复习

function test1() {
        function test2() {
          var b = 2;
          console.log(a); // 1
        }
        var a = 1;
        return test2;
      }
      var c = 3;
      var test3 = test1();
      test3()

test1 定义 ：test1.scope->0:GO
test1 执行：test1.scope->0:test1.AO
1:GO
test2定义：test2.scope->0:test1.AO
1:GO
test1结束：test1.scope->0:GO
test1函数先结束，return了，函数肯定执行完成，test1.AO的地址删除，只有GO的地址

test3(test2) 执行：test2.scope->0:test2.AO
1:test1.AO
2:GO
test3结束：test2.scope->0:test1.AO
1:GO