javascriptvar tree_data = {
    'for-sale' : {name: 'For Sale', type: 'folder'} ,
    'vehicles' : {name: 'Vehicles', type: 'folder'} ,
    'rentals' : {name: 'Rentals', type: 'folder'}   ,
    'real-estate' : {name: 'Real Estate', type: 'folder'}   ,
    'pets' : {name: 'Pets', type: 'folder'} ,
    'tickets' : {name: 'Tickets', type: 'item'} ,
    'services' : {name: 'Services', type: 'item'}   ,
    'personals' : {name: 'Personals', type: 'item'}
}
tree_data['for-sale']['additionalParameters'] = {
    'children' : {
        'appliances' : {name: 'Appliances', type: 'item'},
        'arts-crafts' : {name: 'Arts & Crafts', type: 'item'},
        'clothing' : {name: 'Clothing', type: 'item'},
        'computers' : {name: 'Computers', type: 'item'},
        'jewelry' : {name: 'Jewelry', type: 'item'},
        'office-business' : {name: 'Office & Business', type: 'item'},
        'sports-fitness' : {name: 'Sports & Fitness', type: 'item'}
    }
}

tree_data['for-sale']['additionalParameters'] 这两个中括号中的内容如何去理解！
谢谢！