Answers for "change value of a field on change another javascript"

jQuery checkbox changed event

//jQuery listen for checkbox change
$("#myCheckBoxID").change(function() {
    if(this.checked) {
        //I am checked
    }else{
        //I'm not checked
    }
});

jquery detect change in textarea content

$('#textareaID').bind('input propertychange', function() {

      $("#yourBtnID").hide();

      if(this.value.length){
        $("#yourBtnID").show();
      }
});

javascript input value change

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