TypeScript require one parameter or the other, but not neither

Question

Say I have this type:

export interface Opts {
  paths?: string | Array,
  path?: string | Array

        

Answer 1

If you already have that interface defined and want to avoid duplicating the declarations, an option could be to create a conditional type that takes a type and returns a union with each type in the union containing one field (as well as a record of never values for any other fields to dissalow any extra fields to be specified)

export interface Opts {
    paths?: string | Array,
    path?: string | Array
}

type EitherField =
    TKey extends keyof T ? { [P in TKey]-?:T[TKey] } & Partial, never>>: never
export const foo = (o: EitherField) => {};
foo({ path : '' });
foo({ paths: '' });
foo({ path : '', paths:'' }); // error
foo({}) // error

Edit

A few details on the type magic used here. We will use the distributive property of conditional types to in effect iterate over all keys of the T type. The distributive property needs an extra type parameter to work and we introduce TKey for this purpose but we also provide a default of all keys since we want to take all keys of type T.

So what we will do is actually take each key of the original type and create a new mapped type containing just that key. The result will be a union of all the mapped types that contain a single key. The mapped type will remove the optionality of the property (the -?, described here) and the property will be of the same type as the original property in T (T[TKey]).

The last part that needs explaining is Partial, never>>. Because of how excess property checks on object literals work we can specify any field of the union in an object key assigned to it. That is for a union such as { path: string | Array } | { paths: string | Array } we can assign this object literal { path: "", paths: ""} which is unfortunate. The solution is to require that if any other properties of T (other then TKey so we get Exclude) are present in the object literal for any given union member they should be of type never (so we get Record, never>>). But we don't want to have to explicitly specify never for all members so that is why we Partial the previous record.