TextArea AutoResizer (Bootstrap jQuery/Zepto Plugin)

autoresizer.js

var AutoResizer = function (textArea, options) {
  var self = this;

  this.$textArea = $(textArea);
  this.minHeight = this.$textArea.height();

  this.options = $.extend({}, $.fn.autoResizer.defaults, options)

  this.$shadowArea = $('<div></div>').css({
      position: 'absolute',
      top: -10000,
      left: -10000,
      fontSize: this.$textArea.css('fontSize') || 'inherit',
      fontFamily: this.$textArea.css('fontFamily') || 'inherit',
      lineHeight: this.$textArea.css('lineHeight') || 'inherit',
      resize: 'none'
    }).appendTo(document.body);

  var startWidth = this.$textArea.width() || $(window).width();
  this.$shadowArea.width(startWidth);
  
  if (this.options.resizeOnChange) {
    function onChange() {
      window.setTimeout(function() {
        self.checkResize();
      }, 0);
    }
    this.$textArea.change(onChange).keyup(onChange).keydown(onChange).focus(onChange);
  }

  this.checkResize();
};

 AutoResizer.prototype = {
  constructor: AutoResizer,
  checkResize: function() {
    // No sense in auto-growing non-visible textarea, which height of 0 implies
    if (this.$textArea.height() === 0) {
      return;
    }
    // If this is first time we've seen text area rendered, remember the height
    if (this.minHeight === 0) {
      this.minHeight = this.$textArea.height();
    }
    // If the text area has changed in size past a certain threshold of difference
    // like when it becomes visible or viewport changes
    if (this.$textArea.width() !== 0 && Math.abs(this.$shadowArea.width() - this.$textArea.width()) > 20) {
      this.$shadowArea.width(this.$textArea.width());
    }

    var val = this.$textArea.val().replace(/</g, '&lt;')
            .replace(/>/g, '&gt;')
            .replace(/&/g, '&amp;')
            .replace(/\n/g, '<br/>&nbsp;');

    if ($.trim(val) === '') {
        val = 'a';
    }
    this.$shadowArea.html(val);
    var nextHeight =  Math.max(this.$shadowArea[0].offsetHeight + 10, this.minHeight);
    if (!this.prevHeight || nextHeight != this.prevHeight) {
      this.$textArea.css('height', nextHeight);
      this.prevHeight = nextHeight;
    }
  }
};


$.fn.autoResizer = function ( option ) {
  return this.each(function () {
    var $this = $(this)
      , data = $this.data('autoresizer')
      , options = typeof option == 'object' && option
    if (!data) $this.data('autoresizer', (data = new AutoResizer(this, options)))
    if (typeof option == 'string') data[option]()
    else data.checkResize()
  })
}

$.fn.autoResizer.defaults = {
  resizeOnChange: true
};

$.fn.autoResizer.Constructor = AutoResizer;

Right now the shadow doesn't mimic the padding of the text area, so I imagine the magic number has to do with that. It was good enough for me, but you should try displaying both shadow and text area at same time and you'll see how they differ (and then you can mirror a few more properties over).

I happened to notice this commit via the whole svbtle/obtvse thing:
natew/obtvse@648e9a1

 textarea.onkeydown = function() {
    textarea.style.height = ""; /* Reset the height*/
    textarea.style.height = textarea.scrollHeight + "px";
};

Could it really be that simple?

Here's a jsfiddle: http://jsfiddle.net/lerouxb/vRSP2/

Apart from being simpler it also has the benefit of not momentarily flashing a scrollbar. It isn't very smart about sizing back down again, though.

There are some issues with backspace.

There are some issues with backspace.

On Sat, Mar 24, 2012 at 3:19 PM, Le Roux Bodenstein < ***@***.*** > wrote: Here's a jsfiddle: http://jsfiddle.net/lerouxb/vRSP2/ Apart from being simpler it also has the benefit of not momentarily flashing a scrollbar. It isn't very smart about sizing back down again, though. --- Reply to this email directly or view it on GitHub: https://gist.github.com/2156602

## Cordialement Djordje Lukic

Ok, sorry for the double post :)

and now? http://jsfiddle.net/lerouxb/vRSP2/

Why dose the content jump as you continue to press enter? Tried it on facebook post comment textarea and it was seamless.