code: private void eval( CssFormatter formatter ) {
          if( type != UNKNOWN ) {
              return;
          }
          try {
              ScriptEngineManager factory = new ScriptEngineManager( getClass().getClassLoader() );
              ScriptEngine engine = factory.getEngineByName( "JavaScript" );
              engine.setContext( new JavaScriptContext( formatter, this ) );
  
              String script = toString();
              script = SelectorUtils.replacePlaceHolder( formatter, script, this );
              script = script.substring( 1, script.length() - 1 );
              result = engine.eval( script );
              if( result instanceof Number ) {
                  type = NUMBER;
              } else if( result instanceof Boolean ) {
                  type = BOOLEAN;
              } else if( result instanceof Collection ) {
                  type = LIST;
              } else if( result instanceof Map ) {
                  result = ((Map)result).values();
                  type = LIST;
              } else {
                  type = STRING;
              }
          } catch( Exception ex ) {
              throw createException( ex );
          }
      }
docs: Execute the JavaScript
  @param formatter current formatter