js 将一维数组转为二维数组并分组

let arr = [{a:'W',b:'W01'},{a:'W',b:'W02'},{a:'WC',b:'WC01'},{a:'WC',b:'WC02'},{a:'WC',b:'WC02'},{a:'WC',b:'WC02'}]
let map= {};
arr.forEach(item=>{
       if(map[item.a]){
           map[item.a].push(item.b);
       }else{
           map[item.a] = [item.b];
       } 
   })
   console.log(map);

输出效果如图