ajax请求在firefox中工作，在chrome中不起作用

Question

function ajx(url) {

  var xhri = xhrRequest('xml');
  var ajxOut = '';
  var out = '';

  xhr[xhri].open('GET', url, true);
  xhr[xhri].setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset='+document.characterSet); 
  xhr[xhri].onreadystatechange = function() {

       console.log(xhr[xhri].readyState);

    if (xhr[xhri].readyState == 4 && (xhr[xhri].status == 200 || xhr[xhri].status == 205)) {
      doSomething();
    }
  }

使用此函数的ajax调用会导致chrome和firefox的结果不同：在firefox中，会调用doSomething，因为onreadystatechange会被多次调用并最终传递readyState == 4。但是，在chrome中，onreadystatechange被调用一次并记录readyState == 0。怎么会这样？