typescript - Type union not checking for excess properties

Question

let's imagine a have an object that either have properties A and B or C, e.g.:

const temp = {
  A: 1,
  B: 2,
}

or

const temp = {
  C: 3,
}

And intuitively I see this type as:

type T =  {A: number, B: number} | {C: number};

const valid: T = {A: 1, B: 2};
const alsoValid: T = {C: 3};

// Should complain but it does not
const invalid: T  = {A: 1, B: 2, C: 3};
// Also should complain
const alsoInvalid: T = {A:1, C: 3};

But TS treats such type as {A?: number, B?: number, C?: number} and basically | makes fields optional but I want TS to complain about inconsistent type when I add C property to A and B

---

How can I archive the desirable type?

Answer 1

This is a bit of a quirk in how unions work in conjunction with excess property checks. {A:1, C: 3} is actually compatible with {C: number} except for excess property checks:

const o = {A:1, C: 3};
const ok: {C: number} = o; // No direct literal assignment, no excess property checks
const nok: {C: number} = { A:1, C: 3}; // Excess property checks kick in 

Playground Link

And the quirk of excess property checks is that for unions, it allows any property from any union constituent to be present in the assigned object literal.

You can get an error if the union constituents are incompatible one with another:

type T =  {A: number, B: number} | {C: number, A?: undefined, B?: undefined };

const valid: T = {A: 1, B: 2};
const alsoValid: T = {C: 3};

// Error
const invalid: T  = {A: 1, B: 2, C: 3};
//Error
const alsoInvalid: T = {A:1, C: 3};

Playground Link

You can also use the StrictUnion from here if the union has a lot of memebers

type UnionKeys<T> = T extends T ? keyof T : never;
type StrictUnionHelper<T, TAll> = T extends any ? T & Partial<Record<Exclude<UnionKeys<TAll>, keyof T>, never>> : never;
type StrictUnion<T> = StrictUnionHelper<T, T>

type T =  StrictUnion<{A: number, B: number} | {C: number }>;

const valid: T = {A: 1, B: 2};
const alsoValid: T = {C: 3};

// Error
const invalid: T  = {A: 1, B: 2, C: 3};
//Error
const alsoInvalid: T = {A:1, C: 3};

Playground Link