title: Can you add reactive values to a custom store
body: <p>If in your store you have  an object</p>
  <pre><code>
  [{
  amount: 0, 
  fee: 5, 
  }]
  </code></pre>
  <p>but you have multiple components that need a total value:</p>
  <p>do I have to add the following to each component, I want to use the total value</p>
  <pre><code>$: total = $store.amount + $store.fee
  
  </code></pre>
  <p>or is there is a way to write reactive equations in the store</p>
  <p>This is the properly example of the store</p>
  <pre><code>import { writable } from 'svelte/store';
  
  let donation = {
      amount: 50, 
          fee: 10,
      continue: false,
      
      
  };
  
  const createStore = () =&gt; {
      const { update, subscribe, set } = writable(donation);
  
      return {
          subscribe,
          update,
          set,
          handleContinue: () =&gt;
              update((current) =&gt; {
                  current.continue = true;
                  return current;
              }),
      };
  };
  
  export const donationCheckoutStore = createStore();
  
  </code></pre>