Why does TypeScript think my type is possibly undefined?

Question

type Props = {a: number}
type FuncSig = [string, Props] | [Props]

function thing(...props: FuncSig) {
    const {a} = typeof props[0] === 'string' ? props[1] : props[0]
}

In the above code, there should be no way that a is not known, but the compiler complains:

Property 'a' does not exist on type 'Props | undefined'.

Answer 1

Unfortunately TS doesn't make narrowing by a property's wide type of a union member, but you can narrow by a tuple's length:

Playground

type Props = {a: number}
type FuncSig = [string, Props] | [Props]

function thing(...props: FuncSig) {
    const {a} = props.length === 2 ? props[1] : props[0]
}