Client Side Error Messages in Oracle APEX

SAVE (button) --> Page Message (Dynamic Action) --> On Click (Event) --> TRUE event

/* General page error message */
apex.message.showErrors([
{
type: "error",
location: "page",
message: "Page error has occurred!",
unsafe: false
}
]);

/* Item error message */
apex.message.showErrors([
{
type: "error",
location: [ "page", "inline" ],
pageItem: "P7_CITYNAME",
message: "Value is required!",
unsafe: false
}
]);

/* Multi Item error message */
apex.message.clearErrors();
var chkErr = 0;
var arr = [
   'P7_CITYNAME',
   'P7_COUNTRYNAME'
];

for (var i in arr) {
  if ($v(arr[i]).length == 0) {
    apex.message.showErrors([
      {
        type: apex.message.TYPE.ERROR,
        location: ["inline"],
        pageItem: arr[i],
        message: "Value is required!",
        unsafe: false
      }
    ]);
    chkErr = 1;
  }
}

if ( chkErr == 0 ) {
  /* Custom dynamic action call when no error occurred */
  apex.event.trigger(document, 'customDA', [{customAttribute:'1'}]);
}