My JavaScript book is out! Don't miss the opportunity to upgrade your beginner or average dev skills.

Tuesday, March 29, 2011

Rewind: getters & setters for all IE with cross browser VBClass!

spoiler: if once again everybody knew except me, you guys should do something to be better indexed in Google ... while if this is totally new and cool, well, you are welcome :)

Sometimes I am stubborn, so stubborn, that even if it was me writing this post, and this one after, I have never given up about IE < 9 getters and setters ... "there must be a way", I have thought during last days, and ... yes, eventually I have found the way!

Test Driven Developed Solution

I have decided the possible behaviour, I have implemented all use cases, and I have successfully validated them against all browsers I could came up, with the exception of a single test, the first one, which fails in those browsers unable to freeze an object ... well, these browsers are disappearing thanks to their suggested, or automatic, updates, so everything is fine.
Here the unit test that should cover 100% of the used JavaScript and/or the used VBScript where available (only IE < 9 so please don't freak out yet!)

The Horrendous VBScript for IE

This wannabe Web programming language has been there, and hated, for ages ... but actually, if we learn all its gotchas we may end up thinking it is not that bad!
The main problem is to learn this "whatever it is language" and trust me: you don't wanna do that in this HTML5 era ... do you ?!

Not Only Get & Let, There Is A "Set" As Well!

Where good old dojo experimental Observable stopped, I didn't.
I kept investigating what the hack was going on behind the scene, forcing my fabulous IE9 to digest such Jurassic web programming language as VBScript is.
What I have discovered yesterday evening, is that when we set a property to a VBScript "unknown" object the Let definition is invoked only with primitives, where primitives are all those JavaScript variables which typeof is not equal to "object", with the exception of null value, and "function".
Once discovered this, the door to a proper implementation able to behave the same in all browsers became easy, and this is why I have introduced a new Class definition, the VBClass

What The Hell Is VBClass

VBClass is a global function able to create global factories, and these are the rules for a VBClass definition object:

  1. the definition of a VBClass IS static, which means once we have defined the structure of our instances, we cannot add, delete, or change arbitrary the number of properties

  2. since VBScript does not accept properties that start with an underscore, the convention to define a property "protected" must be different, as example using the underscore at the end as Closure Library does for whatever reason

  3. if a definition property is a function, this will be considered an immutable method of each instance created via this factory

  4. if a definition property has a value, its reference can be changed at any time and with any kind of object, function included, but in latter case it will not be possible to attach method runtime, but it will surely be possible to invoke the function property via call or apply, specifying the current object

A Full Specs VBClass Example

VBClass("FullSpecs", {
constructor: {
value: function (arg) {
// calls the method
method: {
value: function (arg) {
// invokes the setter
this.getSet = arg;
getSet: {
get: function () {
// returns the "protected"
return this.getSet_;
set: function (value) {
// assign the "protected"
this.getSet_ = value;
getSet_: {
// default "protected" value
value: null

genericObject = {},
test = new FullSpecs(genericObject)

test.getSet === genericObject; // true!

Pros And Cons Of VBClass

These are a few of pros about the precedent described VBClass limits/behavior:

  1. we have to think more about what we really need in our class, forgetting the completely dynamic JavaScript behaviour

  2. we are forced to follow a better convention for what we would like to define "protected properties"

  3. we can trust our defined methods, and we have to stick with them. This is a good/common approach if we consider a constructor.prototype runtime change a bad practice

  4. properties are properties, so there is nothing ambiguous about what is a method and what is a property: only the method can be invoked directly through the instance/variable/object, everything else is a property and there is no magic context injection there, neither for set functions

However, there are few cons to consider about this technique: unified behaviour through hosted VBScript objects means slower performances!
This means that VBClass created classes cannot be used for everything and there must be a valid reason to choose them in favour of normal JavaScript functions and their prototype nature.
A good reason could be, as example, the creation of those public object that would like to implement the coolness of a robust, cross platform, getters and setters implementation ... but be careful! If these objects are created hundred times during our application life-cycle, the performance impact could be massive and specially for older IE browsers.
Fortunately, with mobile browsers, IE 6/7 for Windows Phone 7 a part that I have not tested yet, the ideal scenario should fallback into the Object.create implementation, the one used in the main file, the only one needed, hopefully, as soon as users will update their browsers.

How To Use VBClass

Grab the source code or the minified version from my Google Code VBClass Project. Once all VBClass files are in the same folder, you can simply include the VBClass.loader.js on the top of your page and the rest of the magic is done.


Please copy VBClass locally to avoid round-trip and obtain better performances (also the trunk version in Google Code is not gzipped).

Have fun with VBClass ;)


Mr Speaker said...

Ha ha! That is fantastic! You are crazy. Though seeing VBScript again brought back some terrifying memories.

Andrew said...

This is amazing work - well done on figuring it out!

Quick thought: to overcome the frozen object restriction, why not create a new object and assign its prototype to be the VBClass-created object?

Andrea Giammarchi said...

because IE will throw an error if you use VBScript objects as prototype

This solution is not useful if we are using JS objects "as trash bin" ;-)

We must know in advance what we would like to store and conventions like "data" property null by default may do the trick = 123;

with methods: ...

Problem solved!

Matt said...

I'm trying to get this so that it works as a defineProperty polyfill.

Do you have any advice or suggestions in doing this?

Andrea Giammarchi said...

it won't work because VBClasses are static which means per each property you define you need to create a whole new object.

What I have done is to create once a definition object and "create()" the instance at the very end but this would be more a proxy like approach.

If defineProperty/ies were shimmable in IE, I would have written that shim by myself :-)

Raynos said...

So is a pipe dream?

Bah time to find another solution to getter/setters for oldIE :(