Array key error in Laravel 8 array_key_exists() - arrays

Question

I am getting array_key_exists() expects parameter 2 to be array, null given
in the following code. I know it's because $this->items; is null but I am not sure how to fix it.
Warning: array_key_exists() expects parameter 2 to be array, null given
public function __construct($cart = null)
{
if ($cart) {
$this->items = $cart->items;
$this->totalPrice = $cart->totalPrice;
$this->totalQty = $cart->totalQty;
} else {
$this->items = [];
$this->totalQty = 0;
$this->totalPrice = 0;
}
}
public function add($product)
{
$item = [
'id' => $product->id,
'name' => $product->name,
'price' => $product->price,
'qty' => 0,
'image' => $product->image
];
if (!array_key_exists($product->id, $this->items)) {
$this->items[$product->id] = $item;
$this->totalQty += 1;
$this->totalPrice += $product->price;
} else {
$this->totalQty += 1;
$this->totalPrice += $product->price;
}
$this->items[$product->id]['qty'] += 1;
}

Answer 1

It would be helpful to see the full class and your usage of the add() function.
If $this->items is null, I would expect that the $cart object you are passing into the constructor has null items.
Let's assume this is your whole class;
class Cart
{
public function __construct($cart = null)
{
if ($cart) {
$this->items = $cart->items;
$this->totalPrice = $cart->totalPrice;
$this->totalQty = $cart->totalQty;
} else {
$this->items = [];
$this->totalQty = 0;
$this->totalPrice = 0;
}
}
public function add($product)
{
$item = [
'id' => $product->id,
'name' => $product->name,
'price' => $product->price,
'qty' => 0,
'image' => $product->image,
];
if (!array_key_exists($product->id, $this->items)) {
$this->items[$product->id] = $item;
$this->totalQty += 1;
$this->totalPrice += $product->price;
} else {
$this->totalQty += 1;
$this->totalPrice += $product->price;
}
$this->items[$product->id]['qty'] += 1;
}
}
If you consider the snippet below, this code will give the same error you are experiencing.
$product = (object) [
'id' => 1,
'name' => 'product',
'price' => 100,
'qty' => 0,
'image' => 'image.jpg',
];
$existingCart = (object) [
'items' => null, // this is most likely your issue
'totalPrice' => 1000,
'totalQty' => 1,
];
$cart = new Cart($existingCart);
$cart->add($product); // TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given
If you have items in the $cart->items property, the error won't be thrown;
$product = (object) [
'id' => 1,
'name' => 'product',
'price' => 100,
'qty' => 0,
'image' => 'image.jpg',
];
$existingCart = (object) [
'items' => [
'product-1' => $product, // we actually have an array here of products. An empty array would also be fine
],
'totalPrice' => 1000,
'totalQty' => 1,
];
$cart = new Cart($existingCart);
$cart->add($product); // null
Double check your constructor input by using dd() or print_r() in your constructor to check the $cart argument.