Easy prototype inheritance
Last updated 7 years ago by peerigon .
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Easy prototype inheritance.

alamid-class is a lightweight library (~1.2 kb compressed and gzipped) that allows you to write classes in JavaScript intuitively. It embraces the dynamic and prototypal nature of JavaScript instead of pretending to be a compiled language like Java.

This library is not intended to obscure prototypal inheritance. There are some caveats you should know about if you're new to it.


npm status

build status dependencies devDependencies

browser support

Quick Start


The syntax is easy to grasp and expressive. A simple class looks like this:

var Class = require("alamid-class");

var Cat = new Class({
    name: "Jimmy",
    age: 3,
    constructor: function (name, age) { = name ||;
        this.age = age || this.age;
    strollAround: function () {
        console.log("MEEEOOOWWW!!! Need food! Now!");
var cat = new Cat();
console.log(cat instanceof Cat); // true
console.log(; // "Jimmy"
console.log(cat.hasOwnProperty("strollAround")); // false because it is inherited by the prototype


Now you can extend that class with:

var Octocat = Cat.extend({
    mood: "sad",
    constructor: function () {
        this._super("Octocat", 5);
    strollAround: function () {
        console.log("Seeking parents ... but in the meantime:");

The special function this._super() inside a method provides access to the overridden method. In this case it's simply a shortcut for

var octocat = new Octocat();
console.log(octocat instanceof Cat); // true
console.log(; // "Octocat"
console.log(octocat.mood); // "sad"
octocat.strollAround(); // "Seeking parents ... but in the meantime: 
                        // MEEEOOOWWW!!! Need food! Now!"

You can also inherit from existing prototypes just like that:

var MyEventEmitter = Class(EventEmitter).extend({
    mute: false,
    emit: function () {
        if (this.mute) return;
        return this._super.apply(this, arguments);

Additionally all instances provide a [read-only reference]( property) called Class on the function that created the instance:

console.log(cat.Class); // Cat
console.log(octocat.Class); // Octocat


Mixins are always part of a flexible class system. In alamid-class a mixin can be any object which will be merged into the prototype.

var Orphan = {
    seekParents: function () {
        console.log("No parents found. " + + " is feeling " + this.mood + " now...");
var Octocat = Cat.extend(Orphan, {
    mood: "sad",
    constructor: function () {
        this._super("Octocat", 5);

var octocat = new Octocat();
octocat.seekParents(); // "No parents found. Octocat is feeling sad now..."

This way you can easy leverage multiple inheritance:

var Orphan = new Class({});
var Octocat = Cat.extend(Orphan, EventEmitter, {
    mood: "sad",
    constructor: function () {
        this._super("Octocat", 5);
        // Be sure to apply the mixin's constructors with the correct arguments manually;;

var octocat = new Octocat();
octocat.emit("seeking parents");
octocat.seekParents(); // "No parents found. Octocat is feeling sad now..."

If two mixins define a property with the same name, the latter mixin will simply override the former. This especially applies to the constructor, so be sure to call it manually.

A class itself can also augment existing objects:

var someObj = {};
Octocat.mixin(someObj);; // Applying the constructor manually

someObj.seekParents(); // "No parents found. Jimmy is feeling sad now..."


For a nicer debugging experience and better stack traces you can give your classes names. Take a look at the difference:


But in order to set the function's name alamid-class needs to use eval(). Since eval() is slow and you usually don't need class names in production, this feature is only available when = true. In production mode, names are simply ignored.

You can set a name like this:

var Cat = new Class("Cat", {...});

var Octocat = Cat.extend("Octocat", {...});


var Class = require("alamid-class");

new Class(name?, proto1, proto2, ...): Class

Creates a new function that will use the given prototypes.

  • name (optional):
    Specifies the name of the returned function. Works only in dev-mode. Defaults to "AnonymousClass"

  • proto1, proto2, ...:
    Multiple prototypes that will be merged into one prototype (while the latter prototype overrides the former). If the given prototype is typeof function, its prototype is used instead. So, passing func or func.prototype is the same.

class.extend(name?, proto1, proto2, ...): Class

Creates a new function that will inherit from class and implement the given prototypes.

class.mixin(obj): Class

Copies all properties of class.prototype to the given target. Returns class.

  • obj:
    The target object that receives all properties.

Boolean variable that switches alamid-class to dev-mode. Defaults to false. Checkout the example.


Object and arrays as properties

Since object and arrays are copied by reference in JavaScript all instances will share the same object as property. Imagine this class:

var MyClass = new Class({
    myObj: {}

var a = new MyClass();
var b = new MyClass();
console.log(a.myObj === b.myObj); // true

This is a common misconception when using prototypal inheritance. So, if you want an object for each instance you should create it within the constructor like this:

var MyClass = new Class({
    myObj: null,
    constructor: function () {
        this.myObj = {};

var a = new MyClass();
var b = new MyClass();
console.log(a.myObj === b.myObj); // false

How does this._super work?

While this is basically accomplished with John Resig's trick, it has been tweaked so the function's length-attribute isn't modified.

Asynchronous calls of this._super

There is one problem with Resig's technique: If the call on this._super() is asynchronous, this._super may point to another function. So, instead of this:

    somethingAsync: function () {
        var self = this;

        setTimeout(function () {
            self._super(); // this might point to another function than the overridden
        }, 0);

you should do:

    somethingAsync: function () {
        var self = this,
            super = this._super;

        setTimeout(function () {
        }, 0);

Read-only property Class

Every instance provides a read-only reference called Class to the function that created the instance. Some could argue that alamid-class should use the built-in constructor-property for that. The problem is, that constructor always points to the topmost function in the prototype-chain:

function A() {}
function B() {}
B.prototype = Object.create(A.prototype);

console.log(new B().constructor === A); // true
console.log(new B().constructor === B); // false

That's the reason why alamid-class introduces a new read-only reference to the function that has been called by new.

About alamid

alamid-class has been extracted as standalone library from the application framework alamid.


Suggestions and bug-fixes are always appreciated. Don't hesitate to create an issue or pull-request. All contributed code should pass

  1. the tests in node.js by running npm test
  2. the tests in all major browsers by running npm run test-browser and then visiting http://localhost:8080/bundle



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