Question

So objects are more complicated than I initially portrayed them. They have prototypes, which are other objects, and will act as if they have properties they don’t have as long as the prototype has that property. Simple objects have Object.prototype as their prototype.

Constructors, which are functions whose names usually start with a capital letter, can be used with the new operator to create new objects. The new object’s prototype will be the object found in the prototype property of the constructor function. You can make good use of this by putting the properties that all values of a given type share into their prototype. The instanceof operator can, given an object and a constructor, tell you whether that object is an instance of that constructor.

One useful thing to do with objects is to specify an interface for them and tell everybody that they are supposed to talk to your object only through that interface. The rest of the details that make up your object are now encapsulated, hidden behind the interface.

Once you are talking in terms of interfaces, who says that only one kind of object may implement this interface? Having different objects expose the same interface and then writing code that works on any object with the interface is called polymorphism. It is very useful.

When implementing multiple types that differ in only some details, it can be helpful to simply make the prototype of your new type derive from the prototype of your old type and have your new constructor call the old one. This gives you an object type similar to the old type but for which you can add and override properties as you see fit.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.