return Values

If your callback is fired using the typical (err, result) arguments, such as this:

const Soda = Class({ __asyncify: true }, class Soda { pour(callback) { setTimeout( function() { callback( null, "8oz" ); }, 250 ); } });

Then you can access the result using async in this way:

let drink = new Soda(); try { let result = await drink.pour(); console.log(result); } catch (err) { throw err; }

However, if your callback has multiple result arguments, like this:

const Soda = Class({ __asyncify: true }, class Soda { pour(callback) { setTimeout( function() { callback( null, 8, "oz" ); }, 250 ); } });

They will be returned in an array which you can destruct like this:

let drink = new Soda(); try { let [amount, units] = await drink.pour(); console.log(amount, units); } catch (err) { throw err; }

Finally, for ultimate control over the async conversion, you can pre-declare the names of your callback arguments in the __asyncify property, by setting it to an object containing the function names as keys, and the argument names as array items. Then, the result can be awaited as a destructed object with named keys. Here is how to set this up in the class:

const Soda = Class({ __asyncify: { pour: ['amount', 'units'] // declare pour() callback arg names here } }, class Soda { pour(callback) { setTimeout( function() { callback( null, 8, "oz" ); // fire callback as usual }, 250 ); } });

And here is how to await it:

let drink = new Soda(); try { let { amount, units } = await drink.pour(); console.log(amount, units); } catch (err) { throw err; }

The idea here is that the calling code can select which of the arguments it wants. For example, we can omit units and only fetch amount:

let { amount } = await drink.pour();


The MIT License

Copyright (c) 2019 - 2022 Joseph Huckaby

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.




Last updated on 01 Feb 2022

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