如何动态加载Google AnalyticsJavaScript？

// 创建一个异步加载的函数
function loadScript(url, callback) {
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = url;

  if (script.readyState) {  // for Internet Explorer
    script.onreadystatechange = function() {
      if (script.readyState == 'loaded' || script.readyState == 'complete') {
        script.onreadystatechange = null;
        callback();
      }
    };
  } else {  // for other browsers
    script.onload = function() {
      callback();
    };
  }

  document.getElementsByTagName('head')[0].appendChild(script);
}

// 使用上述函数动态加载 Google Analytics JavaScript
loadScript('https://www.googletagmanager.com/gtag/js?id=GA_MEASUREMENT_ID', function() {
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'GA_MEASUREMENT_ID');
});