TypeScript a | b allows combination of both

Question

I was surprised to find that TypeScript won\'t complain at me doing something like this:

type sth = { value: number, data: string } | { value: number, note:          


        

Answer 1

Another option is to use optional never properties to explicitly disallow a mix of fields from the two types in the union:

type sth =
  { value: number, data: string; note?: never; } |
  { value: number, note: string; data?: never; };

const a: sth = { value: 7, data: 'test' };
const b: sth = { value: 7, note: 'hello' };
const c: sth = { value: 7, data: 'test', note: 'hello' };
   // ~ Type '{ value: number; data: string; note: string; }'
   //     is not assignable to type 'sth'.

The ts-essentials library has an XOR generic that can be used to help you construct exclusive unions like this:

import { XOR } from 'ts-essentials';

type sth = XOR<
  { value: number, data: string; },
  { value: number, note: string; }
>;

const a: sth = { value: 7, data: 'test' };
const b: sth = { value: 7, note: 'hello' };
const c: sth = { value: 7, data: 'test', note: 'hello' };
// ~ Type '{ value: number; data: string; note: string; }'
//     is not assignable to type ...

Here's a playground link for that last example.