code: function initialize() {
    const transition = new ExpandableTransition(
      this.ui.content
    );
    this.transition = transition.init();
  
    if ( this.ui.content.classList.contains( ExpandableTransition.CLASSES.EXPANDED ) ) {
      this.ui.target.classList.add( this.classes.targetExpanded );
    } else {
      this.ui.target.classList.add( this.classes.targetCollapsed );
    }
  
    const expandableGroup = closest( this.ui.target, '.' + this.classes.group );
  
    this.isAccordionGroup = expandableGroup !== null &&
      expandableGroup.classList.contains( this.classes.groupAccordion );
  
    if ( this.isAccordionGroup ) {
      Events.on(
        'accordionActivated',
        _accordionActivatedHandler.bind( this )
      );
    }
  }
docs: Initialize a new expandable.