TypeScript HOF cache

cache.ts

type Fun<A extends any[], B> = (...args:A) => B
type Cache<T> = {[key:string]: T}

function cache<P extends any[], R>(cache: Cache<R>, fn: Fun<P, R>, getKey: Fun<P, string>) {
    return (...args: P) => {
        const key = getKey(...args)
        const saved = cache[key]
        if (!!saved) return saved
        else return cache[key] = fn(...args)
    } 
}

  /* 
  How to use it 
  
  for example we have slow fuction that will be cached on each call
  */
  
  function slowFuntion(id:string, offset:number){
      //implementation
      //return number
  }
  
  /*
  create cached function using function above
  */
  
  const memoryCache:Cache<number> = {}
  
  const slowFunctionCached = cache(memoryCache, slowFunction, (id, offset) => id)
  
  
  const result = slowFunctionCached(<id>, <offset>)