Drupal 8, behaviors, and jQuery Once

When porting some front-end code from Drupal 7 to Drupal 8, I ran into an unexpected change in the use of jQuery.once(). In Drupal 7, you’d do this in a behavior:

Javascript

Drupal.behaviors.exampleBehavior = {
  attach: function(context, settings) {
    $('.example', context).once('example-behavior', function() {
      // Do stuff.
    });
  },
  detach: function(context, settings, trigger) {
    $('.example', context).removeOnce('example-behavior', function() {
      // Undo stuff.
    });
  }
};

In Drupal 8, however, you can’t pass in a function to jQuery.once(), as the API for that jQuery plugin has changed. It now acts like jQuery.filter(), in that it filters out any elements that have already been processed so they aren’t processed more than once, and returns a jQuery collection. So, in Drupal 8, the example would be:

Javascript

Drupal.behaviors.exampleBehavior = {
  attach: function(context, settings) {
    $('.example', context).once('example-behavior').each(function() {
      // Do stuff.
    });
  },
  detach: function(context, settings, trigger) {
    $('.example', context).removeOnce('example-behavior').each(function() {
      // Undo stuff.
    });
  }
};

Tags: