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Standardized class syntax is coming in ECMAScript 6, supposedly, but until then I need a class factory that fills the gap.

For me that means (in no particular order):

  • prototypal inheritance under the hood
  • access to overridden properties (super)
  • inheritance of static properties
  • static constructors
  • close correspondence to ES6 syntax
  • ES5/browser compatibility
  • only-pay-for-what-you-use performance
  • excellent test coverage: Build Status

I have no delusions of persuading the world to use this tool. Just try npm search inheritance some time to see how many other people have come up with solutions that work for them.

If you have a special interest in the tired me-too sport of pure-JavaScript class factory implementations, you might find this one interesting for its solutions to each of the requirements listed above, particularly the lazy (just-in-time) population of prototype properties.

Note that I did not mention privacy enforcement as a requirement. If you need a mechanism like the private keyword in other languages, I have a separate project that works seamlessly alongside this one.


From NPM:

npm install cls

From GitHub:

cd path/to/node_modules
git clone git://
cd cls
npm install .


One example will have to suffice for now:

var cls = require("cls");
var BaseClass = cls.extend({
    init: function(a, b) {
        this.sum = a + b;
    getSum: function() {
        return this.sum;
    statics: {
        name: "BaseClass",
        init: function(cls) {
   = new cls(0, 0);
var SubClass = BaseClass.extend({
    init: function(arg) {
        this._super(arg, arg);
        this.sum += 1;
    statics: {
        name: "SubClass"
assert( === "BaseClass");
assert( === "SubClass");
assert(new BaseClass(2).getSum() === 4);
assert(new SubClass(2).getSum() === 5);
assert( !==;
assert( instanceof SubClass);
assert( === 1);

For more complex examples, see test/run.js.