做你想做的事情的几种方法（可能还有更多）：

let map = {
 Argentina : "222",
 Brazil : "333",
 Mexico : "444",
};

let country = "Argentina";
let country2: keyof typeof map = "Argentina";
// let country3: keyof typeof map = "Argentina2"; // Type '"Argentina2"' is not assignable to type '"Argentina" | "Brazil" | "Mexico"'.

console.log(map[country as keyof typeof map]);
console.log((map as any)[country]);
console.log(map[country2]);

Playground链接
另一种方法是显式地为map对象指定一个类型（不太严格

// let map: {[key: string]: string} = {
let map: {[key: string]: any} = {
 Argentina : "222",
 Brazil : "333",
 Mexico : "444",
};

let country = "Argentina";
console.log(map[country]);

Playgrond链接2