TypeScript: Optional merge types

Question

I have type a and type b, but this should work with any amount of types.

type a = {
    first: number
}

type b = {
    second: string
    third: string
}

I want to create a type that optionally merges all those types, so if it would have the second field, it should have the third field also, but it doesn't have to have them both:

Good:

const aa = {
     first: 1,
     second: "hi",
     third: "hello"
}

const ab = {
     first: 1
}

const ac = {
     second: "hi",
     third: "hello"
}

Bad:

const bb = {
     first: 1,
     second: "hi"
}

How could I define such a type?

Are you looking for XOR merging?

MacD
– MacD

2020-09-11 22:02:53 +00:00
Commented Sep 11, 2020 at 22:02 — MacD
– MacD, Commented Sep 11, 2020 at 22:02

Lesiak · Accepted Answer · 2020-09-11 22:15:44Z

13

type None<T> = {[K in keyof T]?: never}
type EitherOrBoth<T1, T2> = T1 & None<T2> | T2 & None<T1> | T1 & T2

type abcombined = EitherOrBoth<a,b>

See more elaborated example at: Can Typescript Interfaces express co-occurrence constraints for properties

Playground link

answered Sep 11, 2020 at 22:15

Lesiak

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4 Comments

The Cubear Guy Over a year ago

Thank you, this is very helpful! Just can the EitherOrBoth type be applied to more than two types?

Lesiak Over a year ago

Yes, you can pass another EitherOrBoth as a type argument. Check linked questions for example.

The Cubear Guy Over a year ago

Thank you! This solves my problem. I've checked the linked question, could the CombinationOf type be applied to any number of types?

Lesiak Over a year ago

The definition in linked question has up to four types, but it is easy to extend it if you need more.

Karan Kumar · Accepted Answer · 2020-09-11 22:01:24Z

0

I am not sure if you could do that. This is what I would do :

You could use "|" operator while defining a variable.

type a = {
    first: number
}

type b = {
    second: string
    third: string
}

const test: a | b ={ first: 3, second: 'string'}

answered Sep 11, 2020 at 22:01

Karan Kumar

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