Increase qty by 1 if item already exist in cart – Reactjs/NextJS

I have found many questions similar to the subject, however, I couldn’t understand the logic, as most of these are asked/explained in the PHP/ MySQL community.

I’m building an eCommerce store, where I need to handle cart items. The problem is already mentioned in the subject. Below is a short snippet of my code.

const [cartItems, setCartItems] = useState([]) // initial state of cartItems array
const handleCartUpdate = (id,title, price, qty) => {
  // first checking if  cart is empty or not
  const cartLength = cartItems.legnth >=1 ? true : false
  // Check if item exist with the id as given in parameters
  if(cartLength) {
    const checkItemExist = cartItems.find(item => == id)
    // Now, if item exist, update the propert 'qty' , My approach is as under:
    if (checkItemExist) { => {
        if( == id) {
          return {product, qty: qty+1
      return product
      setCartItems(...cartItems, product)
  } else {
    setCartItems(...cartItems, {...})

After that, I am unable to understand how to update the cartItems