1. const xhr = new XMLHttpRequest();
    3. xhr.responseType = 'json';
    4. xhr.onreadystatechange = () => {
    5.   if(xhr.readyState === 4) {
    6.     if(xhr.status >= 200 && xhr.status < 300) {
    7.       console.log('success', xhr.response);
    8.     }
    9.   }
    10. }
    12. // true 是否异步
    13. xhr.open('GET', './user.json', true);
    14. xhr.send(null);