1. 3. Prototype-based Programming

2. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.2 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

3. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.3 References  JavaScript: The Definitive Guide, D. Flanagan, O’Reilly, 5th edition  JavaScript Guide & Reference, Mozilla Developer Center, http://developer.mozilla.org/en/docs/JavaScript 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 http://www.ecma- international.org/publications/files/ECMA-ST/Ecma-262.pdf

4. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.4 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

5. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.5 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

6. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.6 Class- vs. Prototype-based  Classes share methods and define common properties  Inheritance along class chain  Instances have exactly the properties and behavior defined by their class  Structure typically cannot be changed at runtime Class-based:Prototype-based:  No classes, only objects  Objects define their own properties and methods  Objects delegate to their prototype(s)  Any object can be the prototype of another object Prototype-based languages unify objects and classes

7. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.7 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

8. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.8 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

9. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.9 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

10. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.10 Syntax Comments: // single line comment /* multi line comment */ Identifiers: First character must be a letter, _, or $; subsequent characters can be digits: i, v17, $str, __proto__ Basic literals: ‘a string’, “another string”, “that’s also a string” 17, 6.02e-32 true, false, null, undefined Object literals: var point = { x:1, y:2 } empty: {} nested: var rect = { upperLeft: { x:1, y:2 }, lowerRight: { x:4, y:5 } } Function literals: var square = function(x) { return x*x; } Array literals: [1,2,3] [] Operators: assignement: = equal: == strict equal: ===

11. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.11 Object Properties var book = { title:’JavaScript’ }; book.title; //=>’JavaScript’ Adding new properties (at runtime) Reading properties book.author = ‘J. Doe’; ‘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

12. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.12 Methods var obj = { counter:1 }; obj.increment = function(amount) { this.counter += amount; }; obj.increment(16); obj.counter; //=> 17 Property and method slots are unified. Functions are first-class objects. var f = obj.increment; typeof f; //=> ‘function’ Methods are just functions that are assigned to properties of an object Accessing (vs. executing) methods At runtime the keyword this is bound to the object of the method

13. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.13 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

14. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.14 Delegation var oldRect = { width:10, height:3 }; var newRect = {}; newRect.__proto__ = oldRect; An object delegates to its prototype object (the Mozilla interpreter allows one to access the prototype through the property __proto__) newRect.width; //=>10 newRect.foo; //=>undefined Mechanism to share data and behavior is called delegation Binding not found in object, then lookup in prototype ‘width’ in newRect; //=>true newRect.hasOwnProperty(‘width’); //=>false Local vs. inherited properties

15. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.15 Delegation of Messages 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! The key aspect of prototype delegation is that this in the prototype is bound to the receiver of the original message.

16. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.16 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

17. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.17 Constructor Functions function Rectangle(w, h) { this.width = w; this.height = h; this.area = function() { return this.width * this.height; }; rect = new Rectangle(3,4); rect.area(); //=>12 The operator new creates an object and binds it to this in the constructor. By default the return value is the new object. Constructors are functions that are used with the new operator to create objects

18. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.18 Constructor.prototype function Rectangle(w, h) { this.width = w; this.height = h; }; Rectangle.prototype.area = function() { return this.width * this.height; }; Instead of creating a new method for each object, add one to the 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)

19. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.19 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();

20. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.20 Object Model Notice, this figure is incomplete... For example: Rectangle.__proto__ === Function.prototype What is Function.__proto__? What is Function.__proto__.__proto__?

21. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.21 Predefined Objects  Global functions: Array, Boolean, Date, Error, Function, Number, Object, String,... eval, parseInt,...  Global objects: Math

22. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.22 Extending Predefined Objects Array.prototype.map = function(f) { 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] Object.prototype.inspect = function() { alert(this); }; 'a string'.inspect(); true.inspect(); (new Date()).inspect(); Extending all objects Extending arrays The last object in the prototype chain of every object is Object.prototype

23. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.23 The arguments object function concat(separator) { var result = ‘’; for (var i = 1; i < arguments.length; i++) result += arguments[i] + separator; return result; }; concat(";", "red", "orange", "blue"); //=>"red;orange;blue;" arguments object 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 you can call a function with more arguments than it is formally declared to accept

24. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.24 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

25. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.25 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

26. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.26 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 function f(x) { var y = 1; return function() { return x + y; }; }; closure = f(2); var y = 99; closure(); //=>3 When the anonymous function is created, the current scope chain is saved. Hence, the scope of f(x) continues to exist even after f(x) returned.

27. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.27 Closures A closure is a function that can have free variables together with an environment that binds those variables (that "closes" the expression).

28. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.28 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) { if (this.check(oldPwrd)) { password = newPwrd; }; login = new Login(‘Doe’,’xyz’); login.username; //=>’Doe’ login.password; //=>undefined login.reset(‘xyz’, 17); login.check(17); //=>true Only check() and reset() have access to the variable password. Notice, password is a local variable of the constructor, not a property of the created object!

29. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.29 Closures and Objects Both functions close over the same environment, the execution context of Login(), enabling them to communicate privately by altering that environment.

30. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.30 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

31. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.31 Roadmap  Class- vs. prototype-based languages  Objects, properties and methods  Delegation  Constructors  Closures  Other prototype-based languages

32. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.32 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

33. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.33 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?

34. © A. Lienhard, O. Nierstrasz PS — Prototype-based Programming 3.34 Can you answer these questions?  What is the prototype of the global object Function?  How would you implement private static properties?

35. © Oscar Nierstrasz ST — Introduction 1.35 Attribution-ShareAlike 3.0 Unported You are free: to Share — to copy, distribute and transmit the work to Remix — to adapt the work Under the following conditions: Attribution. You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to this web page. Any of the above conditions can be waived if you get permission from the copyright holder. Nothing in this license impairs or restricts the author's moral rights. License http://creativecommons.org/licenses/by-sa/3.0/