open(
     method, // 请求的方式，如 GET/POST/HEADER 等，这个参数不区分大小写
     url // 请求的地址，可以是相对地址或者绝对地址
     [, async = true // 默认值为true，即为异步请求，若async=false，则为同步请求
     [, username = null // Basic 认证的用户名密码
     [, password = null]]]
);

const xhr = new XMLHttpRequest();
xhr.timeout = 3000;
xhr.ontimeout = function(event) {
  alert("请求超时！");
};
const formData = new FormData();
formData.append("tel", "18217767969");
formData.append("psw", "111111");
xhr.open("POST", "http://www.test.com:8000/login");
xhr.send(formData);

// 如果是同步请求，则不需要监听事件
xhr.onreadystatechange = function() {
  if (xhr.readyState == 4 && xhr.status == 200) {
    alert(xhr.responseText);
  } else {
    alert(xhr.statusText);
  }
};

// Just getting XHR is a mess!
if (window.XMLHttpRequest) {
  // Mozilla, Safari, ...
  request = new XMLHttpRequest();
} else if (window.ActiveXObject) {
  // IE
  try {
    request = new ActiveXObject("Msxml2.XMLHTTP");
  } catch (e) {
    try {
      request = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (e) {}
  }
}

// Open, send.
request.open("GET", "https://davidwalsh.name/ajax-endpoint", true);
request.send(null);