title: Using variables to build JS Object
body: <p>I want to create an object that has some keys which come from a variable parameter. 
  Let's say for example that prod_id below is a variable containing some value... I want to create an object which has an attribute with key of that 'prod_id' and value of 1.  However this does not work? Is this possible to achieve? if so, how? thanks a heaps</p>
  
  <pre><code>var cart_obj;
    cart_obj = {
    prod_id : 1
  };
  localStorage.setItem("cart", JSON.stringify(cart_obj));
  </code></pre>