double-dot-negative.js

(function($){

  var onKeyDown = function (e) {
    var $input = $(e.target);
    var lastkey = $input.data("ddn-lastkey");
    var lastval = $input.data("ddn-lastval");
    var key = String.fromCharCode(e.charCode);
    if (lastkey === "." && key === "."){
      $input.val(isNaN(lastval) ? "-0" : String(-1 * lastval));
      $input.data("ddn-lastkey", "");
      e.preventDefault();
    }else{
      $input.data("ddn-lastkey", key);
    }
    
    $input.data("ddn-lastval", parseFloat($input.val()));
  };

  $.fn.doubleDotNegative = function() {
      $(this).each(function(){
        var $input = $(this);
        $input.data("ddn-lastval", parseFloat($input.val())||0);
        $input.keypress(onKeyDown);
      });
  };

})(jQuery);

var isAndroid = /android/i.test(window.navigator.userAgent);
if (isAndroid){
  $("input[type=number]").doubleDotNegative();
}