Why XMLHttpRequest status 0 with this link?

Question

I am trying to retrieve some JSON data from a link using javascript. Yet, I am getting status = 0, and I have no idea why. This always worked for me, but now I just get an empty response. I can retrieve the data normally in a browser, so I tried setting the UA header without luck. Maybe I should set another header??

This is my method to retrieve the data:

function retrieveBusData(bus) {
  var file = "https:....";
  var xhReq = new XMLHttpRequest();
  xhReq.timeout = 50000;
  xhReq.ontimeout = function() { console.log("#########--- " + getsecs() + " - TIMEOUT en retrieveBusData"); };
  xhReq.open("GET", file, true);
  xhReq.setRequestHeader("Accept", "application/json");
  xhReq.onreadystatechange = function() {
    if (xhReq.status != 200) {
      printbondiinfo("X");
    } else {
      if (xhReq.readyState == 4) {
        if (xhReq.responseText)
          processData(xhReq.responseText);
        else
          printbondiinfo("??");
      }   
    } 
  } 
  xhReq.send(null);
} 

EDIT

Thanks to @epascarello for pointing me the CORS problem. Indeed, I can see in the console:

Access to XMLHttpRequest at 'https://jeap.rio...' from origin 'null'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header
is present on the requested resource.

Answer 1

I used php as a workaround. I created a simple simple.php file in my own page with this content:

<?php 
  file_get_contents('https://<stupid file without Access-Control-Allow-Origin header>');
?>

and then I changed the corresponding javascript file line for the XMLHttpRequest with

...
var file = "https://<my site>/simple.php";
var xhReq = new XMLHttpRequest();
xhReq.open("GET", file, true);
...

In this way, the javascript XMLHttpRequest is made to a page with Access-Control-Allow-Origin header set (through my own .htaccess file), and php does not care about the Access-Control-Allow-Origin header.