js 方法调用链

const o = {
    fn(){
      console.log(“str-fn") 
      return this;
    },
    fn1(){
      console.log("str-fn1")
      return this;
    },
     fn2() {
      console.log("str-fn2")
      return this;
    }
  }
o.fn().fn1().fn2()  //依次打印 str-fn  str-fn1  str-fn2
o.fn1().fn2().fn()  //依次打印 str-fn1  str-fn2  str-fn