Changing what was handed to you

What it is for

React not re-rendering, a 'copy' that changed the original, a shared config quietly edited by one module: all this.

How to think about it

Does the caller expect this unchanged? Treat parameters as read-only. If you need a changed version, build a new one: [...arr], {...obj}, arr.map(...).

Worked example

function addTax(cart) {

  return cart.map(i => ({ ...i, total: i.price * 1.2 }));

}

const taxed = addTax(cart);

Your turn

Return a copy with one item added, without changing the input.

function withItem(list, item) {
  return [list, item];
}

Check it