creditLookup.ts

interface HasSsn {
    ssn: string
}

interface Credit {
    income: number
    age: number
}

interface HasSsnAndCredit {
    ssn: string
    credit: Credit
}

interface HasNoCredit {
    credit: undefined
}

function hasSsn(v: any): v is HasSsn {
    return v.ssn != null
}

// This is what we need to make it type safe and typo safe
function creditLookup<P>(person: P | P & HasSsn): P & HasNoCredit | P & HasSsnAndCredit {
    if (hasSsn(person)) {
        return Object.assign({ credit: { income: 2000, age: 42 } }, person)
    }
    return Object.assign({ credit: undefined }, person)
}

// This would work as well but we are not safe for putting in bad credit
function creditLookup2<P>(person: P | P & HasSsn): P | P & HasSsnAndCredit {
    if (hasSsn(person)) {
        // the next line is type correct :o
        // return Object.assign({ credit: { inkomst: 2000, age: 42 } }, person)
        return Object.assign({ credit: { income: 2000, age: 42 } }, person)
    }
    return person
}

const data = [
    { ssn: "1234" },
    {name:  "foo"},
    {ssn: "1234", name: "foo"}]

console.log(data.map(creditLookup))
console.log(data.map(creditLookup2))