How to get the previous selected value from a dropdown before onchange

You could include a hidden input field on your page

<input type="hidden" id="previous_value" name"previous_value" value="abc"/>

Then in your script you could access as any other input:

var previous = $('#previous_value').val();

Set it after an onchange event occurs:

$('#statustab_{{ $row['placementkey'] }}').addEventListener('change', function (e) {
    $('$previous_value').val(e.value)
}) 

1. Use the data-* to store the old value
2. Get the data-* as the old value and the selected option as new value

$('select').change(function(){
var oldvar = $(this).attr('data-old') !== typeof undefined? $(this).attr('data-old') :"";
var newval = $("option:selected",this).text();
$(this).attr('data-old',newval)
console.log("prev value :" + oldvar)
console.log("curr value :" + newval)
}).change();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select>
<option>1</option>
<option>2</option>
<option>3</option>
</select>

Save the original value using data() when the element gets focus:

$('.select2-selecting').on('focusin', function(){
    console.log("Saving value " + $(this).val());
    $(this).data('val', $(this).val());
});

And then get the saved old value in your onchange function:

function listingByStatus(ths,key){
         var thisSelect = $('#statustab_'+key).text();
         var statusText = $( "#statustab_"+key+" option:selected" ).text();
         var currentval = $( "#statustab_"+key+" option:selected" ).val();
         var prev = $('.select2-selecting').data('val'); //old value
         console.log("Old value " + prev);
}

Try below code. Hope it will help you.

$('select').change(function(){
  var oldval = $(this).prop('data-old') !== undefined ? $(this).prop('data-old') : "";
  var newval = $("option:selected",this).text();
  console.log("previous value : " + oldval);
  console.log("current value : " + newval);
  $(this).prop('data-old',newval);
}).change();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select>
  <option>Apple</option>
  <option>Mango</option>
  <option>Banana</option>
  <option>Orange</option>
</select>