JavaScript Automation -

i using code change color of traffic lights on button click, how can change run sequence automatically once button clicked , repeat indefinitely?

  var lightstates = {red:0,amber:1,green:2};     var currentstate = lightstates.red;      document.getelementbyid('changebtn').onclick=function(){         changestate();     };       function changestate()     {         clear();       switch(currentstate)       {         case lightstates.red:         {           document.getelementbyid("red").classname ="light red";           currentstate =  lightstates.amber;         }         break;         case lightstates.amber:         {           document.getelementbyid("amber").classname ="light amber";           currentstate = lightstates.green;         } break;          case lightstates.green:         {           document.getelementbyid("green").classname ="light green";           currentstate = lightstates.red;         } break;        }     }      function clear(){        document.getelementbyid("red").classname ="light off";        document.getelementbyid("amber").classname ="light off";        document.getelementbyid("green").classname ="light off";