How to autosize a textarea using Prototype?

The Mechanics of Auto-Resizing

To provide a well-rounded solution, let’s go more in-depth and examine some crucial facets of dynamically resizing textareas. The primary goal is to tweak the height based on contents, avoiding scrollbars and gracefully accommodating all user input, including line breaks and exceeding texts.

Accurate Height Determination

Firstly, to pinpoint the right height, synchronize the content of the textarea to a hidden div, mirroring the textarea’s styling. The scrollHeight of this div is your winning formula for height:

function updateSize(textarea) {
    // Ghost div acts as our reference
    var ghost = $('ghost');
    ghost.setStyle({
        width: textarea.getWidth() + 'px',
        padding: textarea.getStyle('padding'),
    });

    ghost.update(textarea.value.replace(/\n/g, '<br/>'));  // Tackling line breaks
    var newHeight = ghost.scrollHeight + 'px';  // Voila, the perfect height!
    textarea.setStyle({ height: newHeight });
}

var ghost = new Element('div', { id: 'ghost' }).setStyle({
    position: 'absolute',
    visibility: 'hidden',
    wordWrap: 'break-word',  // Keep wrapping on point
    overflow: 'hidden',  // Ditch the overflowing bits, pls
}).insertTo(document.body);  // Now our ghost div is part of the document

$('textarea_id').observe('keyup', function() {
    updateSize(this);  // The magic happens here
});


Real-Time Space Adjustments

Bind keyup and input events for on-demand resizing. Resetting the height to auto before adjusting to the actual height irons out sizing glitches during shrinking:

$('textarea_id').observe('input', function() {
    this.style.height = 'auto';  // Resets any previous height value
    this.style.height = this.scrollHeight + 'px';  // Boom! Instant resizing
});


Height-Check: Preventing a Visual Mess

When you don’t want your textarea to rival Jack's beanstalk, set a maximum height. For overflows within our set height, let the scrollbar handle it:

#textarea_id {
    max-height: 300px;  // We all have our limits, right?
    overflow-y: auto;  // Enable vertical scrolling
}


Getting On with Text Deletion

By observing deletion, the textarea size can be regulated. The trick with resetting height to auto comes in handy to correctly handle content reduction.

Compatibility: The Browser Problem

Not all browsers are created equal. With that said, make sure to put your new autosizing functionality to the test on different browsers.

Styling Tweaks for a Pleasing Experience

Sprinkling some style can enhance the overall look and usability of the autosizing.

Padding and the Art of Visual Comfort

A touch of padding can introduce much-needed breathing space within the textarea:

#textarea_id {
    padding: 10px;  // 10 pixels of pure comfort
    resize: vertical;  // We're all about adaptability
}


A vertical resize remains a clear winner by offering user control and maintaining visual fidelity.

Keeping the Text Flow In Check

Ensure word-wrap is identical for the textarea and its shadow box to prevent height discrepancies:

#textarea_id, #ghost {
    word-wrap: break-word;  // Break it like you mean it!
}


Hidden Helpers: The Ghost Div

With visibility: hidden, auxiliary elements like our hidden ghost div can make sizing a breeze without tossing the layout into chaos:

#ghost {
    visibility: hidden;  // Ghost div - Now you see me, now you don't!
}


Dynamic Resizing: A Height-Free Zone

While explicit heights could be tempting, they're restrictive. For a seamless resizing experience, let's afford our textareas some freedom.