How to tell if visitors to your site are running a WebExtensions-compatible extension

WebExtensionCheck.js

// If we know the extension's ID and a path to one of its web-accessible resources, we can quietly check for presence.

(function (w, a) {
  var checkExt = function (callback) {
    var hazExt = false;
    var img = new Image();
    img.onload = function () {
      hazExt = true;
    };
    // need to know the right protocol for Firefox and Edge extensions
    img.src = 'chrome-extension://' + a.id + '/' + a.path;
    w.setTimeout(function () {
      callback(hazExt);
    }, 100);
  };
  var browser = w.chrome;
  if (typeof browser === 'object') {
    checkExt(function (r) {
      // careful, now: for Chrome this will report true whether or not the extension is disabled
      console.log('Using extension ' + a.id + '? ' + r);
    });
  } else {
    console.log('Not using a compatible browser.');
  }
}(window, {
  'id': 'your_chrome_extension_id_goes_here',
  'path': 'img/icon_48.png'
}));