The difference in syntax for updating an object item

What is the difference between:

 cartItem = {...cartItem, amount: newAmount}

and

cartItem.amount = newAmount

Answer

The first is an assignment to cartItem, while the second is a mutation of the object held by cartItem.

The first creates a new object. The previous value of cartItem referenced an object that could still be referenced by another reference. Demo:

let newAmount = 13;
let cartItem = { name: "cart", amount: 42 };
let myRef = cartItem;
cartItem = {...cartItem, amount: newAmount};
console.log(myRef); // still the old object

// Let's do this again, with the other technique
cartItem = { name: "cart" };
myRef = cartItem;
cartItem.amount = newAmount;
console.log(myRef); // the mutated object

So there is a difference which can be noticeable when you have another reference to the original object.

This other reference, could be a variable of the caller of a function:

function assignment(cartItem, newAmount) {
    cartItem =  {...cartItem, amount: newAmount};
}

function mutation(cartItem, newAmount) {
    cartItem.amount =  newAmount;
}

// scenario 1
let cartItem = { name: "cart", amount: 42 };
assignment(cartItem, 13);
console.log(cartItem); // Has not changed -- "old" object

// scenario 2
mutation(cartItem, 13);
console.log(cartItem); // Has changed -- "old" object has mutated!