Laravel Ajax购物车在数量变化时动态更改小计

我试图在更改数量时动态更改小计，但失败了，我是Ajax新手，我似乎无法理解如何实现这一点，如果有人帮助，我将非常感激

Controller:

public  function quantity(Request $request){
$carts=Cart::all();
$carts->quantity=$request->quantity;
$carts->save();
return response()->json([
    'quantity' => $carts->quantity,
]);
}

刀片：

html:

web.php

Route::get('/checkout/qty','CheckoutController@quantity')->name('checkout.qty');

手推车数据库：

 Schema::create('carts', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned();
            $table->integer('product_id')->unsigned();
            $table->timestamps();
        });

Route::get('/checkout/qty','CheckoutController@quantity')->name('checkout.qty');

 Schema::create('carts', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned();
            $table->integer('product_id')->unsigned();
            $table->timestamps();
        });