forum/app/assets/javascripts/discourse/scripts/browser-detect.js

/* eslint-disable no-var */ // `let` is not supported in very old browsers

(function () {
  if (
    !window.WeakMap ||
    !window.Promise ||
    typeof globalThis === "undefined" ||
    !String.prototype.replaceAll ||
    !CSS.supports ||
    !CSS.supports("aspect-ratio: 1")
  ) {
    window.unsupportedBrowser = true;
  } else {
    // Some implementations of `WeakMap.prototype.has` do not accept false
    // values and Ember's `isClassicDecorator` sometimes does that (it only
    // checks for `null` and `undefined`).
    try {
      new WeakMap().has(0);
      // eslint-disable-next-line no-unused-vars -- old browsers require binding this variable, even if unused
    } catch (err) {
      window.unsupportedBrowser = true;
    }

    var match = window.navigator.userAgent.match(/Firefox\/([0-9]+)\./);
    var firefoxVersion = match ? parseInt(match[1], 10) : null;
    if (firefoxVersion && firefoxVersion < 89) {
      // prior to v89, Firefox has bugs with document.execCommand("insertText")
      // https://bugzil.la/1220696
      window.unsupportedBrowser = true;
    }
  }
})();