success functions is not working. How to use two types of responses in success function

Question

I used moveuploadfile for uploading files and I have following code in my controller:

$fsize = $_FILES[$key]['size'];
if ($fsize > 2621000) {
  echo "sizeError";
  return;
} else {
  if (move_uploaded_file($_FILES[$key]['tmp_name'], $name)) {
    echo 'Success';
  } else {
    echo 'fail';
  }
}

and in uploadWidget, I have following code for success and error functions:

success: function(response) {
    if (response = 'Success') {
      $('.' + input.id + ' .progress-bar').removeClass('active progress-bar-striped').addClass('progress-bar-success')
      console.log(response);
    } else {
      $('.' + input.id + ' .progress-bar').removeClass('active').addClass('progress-bar-danger');
    }

  },
  error: function(error) {
    $('.' + input.id + ' .progress-bar').removeClass('active').addClass('progress-bar-danger');
    $('.' + input.id + ' .percent').html(error);
    console.log(error)
  }

But, Success function is not working properly. To be more exact, when my controller returns sizeError response, it is loading following code:

if (response = 'Success') {
  $('.' + input.id + ' .progress-bar').removeClass('active progress-bar-striped').addClass('progress-bar-success')
  console.log(response);
}

instead of:

else {
  $('.' + input.id + ' .progress-bar').removeClass('active').addClass('progress-bar-danger');
}

Did I make a mistake? How can I fix this issues?