set css class for controls using javascript from code behind

string scriptString = "document.getElementById('ctl00_tvMainNavigationn17')";
scriptString += ".setAttribute('className', 'NodeBold');";
scriptString += "document.getElementById('ctl00_tvMainNavigationn18').setAttribute('className', 'NodeBoldClass');";
scriptString += "document.getElementById('ctl00_tvMainNavigationn19').setAttribute('className', 'NodeBoldClass');";
Page.RegisterStartupScript("onload", scriptString);