$(document).ready(function () {
  $('#id_previous_postcode').change(function () {
    var postcode = $(this).val()
    console.log(postcode)
    $.ajax({
      // could prevent url hard coding by using url tag in form widget attribute and assign url key here to that attr
      url: '/users/prior_addresses_ajax/',
      data: {
        'postcode': postcode
      },
      dataType: 'json',
      success: function (data) {
        var select = document.querySelector('#id_previous_full_address');
        console.log(data)
        data.property_choices.forEach(function (subarray) {
          var option = new Option(subarray[1], subarray[0]);
          select.options.add(option);
        });
      }
    })
  })
})