APEX

I am trying to understand how AJAX Callbacks work.

I want to perform a DA when a Textfield loses focus IF a certain backend condition is met (i.e. I want to Execute Javascript that will check against an AJAX Callback process).

This is the Javascript code to execute:

apex.server.process(
    "MY_PROCESS",
    {
        pageItems: "#P302_COUNTRY_CODE"
    }, 
    {
        dataType: "text",
        success: function( pData ) {
            if (pData === 'YES') {
                console.log('YES');
                return(true);
            } else {
                console.log('NO');
                return(false);
            }
        }
    }
);

And this is the AJAX Callback (PL/SQL):

DECLARE
  pHasProvinces INTEGER;
BEGIN
  SELECT COUNT(*) 
  INTO pHasProvinces
  FROM SUBDIVISION
  WHERE
      SUBDIVISION_LEVEL = 3 AND
      COUNTRY_CODE_ALPHA2 = :P302_COUNTRY_CODE;
  
  IF pHasProvinces > 0 THEN
      htp.p('YES');
  ELSE
      htp.p('NO');
  END IF;
  
END;

I have reviewed the documentation here. But unfortunately, I can not find an example on the PL/SQL process to implement.

Any help with the above code is welcomed. Also I am wondering if htp.p is the way to return a value in Apex18.

Apex 18.2.0.00.12

Oracle 18c XE