callMethodWithArgs.ts

// ts 3.x
// https://stackoverflow.com/questions/54129289/typescript-call-a-member-of-an-object-with-arguments
let fns = {
  set: (foo: string, bar: string) => {
    return { foo, bar };
  }
};

function callMethodWithArgs<
FnsObj extends { [key in MethodKey]: (...args: any[]) => any },
MethodKey extends string,
FnInMethodKey extends FnsObj[MethodKey],
>(obj: FnsObj, methodName: MethodKey, args: Parameters<FnInMethodKey>) {
  return obj[methodName](...args) as ReturnType<FnInMethodKey>;
}

const foobar = callMethodWithArgs(fns, "set", ["foo", "bar"]);