pass parameter to function so that it becomes the key of an object and a value can be returned from an object

Question

I want that it prints "S&P" in the console. Why does it not work?

var agency = "s_p"

function switch_Name(agency) {
    const agencies = {s_p: "S&P", moody: "Moodys", fitch: "Fitch"};
    console.log(agencies.agency);
};

switch_Name();