let flatArr = [
            { id: 1, title: '标题1', parent_id: 0 },
            { id: 2, title: '标题2', parent_id: 0 },
            { id: 3, title: '标题2-1', parent_id: 2 },
            { id: 4, title: '标题3-1', parent_id: 3 },
            { id: 5, title: '标题4-1', parent_id: 4 },
            { id: 6, title: '标题2-1', parent_id: 2 },
        ]
        
        function convert (list) {
          const result = [];
          const map = list.reduce((pre,cur) => {
              pre[cur.id] = cur;
              return pre
          }, {})
        // 1: {id: 1, title: '标题1', parent_id: 0}
        // 2: {id: 2, title: '标题2', parent_id: 0}
        // 3: {id: 3, title: '标题2-1', parent_id: 2}
        // 4: {id: 4, title: '标题3-1', parent_id: 3}
        // 5: {id: 5, title: '标题4-1', parent_id: 4}
        // 6: {id: 6, title: '标题2-1', parent_id: 2}
          for (let item of list) {
              if (item.parent_id === 0) {
                  result.push(item)
                  continue
              }
              if (item.parent_id in map) {
                  const parent = map[item.parent_id]
                  parent.children = parent.children || []
                  parent.children.push(item)
              }
          }
          return result
        }
        console.log(convert(flatArr));

运行结果