Q:

How to autosize a textarea using Prototype?

I'm currently working on an internal sales application for the company I work for, and I've got a form that allows the user to change the delivery address.
Now I think it would look much nicer, if the textarea I'm using for the main address details would just take up the area of the text in it, and automatically resize if the text was changed.
Here's a screenshot of it currently.

Any ideas?

@Chris
A good point, but there are reasons I want it to resize. I want the area it takes up to be the area of the information contained in it. As you can see in the screen shot, if I have a fixed textarea, it takes up a fair wack of vertical space.
I can reduce the font, but I need address to be large and readable. Now I can reduce the size of the text area, but then I have problems with people who have an address line that takes 3 or 4 (one takes 5) lines. Needing to have the user use a scrollbar is a major no-no.
I guess I should be a bit more specific. I'm after vertical resizing, and the width doesn't matter as much. The only problem that happens with that, is the ISO number (the large "1") gets pushed under the address when the window width is too small (as you can see on the screenshot).
It's not about having a gimick; it's about having a text field the user can edit that won't take up unnecessary space, but will show all the text in it.
Though if someone comes up with another way to approach the problem I'm open to that too.

I've modified the code a little because it was acting a little odd. I changed it to activate on keyup, because it wouldn't take into consideration the character that was just typed.
resizeIt = function() {
  var str = $('iso_address').value;
  var cols = $('iso_address').cols;
  var linecount = 0;

  $A(str.split("\n")).each(function(l) {
    linecount += 1 + Math.floor(l.length / cols); // Take into account long lines
  })

  $('iso_address').rows = linecount;
};

A:

Facebook does it, when you write on people's walls, but only resizes vertically.
Horizontal resize strikes me as being a mess, due to word-wrap, long lines, and so on, but vertical resize seems to be pretty safe and nice.
None of the Facebook-using-newbies I know have ever mentioned anything about it or been confused. I'd use this as anecdotal evidence to say 'go ahead, implement it'.
Some JavaScript code to do it, using Prototype (because that's what I'm familiar with):
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
  "http://www.w3.org/TR/html4/loose.dtd">
<html>
    <head>
        <script src="http://www.google.com/jsapi"></script>
        <script language="javascript">
            google.load('prototype', '1.6.0.2');
        </script>
    </head>

    <body>
        <textarea id="text-area" rows="1" cols="50"></textarea>

        <script type="text/javascript" language="javascript">
            resizeIt = function() {
              var str = $('text-area').value;
              var cols = $('text-area').cols;

              var linecount = 0;
              $A(str.split("\n")).each( function(l) {
                  linecount += Math.ceil( l.length / cols ); // Take into account long lines
              })
              $('text-area').rows = linecount + 1;
            };

            // You could attach to keyUp, etc. if keydown doesn't work
            Event.observe('text-area', 'keydown', resizeIt );

            resizeIt(); //Initial on load
        </script>
    </body>
</html>

PS: Obviously this JavaScript code is very naive and not well tested, and you probably don't want to use it on textboxes with novels in them, but you get the general idea.

A:

One refinement to some of these answers is to let CSS do more of the work.
The basic route seems to be:

Create a container element to hold the textarea and a hidden div
Using Javascript, keep the textarea’s contents synced with the div’s
Let the browser do the work of calculating the height of that div
Because the browser handles rendering / sizing the hidden div, we avoid
explicitly setting the textarea’s height.

document.addEventListener('DOMContentLoaded', () => {
    textArea.addEventListener('change', autosize, false)
    textArea.addEventListener('keydown', autosize, false)
    textArea.addEventListener('keyup', autosize, false)
    autosize()
}, false)

function autosize() {
    // Copy textarea contents to div browser will calculate correct height
    // of copy, which will make overall container taller, which will make
    // textarea taller.
    textCopy.innerHTML = textArea.value.replace(/\n/g, '<br/>')
}
html, body, textarea {
    font-family: sans-serif;
    font-size: 14px;
}

.textarea-container {
    position: relative;
}

.textarea-container > div, .textarea-container > textarea {
    word-wrap: break-word; /* make sure the div and the textarea wrap words in the same way */
    box-sizing: border-box;
    padding: 2px;
    width: 100%;
}

.textarea-container > textarea {
    overflow: hidden;
    position: absolute;
    height: 100%;
}

.textarea-container > div {
    padding-bottom: 1.5em; /* A bit more than one additional line of text. */ 
    visibility: hidden;
}
<div class="textarea-container">
    <textarea id="textArea"></textarea>
    <div id="textCopy"></div>
</div>

A:

Here's another technique for autosizing a textarea.

Uses pixel height instead of line height: more accurate handling of line wrap if a proportional font is used.
Accepts either ID or element as input
Accepts an optional maximum height parameter - useful if you'd rather not let the text area grow beyond a certain size (keep it all on-screen, avoid breaking layout, etc.)
Tested on Firefox 3 and Internet Explorer 6

Code:
(plain vanilla JavaScript)
function FitToContent(id, maxHeight)
{
   var text = id && id.style ? id : document.getElementById(id);
   if (!text)
      return;

   /* Accounts for rows being deleted, pixel value may need adjusting */
   if (text.clientHeight == text.scrollHeight) {
      text.style.height = "30px";
   }

   var adjustedHeight = text.clientHeight;
   if (!maxHeight || maxHeight > adjustedHeight)
   {
      adjustedHeight = Math.max(text.scrollHeight, adjustedHeight);
      if (maxHeight)
         adjustedHeight = Math.min(maxHeight, adjustedHeight);
      if (adjustedHeight > text.clientHeight)
         text.style.height = adjustedHeight + "px";
   }
}

Demo:
(uses jQuery, targets on the textarea I'm typing into right now - if you have Firebug installed, paste both samples into the console and test on this page)
$("#post-text").keyup(function()
{
   FitToContent(this, document.documentElement.clientHeight)
});

