car.js

//pseudo-classical
function Car(options) {
  //if new keyword was used to invoke the constructor
  //`this` would be an instance of Car.
  if (!(this instanceof Car)) {
    //ensure constructor is called with new keyword
    return new Car(options)
  }

  this.instanceMethod = function(){};
}

Car.prototype.sharedMethod = function(){};

//functional
var Vehicle = function() {
  var instance = {};
  instance.instanceMethod = function(){};
  return instance;
}

//normally Car should be instantiated with `new`
var myCar = new Car();
//...but our check in the constructor averts the problem if we forget to do so. 
//so we can invoke it directly and get expected result
var myCar = Car();

//Vehicle doesn't need to be instantiated with new, but will still work
var myVehicle = new Vehicle();

//so as a naming convention all classes can be capitalised even if you don't use new keyword.

//if you want to clearly indicate that a function doesn't need new keyword it could be given an name like:
var makeVehicle = function(){
  var instance = {};
  instance.instanceMethod = function(){};
  return instance;
};