js创建namespace(o, ‘a.b.c’)得到o.a.b.c的值，目标属性不存在返回undefined

let obj2 = o = { a: { b: { c: '12', c1: '1s2' }, b1: 2 } }
    // namespace(o, ‘a.b.c’)得到o.a.b.c的值，目标属性不存在返回undefined
    function namespace(o, str) {
      let str2;
      function namespace1(obj1, str) {
        let arr = str.split('.')
        let shi = arr.shift()
        if (typeof obj1[shi] === 'object' && arr.length > 0) {
          namespace1(obj1[shi], arr.join('.'))
        } else {
          str2 = obj1[shi]
        }
      }
      namespace1(o, str)
      return str2
    }
    console.log(namespace(o, 'a.b'))
    console.log(namespace(o, 'a.b.c'))