TypeScriptConditionalType

notgoodbye.ts

type NotGoodbye<T> = T extends "goodbye" ? never : T;
type Salutation = "hello" | "goodbye";
type AllPossibleWords = string
type AllPossibleWordsWithoutGoodbye = NotGoodbye<AllPossibleWords>

function neverSayBye(salutation: NotGoodbye<Salutation>, greetee: string) {
    console.log(`${salutation} ${greetee}`)
}

neverSayBye('goodbye', 'David');
neverSayBye('hello', 'David');

fetch('/random/salutation')
    .then(response => response.json())
    .then((randomSalutation: Salutation) => {
        // The below fails to compile as we cannot assert "goodbye" is
        // not the chosen salutation
        neverSayBye(randomSalutation, 'David');
    });

let someGreeting: AllPossibleWordsWithoutGoodbye = 'hello';
let anotherGreeting: AllPossibleWordsWithoutGoodbye = 'goodbye';