const add = () => {
  const cache = {}
  return num => {
    if (num in cache) {
      return `Из кеша! ${cache[num]}`
    } else {
      const result = num + 10
      cache[num] = result
      return `Вычислено! ${result}`
    }
  }
}

const addFunction = add()
console.log(addFunction(10))
console.log(addFunction(10))
console.log(addFunction(5 * 2))