const arr = [
      {
        id: 1,
        title: "課程 1",
        children: [
          { id: 4, title: "課程 1-1" },
          {
            id: 5,
            title: "課程 1-2",
            children: [
              { id: 6, title: "課程 1-2-1" },
              { id: 7, title: "課程 1-2-2" },
            ],
          },
        ],
      },
      { id: 2, title: "課程 2" },
      { id: 3, title: "課程 3" },
    ];


    function flaten(arr) {
      return arr.reduce((p, v, i) => {
        for (let i = 0; i < p.length; i++) {
          if (p[i].children) {
            delete p[i].children
          }
        }
        return p.concat(v.children ? flaten(v.children).concat(v) : v);
      }, [])
    }
    console.log(flaten(arr))


 // reduce 功能很強(qiáng)大呦；