keyed-items.js

const keyedItems = treeItems.map( ( item ): TreeItem => {
    const fieldValues = fieldNames.map( ( field ): string => item.data && item.data[field] );
    let parentKey = fieldValues.slice( 0, -1 ).reduce( ( id, field ):string=>{
      if ( id && field ) {
        id += `-` + field;
      }
      if ( !id && field ) {
        id = field;
      }
      parentKeys.add( id );
      return id;
    }, `` );

    let key = fieldValues.join( `-` );

    parentKeys.add( parentKey );

    // return the TreeItem with correct key and parentKey values
    // ditch the mesh component of the TreeItem as unneeded baggage
    // eslint-disable-next-line @typescript-eslint/no-unused-vars
    const { mesh, ...keyedItem } = Object.assign( item.data, { id:item.id, name: item.name, parentKey, key } );

    return keyedItem;
  } );