sleect open js code example
Example 1: open select dropdown
$('#MyButton').on("click",function(){
$("#selecttest").attr("size", 8);
$("#selecttest").css("position","fixed");
$('#MyButton').css("margin-left", $("#selecttest").width() + 5);});
$('#selecttest').on("change",function(){
$("#selecttest").attr("size", 1);});
Example 2: open select dropdown
var myDropDown=$("#myDropDown");
var length = $('#myDropDown> option').length;
//open dropdown
myDropDown.attr('size',length);