js 转树结构

     toTree (data) {       var map = {}       var val = []       data.forEach(function (item) {         map[item.id] = item       })       data.forEach(function (item) {         var parent = map[item.parentId]         if (parent) {             (parent.children || (parent.children = [])).push(item)         } else {             val.push(item)         }       })       return val     }