ajax.js

function ajax(params) {
  // 参数
  params = Object.assign({}, { type: "GET", url: "/", success: () => null, error: () => null, headers: {}, timeout: 5000 }, params);
  const http = new XMLHttpRequest();
  // 方法 url
  http.open(params.type, params.url, true);
  // 超时时间
  http.timeout = params.timeout;
  // header
  Object.keys(params.headers).forEach(key => http.setRequestHeader(key, params.headers[key]));
  // ok
  http.addEventListener("load", () => http.status >= 200 && http.status < 300 && params.success(parseResponse(http)));
  // error
  http.addEventListener("error", error => params.error(error));
  http.send();
}

function parseResponse(xmlHttp) {
  if (xmlHttp.responseType === "json") {
    return JSON.parse(xmlHttp.responseText);
  } else {
    // ...
    return xmlHttp.responseText;
  }
}