Question: Is it possible to run some javascript code (having perhaps an `NSString` as input) without using a `UIWebView`? I would like to run this code in a separate thread so UIWebView is not possible and I don't really want to render anything, just get back the results.

Answer: 
I have not tried this, but here is one idea:

Write code to allocate an empty `UIWebView` but doesn't add it as a subview anywhere, and just directly call [`- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script`](http://developer.apple.com/iphone/library/documentation/UIKit/Reference/UIWebView_Class/Reference/Reference.html#//apple_ref/occ/instm/UIWebView/stringByEvaluatingJavaScriptFromString:) on your javascript.

Then use `NSOperation` to run that in parallel.

Any reason that might not work for you? I think it would avoid displaying anything for the `UIWebView`.

As a side note, I can't think of anything you can do in javascript that you can't do more quickly directly in Objective-C. Except maybe parsing javascript that you get externally, which may be considered a no-no by Apple, depending on the details.

Guessing at your goals: If you want to parse a JSON server reply, I recommend the [json-framework library hosted on google code](http://code.google.com/p/json-framework/). It has a nice interface, and it is surprisingly easy to learn and integrate.
I think Parmanoir's article [Taming JavascriptCore within and without WebView](http://parmanoir.com/Taming_JavascriptCore_within_and_without_WebView) may give you some advice.