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How do I make this typescript function return strong typed array instead of string[]

January 5, 2023

I have this function that I’m trying to convert from js to ts, I thought that the type of the variable hi will be ('s'|'bb')[] but actually it’s string[]. How can I get typescript to infer K to be the keys of NewOptions?

export const getWithOverrides = <
  K extends string,
  NewOptions extends { [x in K]?: boolean },
  Overrides extends { [x in K]?: boolean }
>(
  newOptions: NewOptions,
  overrides: Overrides
): K[] =>
  (Object.keys(newOptions) as K[]).filter(key => {
    if (typeof overrides[key] === 'boolean') {
      return overrides[key];
    }
    return newOptions[key];
  });

const hi = getWithOverrides({ s: true, bb: true }, { s: true });

>Solution :

You can do that by removing the K type parameter, requiring that the object types extend Record<string, boolean>, and using keyof with your two object types:

export const getWithOverrides = <
    NewOptions extends Record<string, boolean>,
    Overrides extends Record<string, boolean>
>(
    newOptions: NewOptions,
    overrides: Overrides
): (keyof NewOptions | keyof Overrides)[] =>
    Object.keys(newOptions).filter((key) => {
        if (typeof overrides[key] === "boolean") {
            return overrides[key];
        }
        return newOptions[key];
    });

const hi = getWithOverrides({ s: true, bb: true }, { s: true });
//    ^? const hi: ("s" | "bb")[]

Playground example