为什么在使用＆＃39; var＆＃39;声明变量时，javascript中可配置的属性描述符的默认值为false

Question

为什么属性＆＃39;可配置＆＃39;默认为＆＃39; false＆＃39;当使用＆＃39; var＆＃39;声明变量时如果变量在全局范围内声明而没有＆＃39; var＆＃39;它是可配置的＆＃39; true＆＃39;？

   // variable declared in global scope with 'var' keyword

          var foo = 2;
          Object.getOwnPropertyDescriptor(this, 'foo')

   // will print {value: 2, writable: true, enumerable: true, configurable: false}


   // But with variable declared without var has configurable true

          bar = 2;   

   // will print {value: 2, writable: true, enumerable: true, configurable: true}