autoGrow.js

//for Firefox
//javascript:
(function(){
var ta=document.createElement('textarea');
ta.rows=1;
ta.id='fuga';
ta.style.height='auto';
ta.style.minHeight='none';
document.body.appendChild(ta);
var st=getComputedStyle(ta,'');
var taLineHeight=st.height;
ta.parentNode.removeChild(ta);
javascript:
[].forEach.call(document.getElementsByTagName('textarea'),function(ta){
	var pre=document.createElement('pre');
	var st=getComputedStyle(ta,'');
	var prop='direction fontFamily fontSize fontAdjust fontStyle fontVariant fontWeight lineHeight overflowX padding paddingBottom paddingLeft paddingTop textAlign textDirection textIndent textRendering textShadow textTransform whiteSpace width wordSpacing';
	prop.split(' ').forEach(function(e){pre.style[e]=st[e]});
	with(pre.style){
		/*if(lineHeight=='normal')lineHeight=taLineHeight;*/
		position='fixed';
		maxHeight='none';
		minHeight='none';
		height='auto';
		top=0;
		left=0;
		whiteSpace='pre-wrap';
		visibility='hidden';
	}
	with(ta.style){
		minHeight=st.height;
		overflowY='hidden';
		maxHeight='none';
		height='auto';
	}
	document.body.appendChild(pre);

	ta.addEventListener('input',function(){
		pre.textContent=ta.value+'\n\n';
		ta.style.height=getComputedStyle(pre,'').height;
	},false);
})
})()

//for Opera
//javascript:
[].forEach.call(document.getElementsByTagName('textarea'),function(ta){
	var pre=document.createElement('pre');
	var st=getComputedStyle(ta,'');
	var prop='direction fontFamily fontSize fontAdjust fontStyle fontVariant fontWeight lineHeight overflowX padding paddingBottom paddingLeft paddingTop textAlign textDirection textIndent textRendering textShadow textTransform whiteSpace width wordSpacing';
	prop.split(' ').forEach(function(e){pre.style[e]=st[e]});
	with(pre.style){
		position='absolute';
		maxHeight='none';
		top=0;
		left=0;
		visibility='hidden';
	}
	with(ta.style){
		minHeight=st.height;
		overflowY='hidden';
		maxHeight='none';
		height='auto';
	}
	pre.innerHTML='<span></span>';
	var span=pre.firstChild;
	document.body.appendChild(pre);
	ta.addEventListener('input',function(){
		span.textContent=ta.value;
		var texts=span.textContent.split('\n');
		var mod=(texts[texts.length]=='')?0:texts.filter(function(e){return e==''}).length;
		ta.rows=span.getClientRects().length+mod+1;
	},false);
})