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$(document).ready(function() {
  $(".amount_field, amount_field2").keyup(function() {
    convertToINRFormat($(this).val(),$(this));
  });
  convertToINRFormat($("#amount_field").val(),$("#amount_field"));
  convertToINRFormat($("#amount_field2").val(),$("#amount_field2"));
});

function convertToINRFormat(value, inputField) {
  var number = Number(value.replace(/,/g, ""));
  // India uses thousands/lakh/crore separators
  $('#result').html(number.toLocaleString('en-IN'));

  $('#result1').html(number.toLocaleString('en-IN', {
    maximumFractionDigits: 2,
    style: 'currency',
    currency: 'INR'
  }));

  $(inputField).val(number.toLocaleString('en-IN'));
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input type="text" name="amount" class="amount_field" id="amount_field" value="123456789" />
<input type="text" name="amount" class="amount_field" id="amount_field2" value="987654321" />

<div id="result"></div>
<div id="result1"></div>