Typing key/value pairs generically in function signatures in Typescript

type-key-val-pairs.ts

type Fields = Record<string, number>
type Obj<ObjKey extends string, ObjVals extends Fields> = { [key in ObjKey]: ObjVals }

const obj = { "A": { fieldOne: 1 }, "B": { fieldTwo: 2 } }

const acceptsObj = <TKeys extends string, TFields extends Fields>(obj: Obj<TKeys, TFields>) => {
  return <Key extends TKeys>(key: Key, fields: Obj<TKeys, TFields>[Key]) => undefined;
}

const fn = acceptsObj(obj)

// This is fine
fn("A", { fieldOne: 1 })
// In the value, property 'fieldNope' does not exist
fn("A", { fieldNope: 1 })
// In the keys, 'Nope' does not exist
fn("Nope", { fieldNope: 1 })