index.js

if (navigator && navigator.userAgent) {
  // 检测浏览器版本，并设置 window.__disableNativeFetch 来决定是否开启 fetch
  var userAgent = navigator.userAgent || "";
  var appVersion = navigator.appVersion || "";
  var vendor = navigator.vendor || "";

  var ua = (userAgent + ' ' + appVersion + ' ' + vendor).toLowerCase();

  var match = /(chrome)[ \/]([\w.]+)/.exec(ua)
        || /(webkit)[ \/]([\w.]+)/.exec(ua)
        || /(opera)(?:.*version|)[ \/]([\w.]+)/.exec(ua)
        || /(msie) ([\w.]+)/.exec(ua)
        || /(trident)(?:.*? rv:([\w.]+)|)/.exec(ua)
        || ua.indexOf("compatible") < 0
        && /(mozilla)(?:.*? rv:([\w.]+)|)/.exec(ua) || [];

  var engine = match[0];
  var mainVersion = match[2].split('.')[0];

  // chrome 内核版本大于 46， firefox 版本大于39 才开启 fetch
  if (engine.indexOf('chrome') === 0 && mainVersion >= 46) {
    window.__disableNativeFetch = false;
  } else if (engine.indexOf('mozilla') === 0 && mainVersion >= 39) {
    window.__disableNativeFetch = false;
  } else {
    window.__disableNativeFetch = true;
  }
}