[Typescript] Approaches for Typing Object Parameters

Consider this implementation of returnBothOfWhatIPassIn:

const returnBothOfWhatIPassIn = (params: { a: unknown; b: unknown }) => {
  return {
    first: params.a,
    second: params.b,
  };
};

This time the function takes in a params object that includes a and b.

import { expect, it } from 'vitest';
import { Equal, Expect } from '../helpers/type-utils';

const returnBothOfWhatIPassIn = <T1, T2>(params: { a: T1; b: T2 }) => {
  return {
    first: params.a,
    second: params.b,
  };
};

it('Should return an object where a -> first and b -> second', () => {
  const result = returnBothOfWhatIPassIn({
    a: 'a',
    b: 1,
  });

  expect(result).toEqual({
    first: 'a',
    second: 1,
  });

  type test1 = Expect<
    Equal<
      typeof result,
      {
        first: string;
        second: number;
      }
    >
  >;
});

So the solution 1, we use:

const returnBothOfWhatIPassIn = <T1, T2>(params: { a: T1; b: T2 }) => {
  return {
    first: params.a,
    second: params.b,
  };
};

Solution 2:

interface Params<T1, T2> {
  a: T1;
  b: T2;
}

const returnBothOfWhatIPassIn = <T1, T2>(params: Params<T1, T2>) => {
  return {
    first: params.a,
    second: params.b,
  };
};