How can the drop-down selection be maintained after reloading the page using a URL query parameter?

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One way to maintain the drop-down selection after reloading the page using a URL query parameter is to store the selected value in a cookie or local storage.

Here are the steps:

1. When the value in the drop-down is changed, store the selected value in the cookie or local storage.
2. When the page loads, check if there is a value stored in the cookie or local storage.
3. If there is a value stored, set the value of the drop-down to the stored value.
4. If there is not a value stored, set the value of the drop-down using the URL query parameter.

Here is an example of how to accomplish this using JavaScript and jQuery:

// When selection in drop-down changes, store the selected value in cookie
$('#my-dropdown').on('change', function() {
  var selectedValue = $(this).val();
  Cookies.set('selectedValue', selectedValue); // using the JS Cookies library
});

// When the page loads, set the value of the drop-down
$(document).ready(function() {
  var selectedValue = Cookies.get('selectedValue');
  if (selectedValue) {
    $('#my-dropdown').val(selectedValue); // set value using cookie
  } else if ($.urlParam('value')) {
    $('#my-dropdown').val($.urlParam('value')); // set value using URL query parameter
  }
});

// Helper function to get URL query parameter value
$.urlParam = function(name) {
  var results = new RegExp('[\?&]' + name + '=([^&#]*)').exec(window.location.href);
  if (results == null) {
    return null;
  } else {
    return decodeURI(results[1]) || 0;
  }
}