Get custom attribute value from multiple select option using jQuery

I want to get a custom attribute value from multiple select.

Here is my HTML code:

<select multiple name="tax[]" id="tax_classes" onChange="getSelectedOptions(this)">  
   <option value="2" data-percentage="9.00">CGST(9.00%)</option>  
   <option value="1" data-percentage="18.00">GST(18.00%)</option> 
   <option value="3" data-percentage="9.00">IGST(9.00%)</option> 
   <option value="4" data-percentage="12.00">Tax(12.00%)</option> 
</select>

From above tag, I wanted to get the data-percentage attribute value.

Here is the script what I tried so far:

function getSelectedOptions(tax) 
{
  var options = [], option;
  var len = tax.options.length;
  for (var i = 0; i < len; i++) {
    option = tax.options[i]; 
    if (option.selected) { 
      options.push(option); 
      alert(option.value); 
    }
  }  
} 

I am getting output as 2,1,3,

But my expected output is 9.00, 18.00, 9.00 .

The easiest way to build an array of the data attributes of the selected option elements is to use map().

Also note that inline event handlers are not considered best practice; you should use unobtrusive event handlers instead. As you've already included jQuery in the page, here's an example of how to do that:

$('#tax_classes').on('change', function() {
  var options = $(this).find('option:selected').map(function() {
    return $(this).data('percentage');
  }).get();
  console.log(options);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select multiple name="tax[]" id="tax_classes">
  <option value="2" data-percentage="9.00">CGST(9.00%)</option>
  <option value="1" data-percentage="18.00">GST(18.00%)</option>
  <option value="3" data-percentage="9.00">IGST(9.00%)</option>
  <option value="4" data-percentage="12.00">Tax(12.00%)</option>
</select>

If you want to stick with plain JS, here's how to do that:

document.querySelector('#tax_classes').addEventListener('change', function() {
  var options = Array.from(this.options).filter(o => o.selected).map(o => o.dataset.percentage);
  console.log(options);
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select multiple name="tax[]" id="tax_classes">
  <option value="2" data-percentage="9.00">CGST(9.00%)</option>
  <option value="1" data-percentage="18.00">GST(18.00%)</option>
  <option value="3" data-percentage="9.00">IGST(9.00%)</option>
  <option value="4" data-percentage="12.00">Tax(12.00%)</option>
</select>