闭包返回函数不会改变作用域变量

function testLet(){
    var x=9;
    return{
        y:x,
        getX: function(){
            return ++x;
        },
    }
}

var obj = new testLet();
console.log(obj.y); //9 as expected
console.log(obj.getX()); //10 as expected
console.log(obj.getX()); //11 as expected
console.log(obj.y); //I expect it 11.. but it stays 9

为什么最后一行返回 9 而不是 11 因为 x 在同一范围内？