How to add a custom button to the toolbar that calls a JavaScript function?

I am in the process of developing a number of custom Plugins for CKEditor and here's my survival pack of bookmarks:

• A StackOverflow post linking to and talking about a plugins tutorial that got me started (Tim Down already mentioned this)
• A number of ready-made plugins for FCK and CKEditor that may improve one's understanding of the system
• The Blog of AlfonsoML, one of the developers, lots of valuable stuff there, e.g. Plugin localization in CKEditor
• Interaction between dialog buttons and a IFrame dialog, with input from Garry Yao, one of the developers
• The forums are not as bad as they look, there are some hidden gems there. Make sure you search there, well, if not first, then at least second or third.

For your purpose, I would recommend look at one of the plugins in the _source/plugins directory, for example the "print" button. Adding a simple Javascript function is quite easy to achieve. You should be able to duplicate the print plugin (take the directory from _source into the actual plugins/ directory, worry about minification later), rename it, rename every mention of "print" within it, give the button a proper name you use later in your toolbar setup to include the button, and add your function.

Also there is a nice way allowing one to add the button without creating plugin.

html:

<textarea id="container">How are you!</textarea>

javascript:

editor = CKEDITOR.replace('container'); // bind editor

editor.addCommand("mySimpleCommand", { // create named command
    exec: function(edt) {
        alert(edt.getData());
    }
});

editor.ui.addButton('SuperButton', { // add new button and bind our command
    label: "Click me",
    command: 'mySimpleCommand',
    toolbar: 'insert',
    icon: 'https://avatars1.githubusercontent.com/u/5500999?v=2&s=16'
});

Check out how it works here: DEMO

In case anybody is interested, I wrote a solution for this using Prototype. In order to get the button to appear correctly, I had to specify extraPlugins: 'ajaxsave' from inside the CKEDITOR.replace() method call.

Here is the plugin.js:

CKEDITOR.plugins.add('ajaxsave',
{
    init: function(editor)
    {
    var pluginName = 'ajaxsave';

    editor.addCommand( pluginName,
    {
        exec : function( editor )
        {
            new Ajax.Request('ajaxsave.php',
            {
                method:     "POST",
                parameters: { filename: 'index.html', editor: editor.getData() },
                onFailure:  function() { ThrowError("Error: The server has returned an unknown error"); },
                on0:        function() { ThrowError('Error: The server is not responding. Please try again.'); },
                onSuccess:  function(transport) {

                    var resp = transport.responseText;

                    //Successful processing by ckprocess.php should return simply 'OK'. 
                    if(resp == "OK") {
                        //This is a custom function I wrote to display messages. Nicer than alert() 
                        ShowPageMessage('Changes have been saved successfully!');
                    } else {
                        ShowPageMessage(resp,'10');
                    }
                }
            });
        },

        canUndo : true
    });

    editor.ui.addButton('ajaxsave',
    {
        label: 'Save',
        command: pluginName,
        className : 'cke_button_save'
    });
    }
});

See this URL for a easy example http://ajithmanmadhan.wordpress.com/2009/12/16/customizing-ckeditor-and-adding-a-new-toolbar-button/

There are a couple of steps:

1) create a plugin folder

2) register your plugin (the URL above says to edit the ckeditor.js file DO NOT do this, as it will break next time a new version is relased. Instead edit the config.js and add a line like so

config.extraPlugins = 'pluginX,pluginY,yourPluginNameHere'; 

3) make a JS file called plugin.js inside your plugin folder Here is my code

(function() {
    //Section 1 : Code to execute when the toolbar button is pressed
    var a = {
        exec: function(editor) {
            var theSelectedText = editor.getSelection().getNative();
            alert(theSelectedText);
        }
    },

    //Section 2 : Create the button and add the functionality to it
    b='addTags';
    CKEDITOR.plugins.add(b, {
        init: function(editor) {
            editor.addCommand(b, a);
            editor.ui.addButton("addTags", {
                label: 'Add Tag', 
                icon: this.path+"addTag.gif",
                command: b
            });
        }
    }); 
})();