Uncaught (in promise) cancel using SweetAlert2

Update (Jan 2017): This issue has been fixed in v7: v7 upgrade guide ↗

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You need to add a rejection handler to the Promise. Alternatively, you can use .catch(swal.noop) as a quick way to simply suppress the errors:

swal('...')
  .catch(swal.noop);

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PS. the package you're using is called SweetAlert2, not SweetAlert. In future questions please mention it so you can get more relevant answers.

you will need to catch the action for cancel

swal({
  title: 'Are you sure?',
  text: "You won't be able to revert this!",
  type: 'warning',
  showCancelButton: true,
  confirmButtonColor: '#3085d6',
  cancelButtonColor: '#d33',
  confirmButtonText: 'Yes, delete it!'
}).then(function(json_data) {
  //delete item
}, function(dismiss) {
  if (dismiss === 'cancel' || dismiss === 'close') {
    // ignore
  } 
})

SweetAlert2 rejects the result promise when the cancel button is pressed. You can handle that:

swal({
  …
}).then(function(json_data) {
  …
}, function(dismiss) {
  if (dismiss === 'cancel') { // you might also handle 'close' or 'timer' if you used those
    // ignore
  } else {
    throw dismiss;
  }
})

If you don't need to do anything with the json_data, you might also use the catch method.

new Promise(function(resolve, reject) { is not necessary. $.post() returns a jQuery promise object.

Possible solution substitutes Promise.reject() for new Promise() constructor; removed .then() that was placed as an option to first swal() call; pattern appears to expect a Promise to be returned from preConfirm, though not certain what value is expected to be returned from .done() other than json_data.

swal({
  title: "Confirm",
  input: 'checkbox',
  inputValue: 0,
  type: "warning",
  inputPlaceholder: 'I agree to <a href="#blahblahMore"></a>',
  showCancelButton: true,
  confirmButtonText: 'Confirm',
  showLoaderOnConfirm: true,
  preConfirm: function(result) {
      if (result) {
        return $.post("my.php", {
          Data: data
        })
        .done(
          function(json_data) {
            var data_array = $.parseJSON(json_data);
            var moreDetails = '';
            var resulting = 'error';
            var details = "Transaction Declined"
            if (data_array["trxApproved"] == true) {
              resulting = 'success';
              details = "Confirmed"
              moreDetails = "<br>Approved<b>" + data_array["approved"] + "</b>" +
                "<br>Details Code: <b>" + data_array["detailsCode"] + "</b>";
            }
            swal({
              type: resulting,
              title: details,
              html: "<h1>Details: </h1>" + data_array["messagetext"] + moreDetails
            });
          }
        );
      } else {
          return Promise.reject('You must agree to our Terms & Conditions ');
      }
  },
  allowOutsideClick: false
});