How to check if a item/value already exists in Local Storage
<p>I am working on a shopping cart application were users can click an Add to Cart button that will then add the specific item to local storage. When the user is adding a product to their shopping cart I need to be able to check to see if that specific item/value/product already exists in local storage. If it does, I need to increase only the count of that item/value/product by 1. If not, I need to add an entirely new item/value/product to local storage with a count of 1. </p>

<p>How can I find if an item already exists in local storage and compare it to the id of the current product that a user is attempting to add to their cart? My first few attempts failed miserably and have yet to find anything online that is helping with this issue. Is there a better way of going about this? Any assistance is appreciated. Even a good link to a good page would be extremely helpful. </p>

<p>Below is the code I have to attempt in checking for if the productid being added matches any of the productids in local storage. Basically if the productId that is being added matches the productId of an item in local storage simply add 1 to the quantity. </p>

<pre><code>var retrieverObject = localStorage.getItem('Products');
var retrieveObject = JSON.parse(retrieverObject);

var data = {};
var productId = currentNode.name;
var product = currentNode;

data.productPrice = product.parentNode.previousSibling.previousSibling.id;
data.productId = productId;

var length = retrieveObject.length;
console.log(length);
for(var i = 0; i&lt;length; i++){
  if(retrieveObject[i].productId == data.productId){
    var quantity = retrieveObject[i].quantity;
        retrieveObject.push({"productPrice": data.productPrice, "productId": data.productId, "quantity": quantity++});
  }else{
        retrieveObject.push({"productPrice": data.productPrice, "productId": data.productId, "quantity": 1});
  }
}
console.log(retrieveObject);

localStorage.setItem('Products', JSON.stringify(retrieveObject));
var retrievedObject = localStorage.getItem('Products');
var obj = JSON.parse(retrieverObject);
var len = obj.length;
console.log(len);

  for(var i=0; i&lt;len;i++){
    console.log(obj[i]['productPrice']+", "+obj[i]['productId']);
  }
</code></pre>

<p>}
}</p>

<p>There are a few issues. First, I am not entirely certain that the productId of the retrieved object is being compared to the one that is being added. Secondly, the <code>for(var i = 0; i&lt;length; i++){}</code> definitely does not seem to be doing what was expected and is multiplying the number of items being added by 2. Thirdly, which may relate to the second issue, the retrieveObject.push() is not updating the quantity of the product but is adding an entire new entry to local storage. The given answers did not seem to be working for me before so this is what I have been working on. Any new answers or general help would be great. </p>

<p>PT 2. : So I am having an issue with the first entry into the local storage. Without noting that when there is nothing in local storage and you make a call to get the items in it, it returns null or undefined. So currently I have it set up like this: </p>

<pre><code>if(localStorage.getItem("Products") === null || localStorage.getItem("Products") === undefined){
  var data = {};
  var productId = currentNode.name;
  var product = currentNode;

  data.productPrice = product.parentNode.previousSibling.previousSibling.id;
  data.productId = productId;
  var obj = [];
  obj = obj[data.productId] = {
    productPrice: data.productPrice,
    count: 1
  };
  console.log(obj);
  localStorage.setItem('Products', JSON.stringify(obj));
}

else{
  var retrieverObject = localStorage.getItem('Products');
  var retrieveObject = JSON.parse(retrieverObject);

  var data = {};
  var productId = currentNode.name;
  var product = currentNode;

  data.productPrice = product.parentNode.previousSibling.previousSibling.id;
  data.productId = productId;

  if(retrieveObject[data.productId]){
    retrieveObject[data.productId].count++;
  }else{
    retrieveObject[data.productId] = {
      productPrice: data.productPrice,
      count: 1
};
  }
console.log(retrieveObject);
localStorage.setItem('Products', JSON.stringify(retrieveObject));
}
</code></pre>

<p>This creates a first entry in local storage that looks like this : {"productPrice":"78.34","count":1}, and then when adding others looks like this: {"productPrice":"78.34","count":1,"rJUg4uiGl":{"productPrice":"78.34","count":3}} and works perfectly fine. The issue is getting the first entry to b formatted properly. When I change the code in the first if statement like so: </p>

<pre><code>var obj = [];
      obj[data.productId] = {
        productPrice: data.productPrice,
        count: 1
      }
</code></pre>

<p>I  get an empty [] in local storage but when I console.log the obj it is in the proper format. [rJUg4uiGl: Object]. I have been stuck on this and haven't been able to get it working. Again, any help would be really appreciated.</p>