Creating an index signature in Typescript with required AND optional keys

Question

I'm trying to find a more elegant solution for creating a type that allows certain keys of it's index signature to be optional.

This may be a use case for generics, but I can't seem to crack it.

Currently constructing it like this:

//  Required and optional keys allowed as indexes on final type
type RequiredKeys = 'name' | 'age' | 'city'
type OptionalKeys = 'food' | 'drink'

//  Index types use to combine for final type
type WithRequiredSignature = {
    [key in RequiredKeys]: string
}
type WithOptionalSignature = {
    [key in OptionalKeys]?: string
}

//  Build type with required and optional properties on index signature
type FinalType = WithRequiredSignature & WithOptionalSignature

//  Test objects with functional autocomplete
const test1: FinalType = {
    name: 'Test',
    age: '34',
    city: 'New York'
}

const test2: FinalType = {
    name: 'Test',
    age: '34',
    city: 'New York',
    drink: 'Beer'
}

const test3: FinalType = {
    name: 'Test',
    age: '34',
    city: 'New York',
    food: 'Pizza'
}

Seems fine to me. you could create a utility type for it but that is not strictly needed and you could use Record and Partial instead of custom mapped types: typescriptlang.org/play?#code/… — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Feb 15, 2022 at 16:01
I'm not quite sure what you're asking. I mean, you don't need all those intermediary types and they don't seem to help much vs. just writing them (or even just writing the type directly). But I'm guessing you know you can do that, so can you tell us more about what you're trying to do? What the use case is? — T.J. Crowder
– T.J. Crowder, Commented Feb 15, 2022 at 16:02

Yftach · Accepted Answer · 2022-02-15 16:06:09Z

1

Your solution is fine if you are getting the keys dynamically. If you arent then why not just do an interface?

interface FinalType {
   name: string;
   age: number;
   city: string;
   food?: string;
   drink?: string;
}

answered Feb 15, 2022 at 16:06

Yftach

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