Prototype-based Programming with JavaScript

3. Prototype-based Programming

PS — Prototype-based Programming

Roadmap

> Class- vs. prototype-based languages
> Objects, properties and methods
> Delegation
> Constructors
> Closures
> Other prototype-based languages

© A. Lienhard, O. Nierstrasz 3.2


References

> JavaScript: The Definitive Guide, D. Flanagan, OʼReilly, 5th edition
> JavaScript Guide & Reference, Mozilla Developer Center,
http://developer.mozilla.org/en/docs/JavaScript
> Using Prototypical Objects to Implement Shared Behavior in Object
Oriented Systems, H. Lieberman, OOPSLA’86

> ECMAScript Language Specification — 3rd edition, http://www.ecma-
international.org/publications/files/ECMA-ST/Ecma-262.pdf



Object-oriented Languages

program = objects + messages

Good for encapsulation of state and behavior

Two object-oriented models
Class-based: code reuse through inheritance
Prototype-based: code reuse through delegation



Roadmap

> Delegation
> Constructors
> Closures



Class- vs. Prototype-based

Class-based: Prototype-based:
> Classes share methods and > No classes, only objects
define common properties
> Objects define their own
> Inheritance along class chain properties and methods
> Instances have exactly the > Objects delegate to their
properties and behavior prototype(s)
defined by their class
> Any object can be the
> Structure typically cannot be prototype of another object
changed at runtime

Prototype-based languages unify objects and classes



Prototype-based Languages

No classes ⇒simpler descriptions of objects
Examples ﬁrst (vs. abstraction ﬁrst)

Fewer concepts ⇒simpler programming model

Many languages (but only few used outside research):
> JavaScript, Self, Io, NewtonScript, Omega, Cecil, Lua,
Object-Lisp, Examplars, Agora, ACT-I, Kevo, Moostrap,
Obliq, Garnet



Roadmap

> Delegation
> Constructors
> Closures



What is JavaScript?

> Introduced in 1995 by Netscape
> Minimal object model:
— everything is an object (almost)
— functions are ﬁrst-class objects, closures
> Prototype-based delegation
> Dynamically typed, interpreted
> Platforms: web browsers and servers, Java 6, embedded in various
applications (Flash, Photoshop, ...)
> What it is not:
— No direct relationship to Java
— Not only for scripting web browsers



Syntax

// single line comment
Comments: /* multi
line comment */
First character must be a letter, _, or $; subsequent characters
Identiﬁers:
can be digits: i, v17, $str, __proto__
‘a string’, “another string”, “that’s also a string”
Basic literals: 17, 6.02e-32
true, false, null, undefined
var point = { x:1, y:2 }
empty: {}
Object literals: nested: var rect = {
upperLeft: { x:1, y:2 },
lowerRight: { x:4, y:5 } }

Function literals: var square = function(x) { return x*x; }

[1,2,3]
Array literals: []
assignement: =
Operators: equal: == strict equal: ===



Object Properties

var book = { title:’JavaScript’ };
Reading properties
book.title; //=>’JavaScript’

Adding new properties book.author = ‘J. Doe’;
(at runtime) ‘author’ in book; //=>true

Inspecting objects var result = ‘’;
for (var name in book) {
result += name + ‘=’;
result += book[name] + ‘ ’;
};
//=>title=JavaScript author=J. Doe

Deleting properties delete book.title;
‘title’ in book; //=>false



Methods

Methods are just functions var obj = { counter:1 };
that are assigned to obj.increment = function(amount) {
properties of an object this.counter += amount;
};
At runtime the keyword this is obj.increment(16);
bound to the object of the method obj.counter; //=> 17

Accessing (vs. executing) var f = obj.increment;
methods typeof f; //=> ‘function’

Property and method slots are uniﬁed.
Functions are ﬁrst-class objects.


Roadmap

> Delegation
> Constructors
> Closures



Delegation

Mechanism to share data and behavior is called delegation

An object delegates to its prototype object (the Mozilla interpreter allows
one to access the prototype through the property __proto__)

var oldRect = { width:10, height:3 };
var newRect = {};
newRect.__proto__ = oldRect;

‘width’ in newRect; //=>true Local vs. inherited
newRect.hasOwnProperty(‘width’); //=>false properties

Binding not found in object, newRect.width; //=>10
then lookup in prototype newRect.foo; //=>undefined



Delegation of Messages

The key aspect of prototype delegation is that this in the
prototype is bound to the receiver of the original message.

newRect.width = 100;
oldRect.area = function() {
return this.width * this.height;
};
newRect.area(); //=>300

The method area() is executed in the context of
newRect, the receiver, rather than in oldRect, the
object to which the message area() is delegated!



Roadmap

> Delegation
> Constructors
> Closures



Constructor Functions

Constructors are functions that are used with the new
operator to create objects
function Rectangle(w, h) {
this.width = w;
this.height = h;
this.area = function() {
};
The operator new creates an };
object and binds it to this in
the constructor. By default the rect = new Rectangle(3,4);
return value is the new object. rect.area(); //=>12



Constructor.prototype

> All objects created with a constructor share the same prototype
> Each constructor has a prototype property (which is
automatically initialized when deﬁning the function)

Instead of creating a new function Rectangle(w, h) {
method for each object, add this.width = w;
one to the prototype this.height = h;
};
Rectangle.prototype.area = function() {
};



Constructor.prototype

...
function ColoredRectangle(w, h, c) {
this.width = w;
this.height = h;
this.color = c;
};
ColoredRectangle.prototype = new Rectangle(0,0);
coloredRect = new ColoredRectangle(3,4,'red');
coloredRect.area();



Object Model

Notice, this ﬁgure is incomplete... For example:
Rectangle.__proto__ === Function.prototype
What is Function.__proto__?
What is Function.__proto__.__proto__?


Predeﬁned Objects

> Global functions: Array, Boolean, Date, Error, Function,
Number, Object, String,... eval, parseInt, ...
> Global objects: Math



Extending Predeﬁned Objects

Extending all objects Object.prototype.inspect = function() {
alert(this);
The last object in the };
prototype chain of 'a string'.inspect();
every object is true.inspect();
Object.prototype (new Date()).inspect();

Array.prototype.map = function(f) {
Extending arrays var array = [];
for (var n = 0; n < this.length; n++) {
if (n in this) { array[n] = f(this[n]); };
};
return array;
};
[1.7, -3.1, 17].map(Math.floor); //=>[1, -4, 17]
[1.7, -3.1, 17].map(function(x) { return x+1; });
//=>[2.7, -2.1, 18]



The arguments object

arguments object function concat(separator) {
var result = ‘’;
for (var i = 1; i < arguments.length; i++)
you can call a function result += arguments[i] + separator;
return result;
with more arguments };
than it is formally concat(";", "red", "orange", "blue");
declared to accept //=>"red;orange;blue;"

arguments.callee returns the currently executing function
var f = function() {
if (!arguments.callee.count) {
arguments.callee.count = 0; };
arguments.callee.count++; };
f(); f(); f();
f.count; //=>3



Roadmap

> Delegation
> Constructors
> Closures



Variable Scopes

> The scope of a variable is the region of the program in
which it is deﬁned
> Scopes in JavaScript
— function
— global
— no block-level scope(!)
> Identiﬁer resolution:
lookup along scope chain



Closures

> Functions are lexically scoped (rather than
dynamically). Functions are executed in the scope in
which they are created, not from which they are called.
> Inner functions as closures

When the anonymous function f(x) {
function is created, the var y = 1;
current scope chain is return function() { return x + y; };
};
saved. Hence, the scope
closure = f(2);
of f(x) continues to exist var y = 99;
even after f(x) returned. closure(); //=>3



Closures

A closure is a function that can have free variables together with an
environment that binds those variables (that "closes" the expression).



Closures and Objects

By default all properties and methods are public.
Using closures, properties and methods can be made private.
function Login(string, anotherString) {
this.username = string;
var password = anotherString;
this.check = function(pwrd) {
return password == pwrd;
};
this.reset = function(oldPwrd,newPwrd) {
Only check() and if (this.check(oldPwrd)) {
reset() have access to password = newPwrd;
the variable password. };
};
};
Notice, password is a login = new Login(‘Doe’,’xyz’);
local variable of the login.username; //=>’Doe’
constructor, not a property login.password; //=>undefined
of the created object! login.reset(‘xyz’, 17);
login.check(17); //=>true


Closures and Objects

Both functions close over the same environment, the execution context of
Login(), enabling them to communicate privately by altering that environment.



Other JavaScript Features

> Regular Expressions
> Client-side JavaScript (DOM, AJAX, ...)
> Processing XML with E4X (introduced in v1.6)
> Iterators and Generators (introduced in v1.7)

> JavaScript integration into Java 6 with Rhino



Roadmap

> Delegation
> Constructors
> Closures



Variation Points

> Basic mechanisms
— Object creation: ex-nihilo, cloning, extension
— Object representation (slots in JavaScript, Self, NewstonScript vs.
attributes and methods in Agora, Kevo)
> Delegation
— Double delegation in Io/NewtonScript
— Multiple prototypes (aka. parents) in Self
— Can prototype link be changed at runtime?
> Organization of programs (prototypical instance, traits, ...)

program = objects + messages + delegation



What you should know!

✎ What is the difference between delegation and inheritance?
✎ Which object is modified when changing the value of a property
within a delegated method?
✎ How do you extend all objects created with a specific constructor?
✎ Where do you define properties that are shared between a group
of objects (i.e., static members in Java)?
✎ How does variable scoping work?
✎ What is a closure?



Can you answer these questions?

✎ What is the prototype of the global object Function?
✎ How would you implement private static properties?



License

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Prototype-based Programming with JavaScript

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