Javascript函数返回对象undefined的问题？

    function checkObj(obj, checkProp) {
        // Only change code below this line

        if (obj.hasOwnProperty(checkProp) === true) {
            return obj[checkProp];
        } else {
            return "Not Found";
        }

        // Only change code above this line
    }
    // Only change code above this line
    console.log(checkObj({gift: "pony", pet: "kitten", bed: "sleigh"}, "gift"));

为什么返回的时候只能写成return obj[checkProp];而不能写成return obj.checkProp;会输出Undefined