Question: I am facing an issue with shopping cart. Unable to increase quantity of existing item in cart or add another item. On button click the addToCart function executes which takes a product.

```
const [cartItems, setCartItems] = useState([])

const addToCart = product => {
    console.log(product) // here I am getting the entire product
    const index = cartItems.findIndex(item => item._id === product._id);

    if (index === -1) {
      const updateCart = cartItems.concat({
        ...product,
        quantity: 1
      });
      setCartItems(updateCart);
    } else {
      const updateCart = [...cartItems];
      updateCart[index].quantity += 1;
      setCartItems(updateCart);
    }
  };

```

I am getting only 1 quantity of product and if I add another product or increase quantity, it overwrites.

Answer: 
Your else logic is wrong. You want to update your item quantity from carItems, before you spread it.
Change:

```
const updateCart = [...cartItems];
updateCart[index].quantity += 1;
setCartItems(updateCart);  

```

to

```
      cartItems[index].quantity += 1;
      const updateCart = [...cartItems];
      setCartItems(updateCart);  

```

Edit: See it in action below:

```js
let cartItems = [{
  _id: "1",
  name: "shoe",
  quantity: 1
}];

const addToCart = product => {
  console.log(product) // here I am getting the entire product
  const index = cartItems.findIndex(item => item._id === product._id);

  if (index === -1) {
    cartItems.push({
      ...product,
      quantity: 1
    });
    const updateCart = [...cartItems];
    console.log(updateCart);
  } else {
    cartItems[index].quantity += 1;
    const updateCart = [...cartItems];
    console.log(updateCart);
  }
};

var product1 = {
  _id: "1",
  name: "shoe"
};
var product2 = {
  _id: "2",
  name: "apple"
};
addToCart(product1);
addToCart(product2);
```
Your code should work, is quite good
====================================

* probably testing method failed
* problems were related to different parts of code

```js
let cartItems = [
  {
_id: "1",
name: "shoe",
quantity: 1
  }
];

console.log("START", JSON.stringify(cartItems), Array.isArray(cartItems));

const addToCart = product => {
  console.log(product); // here I am getting the entire product
  const index = cartItems.findIndex(item => item._id === product._id);

  if (index === -1) {
const updateCart = cartItems.concat({
  ...product,
  quantity: 1
});
cartItems = updateCart;
console.log("ADD", JSON.stringify(cartItems), Array.isArray(cartItems));
  } else {
// original ... just works ...
// const updateCart = [...cartItems];
// updateCart[index].quantity += 1;

// ... this works, too
// cartItems[index].quantity += 1;
// const updateCart = [...cartItems];

// ... but this is safer (in react) as it replaces item with a new object
const updateCart = [...cartItems];
// new object for replace existing one
updateCart[index] = {
  ...updateCart[index],
  quantity: updateCart[index].quantity + 1
};
cartItems = updateCart;

console.log("INC", JSON.stringify(cartItems), Array.isArray(cartItems));
  }
};

var product1 = {
  _id: "1",
  name: "shoe"
};
var product2 = {
  _id: "2",
  name: "apple"
};
addToCart(product1);
addToCart(product2);
addToCart(product1);
console.log("END", JSON.stringify(cartItems), Array.isArray(cartItems));
```

[working example](https://codesandbox.io/s/wild-cherry-to4p8)

Is good enough? Not for react
-----------------------------

Replacing item [-line] in cart with a new object can be required in react when you're rendering cart using components. f.e.: 

```
cartItems.map( item => <CartOrderLine key={item._id} data={item} /> )

```

`<CartOrderLine />` with the same `data` object ref (mutated `quantity` prop only, not replaced) won't be rerendered!