jquery.autosize.js

// Autosize 1.8 - jQuery plugin for textareas
// (c) 2011 Jack Moore - jacklmoore.com
// license: www.opensource.org/licenses/mit-license.php

(function ($, undefined) {
	var 
	hidden = 'hidden',
	copy = '<textarea style="position:absolute; top:-9999px; left:-9999px; right:auto; bottom:auto; box-sizing:content-box; word-wrap:break-word; height:0 !important; min-height:0 !important; overflow:hidden">',
	// line-height is omitted because IE7/IE8 doesn't return the correct value.
	copyStyle = [
		'fontFamily',
		'fontSize',
		'fontWeight',
		'fontStyle',
		'letterSpacing',
		'textTransform',
		'wordSpacing'
	],
	oninput = 'oninput',
	onpropertychange = 'onpropertychange',
	test = $(copy)[0];

	test.setAttribute(oninput, "return");

	if ($.isFunction(test[oninput]) || onpropertychange in test) {
		$.fn.autosize = function (className) {
			return this.each(function () {
				var 
				ta = this,
				$ta = $(ta),
				mirror,
				minHeight = $ta.height(),
				maxHeight = parseInt($ta.css('maxHeight'), 10),
				active,
				i = copyStyle.length,
				resize,
				boxOffset = $ta.css('box-sizing') === 'border-box' ? $ta.outerHeight() - $ta.height() : 0;

				if ($ta.data('mirror') || $ta.data('ismirror')) {
					// if autosize has already been applied, exit.
					// if autosize is being applied to a mirror element, exit.
					return;
				} else {
					mirror = $(copy).data('ismirror', true).addClass(className || 'autosizejs')[0];

					resize = $ta.css('resize') === 'none' ? 'none' : 'horizontal';

					$ta.data('mirror', $(mirror)).css({
						overflow: hidden, 
						overflowY: hidden, 
						wordWrap: 'break-word',
						resize: resize
					});
				}

				// Opera returns '-1px' when max-height is set to 'none'.
				maxHeight = maxHeight && maxHeight > 0 ? maxHeight : 9e4;

				// Using mainly bare JS in this function because it is going
				// to fire very often while typing, and needs to very efficient.
				function adjust() {
					var height, overflow;
					// the active flag keeps IE from tripping all over itself.  Otherwise
					// actions in the adjust function will cause IE to call adjust again.
					if (!active) {
						active = true;

						mirror.value = ta.value;

						mirror.style.overflowY = ta.style.overflowY;

						// Update the width in case the original textarea width has changed
						mirror.style.width = $ta.css('width');

						// Needed for IE to reliably return the correct scrollHeight
						mirror.scrollTop = 0;

						// Set a very high value for scrollTop to be sure the 
						// mirror is scrolled all the way to the bottom.
						mirror.scrollTop = 9e4;

						height = mirror.scrollTop;
						overflow = hidden;
						if (height > maxHeight) {
							height = maxHeight;
							overflow = 'scroll';
						} else if (height < minHeight) {
							height = minHeight;
						}
						ta.style.overflowY = overflow;

						ta.style.height = height + boxOffset + 'px';
						
						// This small timeout gives IE a chance to draw it's scrollbar
						// before adjust can be run again (prevents an infinite loop).
						setTimeout(function () {
							active = false;
						}, 1);
					}
				}

				// mirror is a duplicate textarea located off-screen that
				// is automatically updated to contain the same text as the 
				// original textarea.  mirror always has a height of 0.
				// This gives a cross-browser supported way getting the actual
				// height of the text, through the scrollTop property.
				while (i--) {
					mirror.style[copyStyle[i]] = $ta.css(copyStyle[i]);
				}

				$('body').append(mirror);

				if (onpropertychange in ta) {
					if (oninput in ta) {
						// Detects IE9.  IE9 does not fire onpropertychange or oninput for deletions,
						// so binding to onkeyup to catch most of those occassions.  There is no way that I
						// know of to detect something like 'cut' in IE9.
						ta[oninput] = ta.onkeyup = adjust;
					} else {
						// IE7 / IE8
						ta[onpropertychange] = adjust;
					}
				} else {
					// Modern Browsers
					ta[oninput] = adjust;
				}

				$(window).resize(adjust);

				// Allow for manual triggering if needed.
				$ta.bind('autosize', adjust);

				// Call adjust in case the textarea already contains text.
				adjust();
			});
		}; 
	} else {
		// Makes no changes for older browsers (FireFox3- and Safari4-)
		$.fn.autosize = function () {
			return this;
		};
	}

}(jQuery));