req.onreadystatechange = callback;
直接将callback赋值给req.onreadystatechange
req.onreadystatechange = callback();
将callback的执行结果赋值给req.onreadystatechange
直接将callback赋值给req.onreadystatechange
req.onreadystatechange = callback();
将callback的执行结果赋值给req.onreadystatechange
第二个系统将callback()当做函数