How to create a custom event in javascript

You can take a callback to find out if your function ends.

for example something like that.

function verificar(callback) {
var interval = setInterval(function () {
    if (oXMLHTTP.readyState == 4) {
        //CREAR NUEVO EVENTO
        callback(true)
    }


   }, 1000);
}

    verificar(function(response){
      alert(response);
    })

It is normal to do it through the event load of XMLHttpRequest :

request.onload = () => {
  if (request.readyState === 4 && request.status === 200) {
    // debes enviar como respuesta: 'CHANGED' si la variable cambió
    if (request.responseText === 'CHANGED') {
      // variable cambió
    }
  }
};

Making requests every second penalizes performance, keep that in mind. Instead, perform verification in processes where you can change the variable , this way you save bandwidth and memory.

There are paradigms such as event-oriented programming or reactive programming. they create a flow of events in a particular way. using a bit of that concept we can make the changes of states detected. in general all frameworks have it.

I do not know what framework you use but this response show how to subscribe to the oXMLHTTP.onreadystatechange event

function ProcessSend() 
    Set oXMLHTTP = CreateObject("MSXML2.XMLHTTP.4.0")
    Set oXMLDoc = CreateObject("MSXML2.DOMDocument")

    oXMLHTTP.onreadystatechange = getRef("HandleStateChange") 

    strEnvelope = "callNo="&callNo&"&exp="&exp

    call oXMLHTTP.open("POST","http://localhost:11883/ServiceCall.asmx/"&posFirm,false)'<< changed true to false here.
    call oXMLHTTP.setRequestHeader("Content-Type","application/x-www-form-urlencoded")


    call oXMLHTTP.send(strEnvelope)
end function

Sub HandleStateChange 
    if(oXMLHTTP.readyState = 4) then
        dim szResponse: szResponse = oXMLHTTP.responseText
        call oXMLDoc.loadXML(szResponse)
        if(oXMLDoc.parseError.errorCode <> 0) then
                'call msgbox("ERROR")
                response = oXMLHTTP.responseText&" "&oXMLDoc.parseError.reason
                'call msgbox(oXMLDoc.parseError.reason)
        else
                response = oXMLDoc.getElementsByTagName("string")(0).childNodes(0).text
        end if

    end if
End Sub

Apparently you try to use the wep API XMLHttpRequest if so, because you do not try the following:

oXMLHTTP.addEventListener("load", function (){
 alert('load successful!');
});