function computeOutput() {
    //get text and create URL
    var textElem = document.getElementById('statusText');
    var text = textElem.value;
    var url = createTwitterThisURL(text);
    var linkText = document.getElementById('linkText').value;
    
    //show output
    document.getElementById('twitterURL').value = url;
    var link = '<a href="'+url+'">'+linkText+'</a>'; 
    //alert('url: '+url+', link: '+linkText);
    document.getElementById('twitterLink').value = link;
    document.getElementById('linkPreview').innerHTML = link; 
}
function createTwitterThisURL(text) {
    if(!text || text.length > 140) {
        alert('Warning: text is null or longer than 140 characters. Can not be tweeted like this!');
    }
    return 'http://twitter.com?status='+encodeURIComponent(text);
}