Use P/Invoke to call JavaScriptCore's C API functions in iOS.

JSCoreiOSViewController.cs

using System;
using MonoTouch.UIKit;
using MonoTouch;
using System.Runtime.InteropServices;

namespace JintiOS
{
	unsafe public partial class JintiOSViewController : UIViewController
	{
		JSContextRef* context;

		unsafe public struct JSStringRef
		{
		};

		unsafe public struct JSValueRef
		{
		};

		unsafe public struct JSContextRef
		{
		};

		unsafe public struct JSClassRef
		{
		};

		unsafe public struct JSObjectRef
		{
		};

		public enum JSPropertyAttributes
		{
			None         = 0,
			ReadOnly     = 1 << 1,
			DontEnum     = 1 << 2,
			DontDelete   = 1 << 3
		};

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSGlobalContextCreate")]
		public extern static JSContextRef* JSGlobalContextCreate (JSClassRef *globalObjectClass);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSContextGetGlobalObject")]
		public extern static JSObjectRef* JSContextGetGlobalObject (JSContextRef *ctx);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSStringCreateWithUTF8CString")]
		public extern static JSStringRef* JSStringCreateWithUTF8CString (string input);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSValueMakeNumber")]
		public extern static JSValueRef* JSValueMakeNumber (JSContextRef *ctx, double number);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSEvaluateScript")]
		public extern static JSValueRef* JSEvaluateScript (JSContextRef *ctx, JSStringRef *script, JSObjectRef *thisObject, JSStringRef *sourceURL, int startingLineNumber, JSValueRef **exception);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSValueToNumber")]
		public extern static double JSValueToNumber (JSContextRef *ctx, JSValueRef *value, JSValueRef **exception);

		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSObjectSetProperty")]
		public extern static void JSObjectSetProperty (JSContextRef *ctx, JSObjectRef *obj, JSStringRef *propertyName, JSValueRef *value, JSPropertyAttributes attributes, JSValueRef **exception);


		public delegate JSValueRef* JSObjectCallAsFunctionCallback (JSContextRef *ctx, JSObjectRef *function, JSObjectRef *thisObject, UIntPtr argumentCount, JSValueRef** arguments, JSValueRef **exception);
		[DllImport (Constants.JavaScriptCoreLibrary, EntryPoint = "JSObjectMakeFunctionWithCallback")]
		public extern static JSObjectRef* JSObjectMakeFunctionWithCallback (JSContextRef *ctx, JSStringRef *name, JSObjectCallAsFunctionCallback callAsFunction);


		public JintiOSViewController (IntPtr handle) : base (handle)
		{
		}

		public override void ViewDidLoad ()
		{
			base.ViewDidLoad ();
			context = JSGlobalContextCreate ((JSClassRef*)IntPtr.Zero);
		}

		unsafe partial void ButtonClicked (MonoTouch.Foundation.NSObject sender)
		{
			var button = sender as UIButton;

			var cubeAddFunc = new JSObjectCallAsFunctionCallback (
				delegate(JSContextRef* ctx, JSObjectRef* function, JSObjectRef* thisObject, UIntPtr argumentCount, JSValueRef** arguments, JSValueRef** exception) {
				var x = JSValueToNumber (ctx, arguments [0], (JSValueRef**)IntPtr.Zero);
				var y = JSValueToNumber (ctx, arguments [1], (JSValueRef**)IntPtr.Zero);
				return JSValueMakeNumber (ctx, x * x * x + y);
			});

			var jsCubeAddFunc = 
				JSObjectMakeFunctionWithCallback (context,
				                                  JSStringCreateWithUTF8CString ("cubeAndAdd"),
				                                  cubeAddFunc);
			
			JSObjectSetProperty (context,
			                     JSContextGetGlobalObject (context),
			                     JSStringCreateWithUTF8CString ("cubeAndAdd"),
			                     (JSValueRef*)jsCubeAddFunc,
			                     JSPropertyAttributes.None,
			                     (JSValueRef**)IntPtr.Zero);

			JSStringRef* script = JSStringCreateWithUTF8CString (@"
                    cubeAndAdd(number, 5)");

			JSObjectSetProperty (context,
			                     JSContextGetGlobalObject (context),
			                     JSStringCreateWithUTF8CString ("number"),
			                     JSValueMakeNumber (context, 3),
			                     JSPropertyAttributes.None,
			                     (JSValueRef**)IntPtr.Zero);

			var returns = JSValueToNumber (context, 
			                               JSEvaluateScript (context, script, (JSObjectRef*)IntPtr.Zero, (JSStringRef*)IntPtr.Zero, 0, (JSValueRef**)IntPtr.Zero),
			                               (JSValueRef**)IntPtr.Zero);

			button.SetTitle (returns.ToString (), UIControlState.Normal);
		}
	}
}