Thursday, February 12, 2009

After the Array, subclassed String

As I wrote months ago in this documentation about JavaScript prototypal inheritance, subclass native data type is not that simple (almost not possible at all) but since JS is loads of weird bits and bops, I often try to break its documented limits.

It was the Array, some time ago, now it is about time for String.

The concept is similar, if you subclass a native data type its methods will return that native data type like instance, so concat, charAt, toString, etc etc, will return a String unless we do not override every inherited method (so performances wont be that good).

Subclassed String

(function(Function, slice, push){
// from WebReflection: Subclassed String
function String(String){
if(arguments.length) // clever constructor, accepts more than a string as argument
String.prototype = new Function;
(new String) + ""; // exception in FireFox
var join = Array.prototype.join;
String.prototype.toString = String.prototype.valueOf = function(){
return, "");
}catch(e){ // no way to retrieve the length with FireFox
String.prototype.toString = String.prototype.valueOf = function(){
for(var Array = [], i = 0; Array[i] = this[i]; i++);
return Array.join("");
(window.$ || ($ = {})).String = String; // let's put this in a namespace
})(String, Array.prototype.slice, Array.prototype.push);

That's it, every String.prototype.method should act as it has been called via native String with probably every browser.


concat, charAt, charCodeAt, toLowerCase, etc, etc will be almost the same of native strings but the constructor will be a bit slower (instances instead of regular "strings").
On the other hand, in every browser the returned instance will be an Array like String, so ...

new $.String("Here", " ", "we", " ", "are!")[0];

will be exactly the char "H" in every compatible browser.


To create a valid alternative that will NOT be an instanceof String we could use the same Stack trick:

// every browser
(function(Function, join, push){
function String(String){
String.prototype.length = 0;
String.prototype.valueOf = String.prototype.toString = function(){
return, "");
// here we can add every String prototype to the $.String prototype
(window.$ || ($ = {})).String = String;
})(String, Array.prototype.join, Array.prototype.slice, Array.prototype.push);

Have fun ;-)

No comments:

Post a Comment