when dropdown change then selected value code example

Example 1: .on change get value

$('select').on('change', function() {
  alert( this.value );
});

Example 2: change value of drop down using jquery

$('#ddlID').val('New_OptionValue').change()

Example 3: jQuery change select value

$("#mySelectElementID").val("my_new_value"); //change selected option/value with jQuery