It's pretty easy to do in Typescript and loses no type safety (without needing to assert or cast anything)

    type T = { [k: string]: number };
    
    const t: T = {
      one: 1,
      two: 2,
      three: 3,
    };
    
    const makeUpperCaseKeys = (v: T): T => {
      const keys = Object.keys(v);
      return keys.reduce((p, c) => {
        const key = c.charAt(0).toUpperCase() + c.slice(1);
        return { ...p, [key]: v[c] };
      }, {});
    };
    
    console.log(makeUpperCaseKeys(t));
    // {
    //   One: 1,
    //   Two: 2,
    //   Three: 3
    // }