Monday, December 10, 2007

FireFox, Safari, and Opera ... JavaScript Conditional Comment

I've still written a comment in John Resig blog about Re-Securing JSON and FireFox or Safari 3 const behaviour to create an immutable function, called Native, and to retrieve original Array, Object, String or other constructors.

This is the function:

const Native = (function(){
const NArray = Array,
NBoolean = Boolean,
NDate = Date,
NError = Error,
NMath = Math,
NNumber = Number,
NObject = Object,
NRegExp = RegExp,
NString = String;
return function(Native){
case Array:return NArray;
case Boolean:return NBoolean;
case Date:return NDate;
case Error:return NError;
case Math:return NMath;
case Number:return NNumber;
case Object:return NObject;
case RegExp:return NRegExp;
case String:return NString;

// Example
Array = Native(Array);

At the same time I've talked about IE behaviour and its missed support for const.

While I was thinking about that I imagined a way to use a piece of code, in this case the keyword const, compatible with every browser.

It should sounds simple, just use the well know trick IE=/*@cc_on ! @*/false; followed by if(IE) ... but ... hey, I need everytime to create a double version of the "same script" ... is it good?

Try to imagine a variable that should be a constant for every compatible browser but at the same time should be declared just one time ...

const MyConstant = 1;

With every day practices we should use a try catch or some strange trick to evaluate a const declaration ... don't we?

But we have a particular behaviour of the conditional comment ... it should contains a comment itself too, sounds cool?

/*@cc_on // @*/ const
Native = function(){
// whatever You need

Can anyone trigger an error with above piece of code? :-)

In this way FireFox 2+, Safari 3+ (I don't know about 2), and Opera 9+ could work without problems disabling Native variable re-declaration while every IE browser will ignore the const keyword.

I suppose this trick is not so new but never as this time it should be useful to make code more slim and efficient.

Instant Update
The usage of const inside the private scope of Native function declaration is not so useful (just a little bit of paranoia :D) but in these cases we could use a better trick to create local variables with internet explorer too.

This is an example:

Native = (function(){
/*@cc_on var // @*/ const
NArray = Array,
NBoolean = Boolean;
return function(Native){
return Native === Array ? Narray : NBoolean;

Above example shows how should be possible to initializzate multiple variables using a comma and respecting the private scope.
Internet Explorer will use var while every other browser will try to use const if it's compatible.

Sounds even better? I hope so :-)

P.S. Native function is quite interesting, imho, that's why I choosed to add them in

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