Defined Partial Record

Using a Record with union types as keys doesn't allow you to make an object with only some of them

const record: Record<'a' | 'b' | 'c', number> = { a: 42, b: 10 } 
// error: Property 'c' is missing in type '{ a: number; b: number; }' 
// but required in type 'Record<"a" | "b" | "c", number>'

Using a Partial Record with union types as keys allows you to make an object without all union members, but makes all keys and values optional, potentially leaving them undefined

const partial: Partial<Record<'a' | 'b' | 'c', number>> = { a: 42 } 
const partialType = typeof partial // { a?: number | undefined, b? : number | undefined, c? : number | undefined }
const operation = 0 + partial.a // error: 'partial.a' is possibly 'undefined'
const access = partial.c // possible, type doesn't know that there is no such key

You need to make a type that takes the best of both worlds, creates all combinations of all the types in the union, so using a key that exists in the object gives you a defined type, but using a key that exists in the union and not in the object throws an error

const best: DefinedPartial<Record<'a' | 'b' | 'c', number>> = { a: 42 } 
const sum = 0 + best.a // 42
const error = best.b // error: property 'b' does not exist on type '{ a: number; }'