union-annotation.ts

// The `name` parameter can be either a string or null
const sayHappyBirthdayOnFacebook = (name: string | null) => {
    if (name === null) {
        console.log('Happy birthday!');
    } else {
        console.log(`Happy birthday ${name}!`);
    }
};

sayHappyBirthdayOnFacebook(null); // => "Happy birthday!"
sayHappyBirthdayOnFacebook('Jeremy'); // => "Happy birthday Jeremy!"