Ensure a function is invoked only when online

underscore.whenOnline.js

_.mixin({
   whenOnline: function(fn, context) {
    return function() {
      var args = arguments;
      context = context || this;
      if (navigator.onLine) {
        fn.apply(context, args);
      } else {
        window.addEventListener('online', function online() {
          fn.apply(context, args);
          window.removeEventListener('online', online);
        });
      }
    };
  },
});