jQuery code to auto expand textarea input field on a form. The Plugin makes use of the scrollTop DOM property to gain the true height of the text and then applies that directly (or via animation) to the textarea.

Plugin Usage

$('textarea#comment').autoResize({
    // On resize:
    onResize : function() {
        $(this).css({opacity:0.8});
    },
    // After resize:
    animateCallback : function() {
        $(this).css({opacity:1});
    },
    // Quite slow animation:
    animateDuration : 300,
    // More extra space:
    extraSpace : 40
});

Online Demo

Download Plugin

Source