What is the difference between `x instanceof Function` and `typeof x === 'function`

I want to create this hook in React (ts)

import { useState, useDeferredValue, useEffect } from "react";type InitialValue<T> = T | (() => T);function getStoredValue<T>(key: string, initialValue: InitialValue<T>) {  const savedValue = localStorage.getItem(key);  if (savedValue) return JSON.parse(savedValue) as T;  // return initialValue instanceof Function ? initialValue() : initialValue;  return typeof initialValue === "function" ? initialValue() : initialValue;}export default function useLocalStorage<T>(key: string, _default_value: InitialValue<T>) {  const [value, setValue] = useState(() => getStoredValue(key, _default_value));  const deferredValue = useDeferredValue(value);  useEffect(() => {    if (deferredValue) localStorage.setItem(key, JSON.stringify(deferredValue));    console.log("saved");  }, [deferredValue]);  return [value, setValue] as const;}

how can i check for the type of initialValue in getStoredValue function

if i use initialValue instanceof Function it works

but if i use typeof initialValue === "function" i get this red-underline under initialValue() that says:

This expression is not callable.  Not all constituents of type '(() => T) | (T & Function)' are callable.    Type 'T & Function' has no call signatures.ts(2349)

i think to use typeof is better but why i get such a message from typescript