Why should you not add functionality to a JavaScript constructor but instead via prototype?

Question

Im look at Addy Osmani's chapter on the constructor pattern: http://addyosmani.com/resources/essentialjsdesignpatterns/book/#constructorpatternjavascript and I came across the following:

function Car( model, year, miles ) {

  this.model = model;
  this.year = year;
  this.miles = miles;

  this.toString = function () {
    return this.model + " has done " + this.miles + " miles";
  };
}

// Usage:

// We can create new instances of the car
var civic = new Car( "Honda Civic", 2009, 20000 );
var mondeo = new Car( "Ford Mondeo", 2010, 5000 );

// and then open our browser console to view the 
// output of the toString() method being called on 
// these objects
console.log( civic.toString() );
console.log( mondeo.toString() );

He said this was not a great thing to do with regards to the this.toString function as it isn't very optimal and isn't shared between all instances of the car type. But he doesn't explain what exactly this means and why it's a bad thing. He recommends to do the following:

function Car( model, year, miles ) {

  this.model = model;
  this.year = year;
  this.miles = miles;

}


// Note here that we are using Object.prototype.newMethod rather than 
// Object.prototype so as to avoid redefining the prototype object
Car.prototype.toString = function () {
  return this.model + " has done " + this.miles + " miles";
};

// Usage:

var civic = new Car( "Honda Civic", 2009, 20000 );
var mondeo = new Car( "Ford Mondeo", 2010, 5000 );

console.log( civic.toString() );
console.log( mondeo.toString() );

Can someone explain why using the prototype object to add in this functionality is optimal/better?

Answer 1

Functions (well, more generally, properties) on the prototype object are shared by all instances. That single "toString" function will remain just one solitary object no matter how many "Car" instances are made. When you do the assignment in the constructor, a new function object is created for each one.

Obviously, if each instance needs a property that may vary from those of other instances, then you need a per-instance property.