使用Promise封装ajax方法

  const ajax = function (obj) {
    return new Promise((resolve, reject) => {
      let method = obj.method || "GET";
      let xhr;

      // 创建xhr
      if (window.XMLHttpRequest) {
        xhr = new XMLHttpRequest();
      } else {
        xhr = new ActiveXObject("Microsoft.XMLHTTP");
      }

      // 超时
      xhr.ontimeout = function () {
        reject({
          errorType: "timeout_error",
          xhr: xhr
        })
      };

      // 错误
      xhr.onerror = function () {
        reject({
          errorType: "error",
          xhr: xhr
        })
      };

      xhr.onreadystatechange = function () {
        if (xhr.readyState === 4) {
          if (xhr.status >= 200 && xhr.status < 300 || (xhr.status === 304)) {
            resolve(xhr.responseText);
          } else {
            reject({
              errorType: "error",
              xhr: xhr
            })
          }
        }
      };

      if (method === "POST") {
        xhr.open("POST", obj.url, true);
        xhr.responseType = "json";
        xhr.setRequestHeader("Accept", "application/json");
        xhr.send(obj.data);
      } else {
        let query = "";
        for (let key in obj.data) {
          query += `&${encodeURIComponent(key)}=${encodeURIComponent(obj.data[key])}`;
        }
        // 去掉query头的&符号
        query.substring(1);
        xhr.open("GET", obj.url + "?" + query, true);
        xhr.send();
      }
    })
  };