overload.js

Function.prototype.addMethod = function(name, method) {
  var existingMethod = this.prototype[name];

  this.prototype[name] = function() {
    if (method.length == arguments.length) {
      return method.apply(this, arguments);

    } else if (typeof existingMethod == "function") {
      return existingMethod.apply(this, arguments);
    }
  }
}

tests.js

var assert = console.assert || require("assert").ok;

function Car() {
  this.velocity = 10;
}

Car.addMethod("run", function(){
  return "Running at " + this.velocity;
});

Car.addMethod("run", function(velocity) {
  return "Running at " + velocity;
});

Car.addMethod("stop", function() {
  return "Stopped";
});

Car.addMethod("stop", function(seconds) {
  return "Stopping in " + seconds + " seconds";
});

function SuperCar() {
  Car.apply(this, arguments);
}

SuperCar.prototype = Object.create(Car.prototype);
SuperCar.prototype.constructor = SuperCar;

SuperCar.addMethod("run", function(velocity) {
  return "Running at " + velocity * 2;
});

SuperCar.addMethod("run", function(velocity, distance) {
  return "Running at " + velocity * 2 + " for " + distance;
});

var car = new Car;
var superCar = new SuperCar;

assert(car.run() == "Running at 10", "Car run no arguments");
assert(car.run(20) == "Running at 20", "Car run with one argument");

assert(superCar.run() == "Running at 10", "SuperCar inheriting the run with no arguments");
assert(superCar.run(20) == "Running at 40", "SuperCar overwriting the run with one argument");

assert(superCar.run(20, 3) == "Running at 40 for 3", "SuperCar creating a new overloaded method");
assert(typeof car.run(20, 30) == "undefined", "Car should not know the new overloaded method");

assert(superCar.stop() == "Stopped", "Inheritance of methods");
assert(superCar.stop(10) == "Stopping in 10 seconds", "Inheritance of overloaded methods");

usage.js

function Car() {
  this.velocity = 10;
}
 
Car.addMethod("run", function() {
  return "Running at " + this.velocity;
});
 
Car.addMethod("run", function(velocity) {
  return "Running at " + velocity;
});
 
Car.addMethod("run", function(velocity, distance) {
  return "Running at " + velocity + " for " + distance + "km";
});

var car = new Car;
car.run() // "Running at 10"
car.run(20) // "Running at 20"
car.run(30, 3) // "Running at 30 for 3km"