typescript-2.8-taggedish-unions.ts

// Declare your union variants
// Easy to name variant contructors + define tags in one place

function Age(age: number) {
    return { tag: 'Age' as 'Age', age }
}

function Name(name: string) {
    return { tag: 'Name' as 'Name', name }
}

// Unify them

type TaggedUnion =
    | ReturnType<typeof Age>
    | ReturnType<typeof Name>;



// Can invoke with either variant
ok(Age(0));
ok(Name(name));

// Can switch on TaggedUnion.tag
// Use a return type and --strictNullChecks if you want exhaustiveness checking .-.
function ok(j: TaggedUnion): null {
    switch (j.tag) {
        case 'Age': {
            console.log(j.age);
            return null;
        }
        case 'Name': {
            console.log(j.name);
            return null;
        }
    }
    // Exhaustive check; this isn't reachable
}