code: function transmitDraftId( calypsoPort ) {
  	// Bail if we are not writing a new post.
  	if ( ! /wp-admin\/post-new.php/.test( location.href ) ) {
  		return;
  	}
  
  	const unsubscribe = subscribe( () => {
  		const currentPost = select( 'core/editor' ).getCurrentPost();
  
  		if ( currentPost && currentPost.id && currentPost.status !== 'auto-draft' ) {
  			calypsoPort.postMessage( {
  				action: 'draftIdSet',
  				payload: { postId: currentPost.id },
  			} );
  
  			unsubscribe();
  		}
  	} );
  }
docs: Monitors Gutenberg store for draft ID assignment and transmits it to parent frame when needed.
  
  @param {MessagePort} calypsoPort Port used for communication with parent frame.