Seeking for an advice from TS gurus.
Here is base code

export const LETTERS = {
    // internal
    a: 'a',
    // external
    b: 'b',
    c: 'c',
} as const;

export const internalLetters = [
    LETTERS.a,
] as const;

export type LETTER_TYPE = typeof LETTERS[keyof typeof LETTERS];

Now assume I want to have a helper function which accept any letter and defines if it is internal

const isInternal = (letter: LETTER_TYPE) => internalLetters.includes(letter);

And now the error appears:

Can you explain what is wrong with such code and how to achieve my goal?
Also I’m not sure if title for the question is clear itself.