XMLHttpRequest_POST.js

var request = new XMLHttpRequest();
 
request.onreadystatechange = function() {
  if(request.readyState === 4) {
    if(request.status === 200) { 
      console.log(request.responseText);
    } else {
      console.error(request.status, request.statusText, request.responseText);
    } 
  }
}

request.open('POST', 'http://hostname:post/api/endpoint', true);
request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

request.send('foo=bar');