var resizingTextareas = [].slice.call(document.querySelectorAll('textarea[autoresize]'));

​

resizingTextareas.forEach(function(textarea) {

  textarea.addEventListener('input', autoresize, false);

});

​

function autoresize() {

  this.style.height = 'auto';

  this.style.height = this.scrollHeight+'px';

  this.scrollTop = this.scrollHeight;

  window.scrollTo(window.scrollLeft,(this.scrollTop + this.scrollHeight));

}