Prototypes Continued

The Prototype Property

While the prototype chain is pretty awesome, it can only be used to access certain properties/methods/functions (Which term or terms is best to use here?). There are really two ways to figure out which properties can be accessed: 1. Exactly what we did previously; typing the (prototype?) object name followed by a period . 2. Entering the name followed by __.proto.__ or .prototype

• __.proto.__ (two underscores on each side) is only used on the base of your prototype chain, person1 in our case (correct?). You can use it to get a better understanding of the prototype chain's path.

• .prototype is used in every other link of the chain (correct?). Without it, you won't be able to use functions such as isPrototypeOf or valueOf. Please note that these are advanced topics and you probably won't need to go this much in depth for awhile.

Object.create()

Remember earlier how we used something called Object.create() to create objects? Let's learn more about it now.

• Let's create a new object instance calledperson2.

  var person2 = Object.create(person1);

• At first glance you may think something's up, it looks just like person1! While the data looks exactly the same (name, age, etc.), it's not really a copy, person2 is just using person1's prototype as a prototype object.

• You can confirm this by running person2.__proto__.

  

  See, the Person object, person1's prototype, is returned.

The Constructor Property

There's also a .constructor property that allows you to see the properties available to object instances created using that constructor. For example, both person1.constructor and person2.constructor return:

Constructor

the Person() constructor.

If for some reason you don't have a reference to the original constructor and want to create a new reference, you're in luck! You can create a new instance like so:

var person3 = new person1.constructor('Karen', 'Stephenson', 26, 'female', ['playing drums', 'mountain climbing']);

The .constructor property can also help you find the constructor name based on an instance name, for example:

person1.constructor.name

returns "Person".

Modifying Prototypes

If you really needed to modify the .prototype property, you can. Let's add a goodbye message to our code.

Person.prototype.farewell = function() {
  alert(this.name.first + ' has left the building. Bye for now!');
};

person1.farewell() returns the following:

Farewell

You shouldn't use this concept often however. For example:

Person.prototype.fullName = 'Bob Smith';

This doesn't work because it's not that flexible. Even something like this

Person.prototype.fullName = this.name.first + ' ' + this.name.last;

would give you undefined because the global scope is being referenced rather than the function scope.

Moral of the story is: If you really want to define properties on the prototype make sure they are static, constant properties. Otherwise, it's generally best to just define them on the constructor.