'AMap.PlaceSearch'这个为什么要用中括号

AMap.service(['AMap.PlaceSearch'], function(){

            btn.onclick = function(){
                new AMap.PlaceSearch({
                    pageSize:5,
                    pageIndex:1,
                    city:'010',
                    citylimit:true,
                    map:map, //展示在哪个地图里
                    panel:'setCenterNode' //放在哪个元素下
                }).search(searchIpt.value);
            }
            
        })