Consider the below code.

type UnionType = 'foo' | 'bar' | 'baz'

const obj = {
    foo: 'huh',
    bar: 'hmm'
}

function func(input: UnionType) {
    if(input in obj) {
        input
    }
}

Playground link.

input is not being narrowed to 'foo' | 'bar'. Why is that?

Is there a way to make it narrow the union down, without explicitly re-stating the object keys?