function is_nan (val) {
  // http://kevin.vanzonneveld.net
  // +   original by: Onno Marsman
  // +      input by: Robin
  // *     example 1: is_nan(NaN);
  // *     returns 1: true
  // *     example 2: is_nan(0);
  // *     returns 2: false
  var warningType = '';

  if (typeof val === 'number' && isNaN(val)) {
    return true;
  }

  //Some errors for maximum PHP compatibility
  if (typeof val === 'object') {
    warningType = (Object.prototype.toString.call(val) === '[object Array]' ? 'array' : 'object');
  }
  else if (typeof val === 'string' && !val.match(/^[\+\-]?\d/)) {
    //simulate PHP's behaviour: '-9a' doesn't give a warning, but 'a9' does.
    warningType = 'string';
  }
  if (warningType) {
    throw new Error('Warning: is_nan() expects parameter 1 to be double, ' + warningType + ' given');
  }

  return false;
}

is_nan(NaN);

true

is_nan(0);

false