A category on UIWebView that lets you return any JSON-compatible content from Javascript code. (This means arrays, objects/dictionaries, strings, numbers, and nulls.)

UIWebView+JSONObjectByEvaluatingJavascript.h

//
//  UIWebView+JSONObjectByEvaluatingJavascript.h
//  Feeder
//
//  Created by Brent Royal-Gordon on 7/1/13.
//  Copyright (c) 2013 Architechies. All rights reserved.
//

@interface UIWebView (Additions)

- (id)JSONObjectByEvaluatingJavascript:(NSString*)javascript parameters:(NSDictionary*)params options:(NSJSONReadingOptions)options;

@end

UIWebView+JSONObjectByEvaluatingJavascript.m

//
//  UIWebView+JSONObjectByEvaluatingJavascript.m
//  Feeder
//
//  Created by Brent Royal-Gordon on 7/1/13.
//  Copyright (c) 2013 Architechies. All rights reserved.
//

@implementation UIWebView (JSONObjectByEvaluatingJavascript)

- (id)JSONObjectByEvaluatingJavascript:(NSString*)javascript parameters:(NSDictionary*)params options:(NSJSONReadingOptions)options {
    NSArray * keys = params.allKeys ?: @[];
    NSArray * values = [params objectsForKeys:keys notFoundMarker:[NSNull null]] ?: @[];
    
    NSString * parameterNames = [keys componentsJoinedByString:@", "];
    NSData * parameterData = [NSJSONSerialization dataWithJSONObject:values options:0 error:NULL];
    NSString * parameterValues = [[NSString alloc] initWithData:parameterData encoding:NSUTF8StringEncoding];
    
    javascript = [NSString stringWithFormat:@"JSON.stringify((function(%@) { %@ }).apply(window, %@))", parameterNames, javascript, parameterValues];
    
    NSString * jsonString = [self stringByEvaluatingJavaScriptFromString:javascript];
    NSData * jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
    return [NSJSONSerialization JSONObjectWithData:jsonData options:options|NSJSONReadingAllowFragments error:NULL];
}

@end