Using Prototype (documentation) for some Ajax work, I found myself repeatedly doing something like

new Ajax.Request(some_url, {method:post, parameters: some_parameters, onComplete:function(transport) {
  eval(transport.responseText);
}});
The effect is that the response text, the output of the backend (at some_url), is evaluated as JavaScript code on the requesting page.

Too much duplicated code, though.

With Ajax.Updater, you can specify the option evalScripts:true to have script tag contents in the response text evaluated, but Ajax.Updater also updates some specified element with the non-script output. If I would use this, I would need some dummy element, and also to wrap the backend output in script tags.

Instead, I subclassed Ajax.Request with an Ajax.Eval class that does exactly what I want and nothing more. It takes an URL and some parameters, and evaluates the output as JavaScript.

Code:

Ajax.Eval = Class.create();
Object.extend(Object.extend(Ajax.Eval.prototype, Ajax.Request.prototype), {
  initialize: function(url, pars) {
    this.transport = Ajax.getTransport();
    this.setOptions({method:post, parameters:pars});
    this.options.onComplete = (function(transport) {
      eval(transport.responseText);
    });
    this.request(url);
  }
});
Now, I can simply do

new Ajax.Eval(some_url, some_parameters);