new XMLHttpRequest().responseType empty

Why is responseType empty?

JavaScript

var xhr = new XMLHttpRequest();
xhr.open(method,url,true);
xhr.onreadystatechange = function()
{
 if (xhr.readyState=='4')
 {
  if (xhr.status==200)
  {
   console.log(xhr.responseType);//[empty]
  }
 }
}

PHP

header('Content-Type: application/json');
//[Validated JSON Data]

No frameworks.