Client-Side Web TechnologiesJavaScript Part II

Object Constructors• Any function called with the new operator acts as a

constructor function

• When a constructor is called the following takes

place:

• A new object instance is created

• The this variable inside the constructor function is

assigned to point to the newly created instance

• Code inside the constructor is executed

• The new instance is returned

Prototypes• A big disadvantage of constructors is that each

method of an object (being a function and functions

are objects) is created once for each instance

• A prototype is an object containing properties and

methods that are shared among all instances of a

reference type

• Each constructor function has a prototype

property that points to its prototype object

• Each prototype object has a constructor

property that points back to the constructor on

which it is a property

Prototypes (continued)• Each instance of an object created with a

constructor gets an internal pointer to the

constructor’s prototype

• In ECMAScript it is referred to as [ [ Prototype ] ]

• In ECMAScript it can be accessed via the Object.getPrototypeOf() method

• The isPrototypeOf() method can be used to

determine if an instance’s prototype pointer points to a

certain prototype

• Examples at: http://csw08724.appspot.com/Prototypes.html

Prototypes (continued)• When a property is accessed on an object for reading

• The object instance is checked to see if it exists

• Otherwise the prototype is checked to see if it exists on the

prototype

• If the prototypes are chained, then the search will continue all

the way along the chain until the property is found or the end is

reached

• When a property is added to an instance it shadows any

property on the prototype with the same name

• So we can “override” the prototype

• The hasOwnProperty() method can be used to

determine if a property or method exists on the instance

Embracing ECMAScript’s Prototypal Nature

• Some people advocate not trying to force classical

inheritance patterns on ECMAScript objects

• No new operator

• No constructor functions

• We create new objects based on existing ones and

add features to the new objects

• See the PurePrototypes.html example