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在亚历山大的评论之后，您必须修改解决方案

function isInCart(id){
 var items = JSON.parse(localStorage.getItem('actualCart'));
 return !!items.filter(i=>i.id == id).length;
}
$(document).ready(function(){
    var actualCartArray = []; //array of cart Items data
    var cartArray = []; //array of cart Items html
    var total = 0;
    var shipping = 2;

    $(".addToCart").click(function(){
      if(isInCart($(this).data("id"))){
        return;
      }
               var flag = 0;
        //Get all data of item to be added
            var id= $(this).data("id");
            var price= $(this).data("price");
            var image = $(this).data('image');
            var name = $(this).data("name");
            var quantity = $(this).data("quantity");
            var availability = $(this).data("availability");
            var weight = $(this).data("weight");
           actualCartArray.push({id,price,image,name,quantity,availability,weight});

            //check if cart is empty to remove empty cart view
               if( (cartArray.length === 0) ) { 
                $("#empty-cart").css("display","none");}

        //Traverse the cart list to check if item already exists or not
             for(var i = 0; i < cartArray.length; ++i){
                 if (//what to do here to match id){
                 var increment= $( this ).find(".quantity").val();
                  increment++;
                  flag = 1;
                  $( this ).find(".quantity").val(increment);
                    return false;
                   }
                 }

          //Add new Item in cart
              if(flag == 0){
                  var cartItem =  "<li class='newItem'><p class='id'>"+id+"</p><div class='picture'><img src='"+image+"' ></div> <div class='details'><p class='name'>"+name+"</p><span class='weight'>"+weight+"</span><span> /</span> <span class='avail'>"+availability+"</span><p class='price'>"+price+"</p><a type='button' class='inc'>+</a><input type='text' class='quantity' value="+quantity+" ><a type='button' class='dec'>-</a></div> <a type='button' class='removeItem'><i class='fa fa-trash'></i></a></li>"
                     cartArray.push(cartItem);
                     localStorage.setItem('cartItem',JSON.stringify(cartArray));
localStorage.setItem('actualCart',JSON.stringify(actualCartArray));  
                      var getItems = JSON.parse(localStorage.getItem('cartItem'));
                     // $("#cartList").prepend(cartItem);
                    $("#cartList li .id").css('display','none');
                      console.log("Item Added")
                   //increment in badge icon number when new item added in cart
                      var counterInc = parseInt($(".badge").html()) + 1;
                        $(".badge").html(counterInc);
                 }
})

创建并使用函数，如下所示。如果项目是否在 localStorage 购物车中，则传递一个 id 并返回布尔值。

function isInCart(id){
 var items = JSON.parse(localStorage.getItem('cartItem'));
 return !!items.filter(i=>i.id == id).length;
}