A prototype-based inheritance library that makes it easy to create objects and inheritance hierarchies without losing the power of javascript's prototype system.

npm install proto
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A prototype-based inheritance/class library that makes it easy to create objects and inheritance hierarchies without losing the power of javascript's prototype system.

Why Use proto?

  • instanceof works with proto classes
  • constructors are inheritable
  • non-objects can be returned from a constructor (even undefined)!
  • easy access to an object's superclass
  • you don't hafta use the new operator
  • native objects work with proto. proto properly creates classes that inherit from native objects - even all the Error types. Inheriting javascript objects has some limitations (see below)
  • prototype and constructor properties are propertly set
  • proto doesn't use Object.create so it should work with older browsers ( testers welcome! )
  • proto is small: ( 896 bytes minified and in AMD format )
  • proto is lightweight. It doesn't attempt to emulate class-based languages or create any fancy features you probably don't actually need (interfaces, abstract classes, etc)


var Person = proto(function() {       // prototype builder
    this.init = function(legs, arms) {      // constructor
        this.legs = legs
        this.arms = arms

    this.getCaughtInBearTrap = function() { // instance method
        this.legs -= 1
    this.limbs = function() {
        return this.arms + this.legs

var Girl = proto(Person, function() {       // inheritance
    this.haveBaby = function() {
        return Person(2,2)

var g = Girl(2,2)                          // instantiation
console.log("Girl has "+g.limbs()+" limbs")
console.log(": (")

var newPerson = g.haveBaby()
console.log("New person has" +newPerson.limbs()+" limbs : )")


npm install proto


Accessing proto:

var proto = require('proto') // node.js

define(['proto'], function(proto) { ... } // amd

proto; // defines proto globally if you really
       //   want to shun module-based design

Using proto:

var Parent = proto(function() {
    this.init = function(v) {   // constructor
        if(v > 0) {
            this.x = v                // you can normally access the object with this inside methods
        } else if(v !== undefined) {
            return true                  // you can return non-object values
        } else {
            return proto.undefined    // return undefined by using a special constructor return value

    this.anythingElse = 5   // static properties can be accessed by the class and the instance

    var that = this // private functions don't have access to the correct 'this', so pass it in
    var privateFn = function(me, arg1, etc) {
        that.x = arg1 + etc
    this.doSomething = function() {
        privateFn(this, this.x, 1)

// you can inherit from any object!
// the resulting object factory will generate instances inheriting from:
    // [if you inherit from]
        // [a function]: that function's prototype
        // [anything else]: that object itself
var Child = proto(Parent, function(superclass) {
    this.init = function() {, arguments) // super-class method call
        this.r = 10
        return this

    // create static methods just like instance methods - you can access them from the constructor
    this.staticMethod = function(x) {
        return this.constructor(x+12)        // uses its own constructor to create a Child object

var object = Child(1)                // instantiation
object.doSomething()                 // method call (as usual)
var object2 = Child.staticMethod(1)  // static method call

Creating a custom Error object:

var CustomError = proto(Error, function(superclass) { = 'CustomError'

    this.init = function(msg, properties) {, msg)
        for(var n in properties) {
            this[n] = properties[n]

Limitations of proto

  • Inheriting from Error and other exception types doesn't automatically set a correct name property, so you need to set it in the contructor manually.
  • Objects inheriting from String can't use the toString method.
  • Inheriting from Array doesn't work.
  • Inheriting from RegExp doesn't work either (the results can't use the test or `match methods).
  • You can't properly access any non-writable properties of a function from the returned proto-object factory though the properties will work correctly on instances. This includes: name, length, arguments, and caller.
  • Some properties are read-only and so can't be reset on the prototype object. An example is name on firefox.


  • Browser testing
    • Chrome [ ]
    • Firefox [ ]
    • IE10 [ ]
    • IE9 [ ]
    • IE8 [ ]
    • Opera [ ]

How to Contribute!

Anything helps:

  • Creating issues (aka tickets/bugs/etc). Please feel free to use issues to report bugs, request features, and discuss changes
  • Updating the documentation: ie this readme file. Be bold! Help create amazing documentation!
  • Submitting pull requests.

How to submit pull requests:

  1. Please create an issue and get my input before spending too much time creating a feature. Work with me to ensure your feature or addition is optimal and fits with the purpose of the project.
  2. Fork the repository
  3. clone your forked repo onto your machine and run npm install at its root
  4. If you're gonna work on multiple separate things, its best to create a separate branch for each of them
  5. edit!
  6. If it's a code change, please add to the unit tests (at test/protoTest.js) to verify that your change
  7. When you're done, run the unit tests and ensure they all pass
  8. Commit and push your changes
  9. Submit a pull request:


Change Log

  • 1.0.8 - if a static property can't be written (because it's read only or for some other reason throws an exception when being set), it will now silently not set, instead of throwing an exception
  • 1.0.7 - getting rid of useless line in stack trace
  • 1.0.6 - fixing custom error name in stacktraces
  • 1.0.5 - fixing github dependencies


Released under the MIT license:

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