<script type="text/javascript" language="javascript">
<!--
//initializing the WebPlayer
var u = new UnityObject2();
u.initPlugin(jQuery("#unityPlayer")[0], "Example.unity3d");

function SaySomethingToUnity()
{
    u.getUnity().SendMessage("MyObject", "MyFunction", "Hello from a web page!");
}
-->
</script>

public static void Evaluate(string js, System.Action<string> callback);

public class JSEvaluator : MonoBehaviour {

    private int id;
    Dictionary<int, Action<string>> callbacks;

    public static void Evaluate(string js, Action<string> callback) {
        ++id;
        callbacks[id] = callback;
        // ':' is separator to know where id ends
        Application.ExternalCall("evaluate", id.ToString() + ":" + js);
    }

    private void OnJSResult(string result) {
        var stringSeparators = new string[] {":"};
        var res = source.Split(stringSeparators, StringSplitOptions.None);
        var i = Convert.ToInt32(res[0]);
        var cb = callbacks[i];
        var rem = res[0] + ":";
        cb(result.Remove(result.IndexOf(rem), rem.Length););
        callbacks.Remove(i);
    }

}

function evaluate(input) {
    var inp = input.split(":");
    var id = parseInt(inp[0]);
    inp.shift();
    var arg = inp.join(":");
    var result = eval(arg);
    // for UnityPlayer you need to u.getUnity().SendMessage
    SendMessage("GameObjectName", "OnJSResult", result);
}