判断是不是空数组

      const a = {};
      //1.JSON.stringify()
      console.log(JSON.stringify(a) === "{}"); //true
      //2. es6的方法Object.keys().length
      console.log(Object.keys(a)); //[] 返回的是空数组
      console.log(Object.keys(a).length); //0
      //3.for-in循环
      var test = function (obj) {
        for (const item in obj) {
          return false;
        }
        return true;
      };
      console.log(test(a)); //true