Duda con cambio de estado

[metacortex]

Código javascript:

function alertContents() {
  if (http_request.readyState == 1) {
    document.getElementById('wpsi-demo-contenedor').innerHTML="Cargando, por favor espere";
  }
  if (http_request.readyState == 4) {
    if (http_request.status == 200) {
      result = http_request.responseText;
      document.getElementById('wpsi-demo-contenedor').innerHTML = result;            
    } else {
      alert('There was a problem with the request.');
    }
  }
}