function Foo () {
      }

      Foo.prototype = {
        m1: function () {
          console.log('m1');
        },
        m2: function () {
          console.log('m2');
        },
        $eval: function (exp) {
          var val;
          with (this) {
            val = eval(exp);
          }
          return val;
        }
      };

      var scope = new Foo();
      scope.value = 'world!';
      scope.$eval('m1()');
      console.log(scope.$eval('"hello " + value'));
      console.log(scope.$eval('"hello!"'));
      

不用with和eval，如何实现一个Foo对象，可以对'"hello " + value'，'m1()'等等进行计算