var metas = document.getElementsByTagName('meta');
var i;
if (navigator.userAgent.match(/iPhone/i)) {
  for (i = 0; i < metas.length; i++) {
    if (metas[i].name == 'viewport') {
      metas[i].content =
        'width=device-width, minimum-scale=1.0, maximum-scale=1.0';
    }
  }
  document.addEventListener('gesturestart', gestureStart, false);
}

function gestureStart() {
  for (i = 0; i < metas.length; i++) {
    if (metas[i].name == 'viewport') {
      metas[i].content =
        'width=device-width, minimum-scale=0.25, maximum-scale=1.6';
    }
  }
}