Return type same as input type in TypeScript

Question

I have two functions that do the same thing with the only difference that the input types and return types are different. I was wondering how I could "merge" these functions into one and one of the ideas was to use a union type but my constrain is that when the input is one member of the union the returned value has to be the same.

const getViewStyle = (styles: ViewStyle[]): ViewStyle => {
  return Object.assign({}, ...styles);
};

const getTextStyle = (styles: TextStyle[]): TextStyle => {
  return Object.assign({}, ...styles);
};

Answer 1

You can do this:

const flattenObjectsToSingle = <T>(items: T[]): T => { ...

Answer 2

A mix of generics and inferred types would be the best. The accepted answer would let the linter of typescript keep complaining if you expect one of the types to be returned when you are using the function.

So, in my opinion, this would be the best answer:

const getStyle = <T extends TextStyle | ViewStyle >(styles: T[]): T => { 
      return Object.assign({}, ...styles);
};

What this basicly say is that The function expect an array of TextStyle or ViewStyle and return one record of what ever type is given.

Credit goes to Ben Awad video on YouTube: https://www.youtube.com/watch?v=nViEqpgwxHE