<p>Is it possible to highlight text inside of a textarea using javascript? Either changing the background of just a portion of the text area or making a portion of the text <em>selected</em>?</p>

Try this piece of code I wrote this morning, it will highlight a defined set of words:
```
```
<html>
    <head>
        <title></title>
        <!-- Load jQuery -->
        <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js"></script>
        <!-- The javascript xontaining the plugin and the code to init the plugin -->
        <script type="text/javascript">
            $(function() {
                // let's init the plugin, that we called "highlight".
                // We will highlight the words "hello" and "world", 
                // and set the input area to a widht and height of 500 and 250 respectively.
                $("#container").highlight({
                    words:  ["hello","world"],
                    width:  500,
                    height: 250
                });
            });

            // the plugin that would do the trick
            (function($){
                $.fn.extend({
                    highlight: function() {
                        // the main class
                        var pluginClass = function() {};
                        // init the class
                        // Bootloader
                        pluginClass.prototype.__init = function (element) {
                            try {
                                this.element = element;
                            } catch (err) {
                                this.error(err);
                            }
                        };
                        // centralized error handler
                        pluginClass.prototype.error = function (e) {
                            // manage error and exceptions here
                            //console.info("error!",e);
                        };
                        // Centralized routing function
                        pluginClass.prototype.execute = function (fn, options) {
                            try {
                                options = $.extend({},options);
                                if (typeof(this[fn]) == "function") {
                                    var output = this[fn].apply(this, [options]);
                                } else {
                                    this.error("undefined_function");
                                }
                            } catch (err) {
                                this.error(err);
                            }
                        };
                        // **********************
                        // Plugin Class starts here
                        // **********************
                        // init the component
                        pluginClass.prototype.init = function (options) {
                            try {
                                // the element's reference ( $("#container") ) is stored into "this.element"
                                var scope                   = this;
                                this.options                = options;

                                // just find the different elements we'll need
                                this.highlighterContainer   = this.element.find('#highlighterContainer');
                                this.inputContainer         = this.element.find('#inputContainer');
                                this.textarea               = this.inputContainer.find('textarea');
                                this.highlighter            = this.highlighterContainer.find('#highlighter');

                                // apply the css
                                this.element.css('position','relative');

                                // place both the highlight container and the textarea container
                                // on the same coordonate to superpose them.
                                this.highlighterContainer.css({
                                    'position':         'absolute',
                                    'left':             '0',
                                    'top':              '0',
                                    'border':           '1px dashed #ff0000',
                                    'width':            this.options.width,
                                    'height':           this.options.height,
                                    'cursor':           'text'
                                });
                                this.inputContainer.css({
                                    'position':         'absolute',
                                    'left':             '0',
                                    'top':              '0',
                                    'border':           '1px solid #000000'
                                });
                                // now let's make sure the highlit div and the textarea will superpose,
                                // by applying the same font size and stuffs.
                                // the highlighter must have a white text so it will be invisible
                                this.highlighter.css({

                                    'padding':          '7px',
                                    'color':            '#eeeeee',
                                    'background-color': '#ffffff',
                                    'margin':           '0px',
                                    'font-size':        '11px',
                                    'font-family':      '"lucida grande",tahoma,verdana,arial,sans-serif'
                                });
                                // the textarea must have a transparent background so we can see the highlight div behind it
                                this.textarea.css({
                                    'background-color': 'transparent',
                                    'padding':          '5px',
                                    'margin':           '0px',
                                    'font-size':        '11px',
                                    'width':            this.options.width,
                                    'height':           this.options.height,
                                    'font-family':      '"lucida grande",tahoma,verdana,arial,sans-serif'
                                });

                                // apply the hooks
                                this.highlighterContainer.bind('click', function() {
                                    scope.textarea.focus();
                                });
                                this.textarea.bind('keyup', function() {
                                    // when we type in the textarea, 
                                    // we want the text to be processed and re-injected into the div behind it.
                                    scope.applyText($(this).val());
                                });
                            } catch (err) {
                                this.error(err);
                            }
                            return true;
                        };
                        pluginClass.prototype.applyText = function (text) {
                            try {
                                var scope                   = this;

                                // parse the text:
                                // replace all the line braks by <br/>, and all the double spaces by the html version &nbsp;
                                text = this.replaceAll(text,'\n','<br/>');
                                text = this.replaceAll(text,'  ','&nbsp;&nbsp;');

                                // replace the words by a highlighted version of the words
                                for (var i=0;i<this.options.words.length;i++) {
                                    text = this.replaceAll(text,this.options.words[i],'<span style="background-color: #D8DFEA;">'+this.options.words[i]+'</span>');
                                }

                                // re-inject the processed text into the div
                                this.highlighter.html(text);

                            } catch (err) {
                                this.error(err);
                            }
                            return true;
                        };
                        // "replace all" function
                        pluginClass.prototype.replaceAll = function(txt, replace, with_this) {
                            return txt.replace(new RegExp(replace, 'g'),with_this);
                        }

                        // don't worry about this part, it's just the required code for the plugin to hadle the methods and stuffs. Not relevant here.
                        //**********************
                        // process
                        var fn;
                        var options;
                        if (arguments.length == 0) {
                            fn = "init";
                            options = {};
                        } else if (arguments.length == 1 && typeof(arguments[0]) == 'object') {
                            fn = "init";
                            options = $.extend({},arguments[0]);
                        } else {
                            fn = arguments[0];
                            options = $.extend({},arguments[1]);
                        }

                        $.each(this, function(idx, item) {
                            // if the component is not yet existing, create it.
                            if ($(item).data('highlightPlugin') == null) {
                                $(item).data('highlightPlugin', new pluginClass());
                                $(item).data('highlightPlugin').__init($(item));
                            }
                            $(item).data('highlightPlugin').execute(fn, options);
                        });
                        return this;
                    }
                });

            })(jQuery);

        </script>
    </head>
    <body>

        <div id="container">
            <div id="highlighterContainer">
                <div id="highlighter">

                </div>
            </div>
            <div id="inputContainer">
                <textarea cols="30" rows="10">

                </textarea>
            </div>
        </div>

    </body>
</html>
```
```
This was written for another post (http://facebook.stackoverflow.com/questions/7497824/how-to-highlight-friends-name-in-facebook-status-update-box-textarea/7597420#7597420), but it seems to be what you're searching for.