expense_form.js

function stripCommas(input) {
  return input.replace(/,/, '');
}

function ExpenseForm($el) {
  $el.on('submit', clean)
  
  function clean() {
    $el.find('input[type=text]').each(function (index, field) {
      field.value = stripCommas(field.value);
    });
  }
}

new ExpenseForm($('#expense-form'));

//vs:

$('#expense-form').on("submit", function (e) {
    $(this).find('input[type=text]').each(function () {
      this.value = this.value.replace(/,/, '')
    });
});