Send alert if item has already been selected

Question

public onSubmit(id, product_name){

var data = {
id,
product_name
};

this.items.push(data);
localStorage.setItem(this.storageKey, JSON.stringify(this.items));
var retrieverObject = localStorage.getItem('this.items.id');
var retrieveObject = JSON.parse(retrieverObject);
if (retrieveObject == id) {
alert('test');
}
}

I want to check if the value has already been submitted. This is part of a shopping cart application in which the product quantity will be updated if the product has already been chosen.

When I use

var retrieverObject = localStorage.getItem('MyDataStorageKey');

it returns:

[{"id":0,"product_name":"Adult Male Bike"},{"id":0,"product_name":"Adult Male Bike"}]

All of the following return null

var retrieverObject = localStorage.getItem('MyDataStorageKey.findIndex(id)');
var retrieverObject = localStorage.getItem('MyDataStorageKey.indexOf("id")');
var retrieverObject = localStorage.getItem('MyDataStorageKey.indexOf(id)');
var retrieverObject = localStorage.getItem('MyDataStorageKey[0].id');
var retrieverObject = localStorage.getItem('MyDataStorageKey[0]');
var retrieverObject = localStorage.getItem('MyDataStorageKey.findIndex("id")');
var retrieverObject = localStorage.getItem('MyDataStorageKey["id"]');
var retrieverObject = localStorage.getItem('MyDataStorageKey.id');