Oct 19 2003
Javascript Talk at St. Louis OOSIG
On October 16, 2003, I spoke on “Objects without Classes”, the non-class-based object model implemented by JavaScript and some other OO languages. The (brief) presentation is available here:
The plain text of the slides (which is, frankly, plenty good enough) follows:
Objects without Classes – JavaScript
An Alternative Object Model
Kyle Cordes
Oasis Digital Solutions Inc.
St. Louis OOSIG
Oct. 16, 2003
Yet Another “About Me” Slide
* Developer, Architect, Consultant, PM, etc.
* Learned OO in C++, early 1990s
* Many other OO languages since: Java, Delphi, C#, Python, Ruby,
JavaScript, etc.)
* Oasis Digital Solutions Inc.
¨ Outsourced development
¨ Services to help teams write better software
* kylecordes.com
Assumptions
* You write software
¨ Not necessarily a lot of software
* You use an OO language
¨ It’s hard to find one that isn’t nowadays
* (More technical than some OOSIG talks)
Agenda
* Objects and Classes
* Classless OO
* Very Brief Introduction to JavaScript / ECMAScript
* Examples
The Point
* Object Oriented does not necessarily mean Classes.
* Classless OO is a useful model, in surprisingly widespread
use, though not in mainstream enterprise development.
* Nonetheless, understanding classless OO can help you create
better designs in the language you use now.
Class-based OO
* C++, Java, C#, Smalltalk, Eiffel, Delphi, many more.
* Objects are divided into types, and don’t change type
* The properties of a set of objects of the same class
are specified in a class definition
* Classes are abstract thing, instances are specific
* Class relationship is static
* This is the only way it can be done, right?
Classless Objects
* Some OO langages don’t have classes
¨ Called Classless or Prototype-based
* Objects share behavior by sharing a common prototype object.
* All objects are instances
* “Class” relationship is dynamic
* Classless OO can be used to build class-based OO, it is more
inclusive
For Example…
* A few years ago, this would be of academic interest only
* But…
* JavaScript (ECMAScript) is in widespread use, and has classless
OO
¨ “Dice Index” of 964
¨ That’s more than C#, for example
Intro to Javascript
* Not much to do with Java
¨ Name came about from Netscape cashing in on initial Java excitement
* Standardized as ECMAScript
* Current spec version is 1.5.
Where is JavaScript Used?
* Most commonly, in client-side web scripting
* Web server implementations
* Very interesting Web Services toolkit from IBM
* In .NET, ships “in the box”
JavaScript Implementations
* Initial Netscape Implementation
* Rhino (written in Java)
¨ Easy to add script scripting to Java apps
* Mozilla
* IE (JScript)
* Opera etc.
* JScript.NET
JavaScript Features
* Dynamic typing, “duck typing”
* Type-safe (dynamic != unsafe)
* Single Inheritance
* Compiled to intermediate representation
¨ Can JIT to machine code, Jscript.NET
* No Classes, Prototype-based
* Functions are first-class objects
A Constructor is just a Function
* function Employee () {
this.name = “”;
this.dept = “general”;
}
* var x = new Employee; // instance
Constructor Parameters
* function Employee (name, dept) {
this.name = name;
this.dept = dept;
}
Methods
* function WFW() {
return this.wage * weeks;
}
* Employee.WageForWeeks = WFW;
Tighter Syntax for Methods
* function Employee () {
this.name = “”;
this.dept = “general”;
this.WageForWeeks =
function (weeks) {
return this.wage * weeks;
}
}
Subclasses
* function Manager () {
this.reports = [];
}
Manager.prototype = new Employee;
* function WorkerBee () {
this.projects = [];
}
WorkerBee.prototype = new Employee;
Subclasses (deeper)
* function SalesPerson () {
this.dept = “sales”;
this.quota = 100;
}
SalesPerson.prototype = new WorkerBee;
* It dynamic, we can change it later (example)
Add A Property
* Add to an instance:
var jim = new Employee;
jim.years_of_service = 12;
* Add to all instances:
Employee.prototype.phone = “”;
* (example)
AOP
* Aspect Oriented Programming
* With a prototype-based, dynamic language, we can add wrapper
/ “advice” etc. without needing new language features or compilers.
Singletons etc.
* There are many times in class based languages where you define
many of classes and create 0 or 1 instances of each, to hold some bit of code.
* With prototype-based OO, many of these situations get much
simpler.
OK, So It’s Different, So What?
* Classless teaches us to think about objects first, then classes
later
¨ Behavior, not just data
¨ Sequence Diagrams before Class Diagrams
* I have been building better designs sooner, since learning
JavaScript, even though I don’t use it much.
Back to The Point
* Object Oriented does not necessarily mean Classes.
* Classless OO is a useful model, in surprisingly widespread
use, though not in mainstream enterprise development.
* Nonetheless, understanding classless OO can help you create
better designs in the language you use now
Links
*
xhref=”http://devedge.netscape.com/central/javascript/” mce_href=”http://devedge.netscape.com/central/javascript/”>http://devedge.netscape.com/central/javascript/
Talk Offer
* I need practice giving talks!
* I’ve give this talk, or one like it
¨ In your organization
¨ For Developers, For Managers
¨ No Cost (in town), No Obligation
The End
Slides will be on my site:
* http://kylecordes.com
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