TypeScript type for reduce

If you want the accumulator value to be indexable by string, Record<string, string> should do the trick. You can pass this as the type argument to reduce

interface MediaQueryProps {
  [key: string]: number;
}

const size: MediaQueryProps = {
  small: 576,
  medium: 768,
  large: 992,
  extra: 1200
};

export default Object.keys(size).reduce<Record<string, string>>((acc, cur) => {
  acc[cur] = `(min-width: ${size[cur]}px)`;
  return acc;
}, {});

Playground link