JSFiddle - React, Tailwind, and code Playground

HTML

<select name="CustomerId" id="CustomerId">
<option value="">--Select--</option>
<option value="1">a</option>
<option value="2">b</option>
<option value="3">c</option>
</select>

JavaScript

$('#CustomerId').change(function(){    
    var val = $(this).val();
    var val = 'eed';
    var index = $('option[value="'+val+'"]', this).index();
    alert(index);
});