Defined Partial Record

by Aleksandr Trutanov @alex-altay

### Defined Partial Record Using a Record with union types as keys doesn't allow you to make an object with only some of them ```ts 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 ```ts const partial: Partial> = { 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 ```ts const best: DefinedPartial> = { a: 42 } const sum = 0 + best.a // 42 const error = best.b // error: property 'b' does not exist on type '{ a: number; }' ```