关于区别浏览器型号

var sBrowser, sUsrAg = navigator.userAgent;

// The order matters here, and this may report false positives for unlisted browsers.

if (sUsrAg.indexOf("Firefox") > -1) {
  sBrowser = "Mozilla Firefox";
  // "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0"
} else if (sUsrAg.indexOf("Opera") > -1 || sUsrAg.indexOf("OPR") > -1) {
  sBrowser = "Opera";
  //"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 OPR/57.0.3098.106"
} else if (sUsrAg.indexOf("Trident") > -1) {
  sBrowser = "Microsoft Internet Explorer";
  // "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; .NET4.0C; .NET4.0E; Zoom 3.6.0; wbx 1.0.0; rv:11.0) like Gecko"
} else if (sUsrAg.indexOf("Edge") > -1) {
  sBrowser = "Microsoft Edge";
  // "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299"
} else if (sUsrAg.indexOf("Chrome") > -1) {
  sBrowser = "Google Chrome or Chromium";
  // "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/66.0.3359.181 Chrome/66.0.3359.181 Safari/537.36"
} else if (sUsrAg.indexOf("Safari") > -1) {
  sBrowser = "Apple Safari";
  // "Mozilla/5.0 (iPhone; CPU iPhone OS 11_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.0 Mobile/15E148 Safari/604.1 980x1306"
} else {
  sBrowser = "unknown";
}

alert("当前浏览器为: " + sBrowser);

这个是 mdn 上的示例，但我不明白的是，如果我是 Chrome，使用 agent.indexOf("Safari") >-1 也判定为 true，实际上就是判断该字符串是否出现，但是这些字符都出现了啊，为什么可以判断是哪个浏览器呢？