Typescript is not warning that a key doesn't exist on type

I have the following ts definition:

interface FormValue {
  fieldA: string;
  fieldB: number;
}

const FIELD_NAMES: Record<string, keyof FormValue> = {
  FIELD_A: 'fieldA',
  FIELD_B: 'fieldB'
} as const;

I want to the to use FIELD_NAMES.FIELD_A, this works fine, but this has an issue, when I use something likeFIELD_NAMES.X typescript is not warning me that X does not exist on FIELD_NAMES.

How can I solve this?

Here is the full playground:

https://www.typescriptlang.org/play?ssl=6&ssc=51&pln=6&pc=21#code/JYOwLgpgTgZghgYwgAgGIHsoFsBqcA2ArigN4CwAUMsjMBPgCYCCAXMgM5hSgDmA3JWq16DAEJsQhLACNoAigF9KlBOhCc0ASQCiAGQAiAfQByTALLaAymwBKEVVAYAeTtxA8ANMgDWEAJ7oMGiYuATEAHzIALzI5FRaekasyADkwoxMKR6CCQaG4qnpYimUCshw7Miq6mDyKmoakBoxqDp5phaWAHStiYZM8tWNEJwATNG5Rh1WXQAafMgA9IvIIOjI0FCYyAD8yhRD6PgQXfjoPAAUTWAAlIMNRydnl9ejd0A

Types and const assertions don't go together very well. If you remove the type and use the satisfies operator to express your constraints, you'll get the behavior you want while still maintaining type safety:

interface FormValue {
  fieldA: string;
  fieldB: number;
}

const FIELD_NAMES = {
  FIELD_A: 'fieldA',
  FIELD_B: 'fieldB'
} as const satisfies Record<string, keyof FormValue>;

const test = FIELD_NAMES.FIELD_A;
const test2 = FIELD_NAMES.X;

console.log(test);
console.log(test2);

^{Playground link}