Generic mapped type constraint with optional member doesnt constraint when one of the member exist

Question

const abc = <T extends object, P extends { [key in keyof T]?: number }>(
  a: T,
  b: P
) => {
  console.log(a, b);
};

const A = { x: "1", y: "2", z: "3" };
const b = { x: 1, y: 2, z: 3 };
const b1 = { x: 1, y: 2 };
const b3 = { m: 5 };
const b4 = { m: 5, x: 1 };

abc(A, b);
abc(A, b1);
abc(A, b3); // Type '{ m: number; }' has no properties in common with type '{ x?: number | undefined; y?: number | undefined; z?: number | undefined; }'
abc(A, b4); // expect type error just like b3 but it is not

since m doesn't exist on A, b4 should error like b3 right? but why it is not error and how to fix it?

here is the sand box codesandbox

Typescript allow extra properties by default, so `{ m: 5 }` is considered valid argument. This post may help you: https://stackoverflow.com/questions/49580725/is-it-possible-to-restrict-typescript-object-to-contain-only-properties-defined — Simon Bruneaud, May 04 '20 at 20:00
@Simon, thanks for the link, quite informative but the solution in it is not working because not only it need exact member(which I want), it also need also to be exact same type(which I dont want) — Acid Coder, May 05 '20 at 04:38

score 0 · Answer 1 · answered May 05 '20 at 04:33

Type '{ m: number; }' has no properties in common with type '{ x?: number | undefined; y?: number | undefined; z?: number | undefined; }.

think its clear, that b4 not error because has either one of x?, y? z? included. thats because you extends from T and T is from the first param which you passed is A.

Acid Coder · Accepted Answer · 2020-09-05T09:21:36.947

turn out this is not about generics at all, this should be better way to represent my question:

const abc = <T extends { [index: string]: string }>(
  a: T,
  b: { [key in keyof T]?: number }
) => {
  console.log(a, b);
};

const A = { x: "1", y: "2", z: "3" };
const b4 = { m: 5, x: 1 };

abc(A, b4); // no error, not "expected"
abc(A, { m: 5, x: 1 }); // exact same value as b4 but throw error as "expected"

solution is you need to pass a fresh value (value that is not a variable)

for explanation and other solution(helper type), read this answer and its comments

update, I found the perfect solution:

const abc = <
  T extends { [index: string]: string },
  U extends { [key in keyof T]?: number }
>(
  a: T,
  b: keyof U extends keyof T ? U : never
) => {
  console.log(a, b);
};

const A = { x: "1", y: "2", z: "3" };
const b4 = { x: 1, y: 2, z: 3 };
const b5 = { m: 2, x: 5 };
const b6 = { x: 1, y: 2, z: 3, m: 2 };
const b7 = { x: 1 };

abc(A, b4); // no error, excat member
abc(A, b5); // error, excess member
abc(A, b6); // error, excess member
abc(A, b7); // no error, one of the member exist
abc(A, {}); // no error, since it is optional

// all work as expected

this work regardless of the value is fresh or not

Generic mapped type constraint with optional member doesnt constraint when one of the member exist

2 Answers2