Typescript | Conditional Types

gistfile1.txt

interface foo {
    foo1: string
    foo2: string
    blubb: string
}

interface bar {
    bar1: number
    bar2: number
    blubb: number
}

function that(accepts: bar | foo) {
    if ('foo1' in accepts) {
        console.log(accepts.blubb)
        onlyFoo(accepts)
        return accepts.foo1 + accepts.foo2
    } else {
        console.log(accepts.blubb)
        return accepts.bar1 + accepts.bar2
    }
}

function onlyFoo(foo: foo) { 
    console.log(foo.foo1)
}

that({
    foo1: "foo1",
    foo2: "foo2",
    blubb: "blubb in foo"
})

that({
    bar1: 2,
    bar2: 1,
    blubb: 0xb1cb
})