word-count.js

// this assumes an existing html tag containing the upper limit
// of the word count, with an id of #word-count- + the id of the tinymce instance
// e.g. <p>Words left: <span id="word-count-textarea-1">200</span></p>
// add as a parameter to the tinyMCE.init({}); function

setup: function(ed) {
    var text = '';
    var span = document.getElementById('word-count-' + ed.id);
    if(span) {
        var wordlimit = span.innerHTML;
        ed.onKeyDown.add(function(e) {
            text = ed.getContent().replace(/(< ([^>]+)<)/g, '').replace(/\s+/g, ' ');
            text = text.replace(/^\s\s*/, '').replace(/\s\s*$/, '');
            wordcount = wordlimit - (text.split(' ').length);
            span.innerHTML = wordcount;
            if(wordcount <= 0 && e.keyCode != 8) {
                return tinymce.dom.Event.cancel(e);
            }
        });
    }
}