Something similar to a Strategy pattern?

gistfile1.js

/*
Let's say I want to create an object using different constructors according
to some environmental condition (browser, available API, etc).
So I have two constructors, for instance for an Ajax object:
*/

function AjaxXMLHTTPRequest() {}
AjaxXMLHTTPRequest.prototype = {
    // some methods
};

function AjaxActiveX() {}
AjaxActiveX.prototype = {
    // same methods with different implementations
};

/*
How would you better use this constructors to abstract their differences?
I'm currently doing this:
*/

function Ajax() {
  (/* some condition*/ ? AjaxXMLHTTPRequest : AjaxActiveX).apply(this, arguments);
}
extend(Ajax, /* some condition*/ ? AjaxXMLHTTPRequest : AjaxActiveX);