Cannot read property 'storageKey' of undefined

Question

I have a shopping cart and I only want the products to be set if they don't already exist in localStorage. If the product is already in localStorage, then no new product should be added. Later I will update the quantity if the product already exists in localStorage but I haven't reached that step yet. When I try to set the item when the id isn't in localStorage, I get the TypeError: Cannot read property 'storageKey' of undefined

 

storageKey = 'MyDataStorageKey';

public onSubmit(id, quantity, product_name){

var data = {
id,
quantity,
product_name,
};
var retrieverObject = localStorage.getItem('items');
var retrieveObject = JSON.parse(retrieverObject);
this.items.push(data);

retrieveObject.forEach(function(el){
alert("This is the storage id " + el.id);
if (el.id == id){
alert(el.id + " same product");
} else {
alert("this id is not in storage ");
//error returns for the following line of code
localStorage.setItem(this.storageKey, JSON.stringify(this.items));
}
});
}