Exclusive Or and the Optional never Trick

[utterances-bot]

Exclusive Or and the Optional never Trick

Effective TypeScript: Exclusive Or and the Optional never Trick

https://effectivetypescript.com/2021/11/11/optional-never/

[aleclofabbro]

Thanks Dan for this blog ! :)
what about
type XOR<A, B> = Exclude<A | B, A & B>

[danvk]

That's a cute idea (and mathematically correct!) but unfortunately TypeScript isn't able to follow along:

type XOR<A, B> = Exclude<A | B, A & B>

type ExclusiveThing = XOR<ThingOne, ThingTwo>;
//   ^? type ExclusiveThing = ThingOne | ThingTwo

The issue is that the Exclude generic filters unions; it won't break up a "base" type like an interface or a string.

[Beraliv]

Hey @danvk! Thank you for mentioning ts-essentials.

This type hasn't been added to TypeScript since 2017, you can read more in microsoft/TypeScript#14094

The utility type existed in ts-essentials for a long time but it didn't have a support of variadic XOR which last year was implemented in ts-xor - microsoft/TypeScript#14094 (comment) (It's also supported in ts-essentials@10)

[userFortyTwo]

Here's a version that is perhaps a little more readable:

type Xor<T, U> = (T & Never<T, U>) | (U & Never<U, T>);

type Never<T, U> = {
    readonly [K in Exclude<keyof U, keyof T>]?: never;
};

Also worth noting that the definition can be easily extended to three or more types using either

type Xor3<T, U, V> = (T & Never<T, U | V>) | (U & Never<U, T | V>) | (V & Never<V, T | U>);

or, equivalently,

type Xor3<T, U, V> = Xor<Xor<T, U>, V>;

EDIT: Or, yeah, just use ts-xor or ts-essentials if you don't mind adding an additional dependency.