example.js

get('hello')(new Map([['hello', 'world']]))
get('hello')({hello: 'world'})
get('hello')({goodbye: 'world'})
get(0)(['hello world'])
get(0)('hello world')
get(0)(null)
get(0)(undefined)

get.ts

export default function get (name: unknown) {
  return function getProperty (keyedFunctor: unknown): null | unknown {
    if (keyedFunctor === undefined || keyedFunctor === null) {
      return null;
    }

    if (typeof keyedFunctor === "object" && "get" in keyedFunctor && typeof keyedFunctor.get === "function") {
      return keyedFunctor.get(name);
    }

    if (typeof keyedFunctor === "object" && (typeof name === "number" || typeof name === "string") && name in keyedFunctor) {
      return keyedFunctor[name];
    }

    return null;
  };
}

output.log

index.ts:9:90 - error TS2339: Property 'get' does not exist on type 'object'.

9     if (typeof keyedFunctor === "object" && "get" in keyedFunctor && typeof keyedFunctor.get === "function") {
                                                                                           ~~~

index.ts:10:27 - error TS2339: Property 'get' does not exist on type 'object'.

10       return keyedFunctor.get(name);
                             ~~~

index.ts:14:14 - error TS7053: Element implicitly has an 'any' type because expression of type 'string | number' can't be used to index type '{}'.
  No index signature with a parameter of type 'string' was found on type '{}'.

14       return keyedFunctor[name];
                ~~~~~~~~~~~~~~~~~~


Found 3 errors.