Show browser confirmation when Meteor methods are pending in the queue

methodQueueWarning.js

window.addEventListener('beforeunload', function (e) {
  const hasOutstandingRequests =
    Meteor.connection._outstandingMethodBlocks.length

  if (hasOutstandingRequests) {
    // Note: modern browsers don't show a custom message but still confirm
    const confirmationMessage =
      'WARNING: There are outstanding requests. Really close this window? If you do you may lose data!'
    e.returnValue = confirmationMessage // Gecko, Trident, Chrome 34+
    return confirmationMessage // Gecko, WebKit, Chrome <34
  }
})