how to apply onchange function in jquery code example

Example 1: .on change get value

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

Example 2: jquery onchange input value

$("input").change(function(){
  alert("The text has been changed.");
});

Example 3: javascript input value change

function updateInput(ish){
    document.getElementById("fieldname").value = ish;
}