Sample type defs for typescript showing how to constrain function parameter combinations.

param_combos.ts

type Payload = Partial<{ [Key in AcceptableCombinations[1]]: number }>;

type EventObject = {
  type: AcceptableCombinations[0];
  payload: Payload;
}

type AcceptableCombinations = 
  | [...[type: "a", key: "x"]] 
  | [...[type: "b", key: "y"]];

function otherThing(...[type, key]: AcceptableCombinations) {
  const payload: Payload = {[key]: key};
  const pp: Payload = {[key]: 7};
  const pq: Payload = {[key]: "x"};
  thing({ type, payload });
}

function thing(obj: EventObject) {
  console.log(JSON.stringify(obj, null, 2));
}

otherThing("a", "x");
otherThing('b', "y");

// @ts-expect-error Invalid second argument given first argument
otherThing('a', "y");
// @ts-expect-error Invalid second argument given first argument
otherThing("b", "x");
// @ts-expect-error `y` is not assignable to type `never` - `"c"` is an invalid first argument.
otherThing("c", "y");
// @ts-expect-error `{z: "z"}` is not assignable to type `{x: "x"}`
otherThing("a", "z");

type Foo = {
  x: number;
  y: string;
}

type Bar = Pick<Foo, "x">;