Add a sort by dropdown in Shopify (the right way). See https://cfxdesign.com/add-a-sort-by-dropdown-in-shopify-the-right-way

sortby-es6.js

// Save existing sort parameters
Shopify.queryParams = {};
if(location.search.length) {
  for(var aKeyValue, i = 0, aCouples = location.search.substr(1).split('&'); i < aCouples.length; i++) {
    aKeyValue = aCouples[i].split('=');
    if (aKeyValue.length > 1) {
      Shopify.queryParams[decodeURIComponent(aKeyValue[0])] = decodeURIComponent(aKeyValue[1]);
    }
  }
}

// Add existing sort parameters to current URL (ES6)
document.querySelector('.sort-by').addEventListener('change', (e) => {
  const { value } = e.currentTarget;
  Shopify.queryParams.sort_by = value;
  location.search = new URLSearchParams(Shopify.queryParams).toString();
});

sortby-tags.html

<script type="text/javascript">
  // Save existing sort parameters
  Shopify.queryParams = {};
  if(location.search.length) {
    for(var aKeyValue, i = 0, aCouples = location.search.substr(1).split('&'); i < aCouples.length; i++) {
      aKeyValue = aCouples[i].split('=');
      if (aKeyValue.length > 1) {
        Shopify.queryParams[decodeURIComponent(aKeyValue[0])] = decodeURIComponent(aKeyValue[1]);
      }
    }
  }

  // Add existing sort parameters to current URL
  document.querySelector('.sort-by').addEventListener('change', function(e) {
    var value = e.currentTarget.value;
    Shopify.queryParams.sort_by = value;
    location.search = new URLSearchParams(Shopify.queryParams).toString();
  });
</script>

sortby.js

// Save existing sort parameters
Shopify.queryParams = {};
if(location.search.length) {
  for(var aKeyValue, i = 0, aCouples = location.search.substr(1).split('&'); i < aCouples.length; i++) {
    aKeyValue = aCouples[i].split('=');
    if (aKeyValue.length > 1) {
      Shopify.queryParams[decodeURIComponent(aKeyValue[0])] = decodeURIComponent(aKeyValue[1]);
    }
  }
}

// Add existing sort parameters to current URL
document.querySelector('.sort-by').addEventListener('change', function(e) {
  var value = e.currentTarget.value;
  Shopify.queryParams.sort_by = value;
  location.search = new URLSearchParams(Shopify.queryParams).toString();
});

sortby.liquid

<div class="sort-by--container">
  <label for="sort-by">Sort by</label> 
  <select id="sort-by" class="sort-by">
    {% for option in collection.sort_options %}
      <option value="{{ option.value }}"{% if option.value == collection.sort_by %} selected{% endif %}>{{ option.name }}</option>
    {% endfor %}
  </select>
</div>