how to sum 2 numbers in javascript before saving it in mongodb database

Question

I'm building a web app using MERN stack. I want to sum 2 numbers then save it mongodb database. Here current num is from database and other is from input field. Here it work by string, but I want it in number type for example: I want : 2 + 2 = 4, but this code work : 2 + 2 = 22

const Inventory = () => {
    const {id}= useParams()
    const [product, setProduct]=useState({})
    const {name, price,quantity, img,descreption,suplier}=product;
    

     useEffect(()=>{
        fetch(`http://localhost:5000/product/${id}`)
        .then(res=>res.json())
        .then(data=> parseInt(setProduct(data)) )
    },[product])

const handleFormQuantity=(event)=>{
       event.preventDefault()
       const currentQuantity = product.quantity;
       console.log(currentQuantity)
      const quantity = parseInt(currentQuantity + event.target.quantity.value)
      const data = {quantity}
      const url = `http://localhost:5000/product/${id}`
      fetch(url,{
          method:'PUT',
          headers:{
            'Content-Type':'application/json'
          },
          body:JSON.stringify(data)
      })
      .then(res=>res.json())
      .then(data=>{
        //   console.log(data)
          alert('user update make succeessfully')
          event.target.reset();
      })
   }
  

             <form onSubmit={handleFormQuantity}>
                <input type="number" name='quantity' placeholder='add item quantity' />
                <input type="submit" value="SUBMIT" />
            </form>