impure addToCart() mutates existing cart. https://jsbin.com/dobefu/edit?html,js,output

impure-add-to-cart.js

// impure addToCart mutates existing cart
const addToCart = (cart, item, quantity) => {
  cart.items.push({
    item,
    quantity
  });
  return cart;
};


test('addToCart()', assert => {
  const msg = 'addToCart() should add a new item to the cart.';
  const originalCart =     {
    items: []
  };
  const cart = addToCart(
    originalCart,
    {
      name: "Digital SLR Camera",
      price: '1495'
    },
    1
  );

  const expected = 1; // num items in cart
  const actual = cart.items.length;

  assert.equal(actual, expected, msg);

  assert.deepEqual(originalCart, cart, 'mutates original cart.');
  assert.end();
});