title: How can I add elements to arrays without overwriting them?
body: <p>I have an issue with saving items into an array in JavaScript/Next.js.</p>
  <p>At the beginning I have an empty array. Then a session is started, and I check if the session is empty or not. If it is empty, I write a response of a get-request into a variable. At the same time I also write the response of the get request into localstorage.</p>
  <p>Basically I now want to write the value of localstorage into the array with the push-method.</p>
  <p>In the next step my program should realize, that the session is not empty anymore and now the new value of the get-request should be stored into localstorage and at the same time it should be added as a new element of the variable. The old elements of the array should not be overwritten. I have now tried a couple of things, but I could not get it to work.</p>
  <p>Does anybody have a hint for me, how to realize this problem? I would be very thankful, if anybody could help me on this one </p>
  <pre class="lang-js prettyprint-override"><code>api.get('products?include=' + mainId, addOrViewCartConfig).then((response) =&gt; {
    if (isEmpty(storedSession)) {
      storeSession(response.data[0].id);
      localStorage.setItem('ID', response.data[0].id);
      var localCart = [];
    };
  
    if (!localCart.indexOf(parseInt(localStorage.getItem('ID')))) {
      localCart.push(parseInt(localStorage.getItem('ID')));
    } else {
      alert('The item you are trying to add is already in your cart!');
      return false;
    }
  
    viewCart();
  }).catch((error) =&gt; {
    console.log('apiError');
  });
  </code></pre>