问.change()不起作用，因为页面已加载

EN

jQuery(document).ready(function($){
  $('#location').change(function(){
    let selected_option = $(this).find(":selected").val();
     $('#locationHeading').text(selected_option)
  }).trigger('change') // <== add this
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<h1 id="locationHeading"></h1>

<select id="location">
<option value="A">A</option>
<option value="B">B</option>
</select>

jQuery(function ($) {
    const loc = $('#location');
    const handler = () => {
        let selected_option = loc.find(":selected").val();
        $('#locationHeading').text(selected_option)
    };
    loc.on('change', handler);
    handler();
});