Common.js

Note: After saving, you may have to bypass your browser's cache to see the changes.

• Firefox / Safari: Hold Shift while clicking Reload, or press either Ctrl-F5 or Ctrl-R (⌘-R on a Mac)
• Google Chrome: Press Ctrl-Shift-R (⌘-Shift-R on a Mac)
• Internet Explorer / Edge: Hold Ctrl while clicking Refresh, or press Ctrl-F5
• Opera: Go to Menu → Settings (Opera → Preferences on a Mac) and then to Privacy & security → Clear browsing data → Cached images and files.

/* Any JavaScript here will be loaded for all users on every page load. */
mw.hook( 'wikipage.editform' ).add( function () {
    mw.loader.using( 'mediawiki.toolbar' ).then( function () {
        mw.toolbar.addButton( {
            imageFile: '//upload.wikimedia.org/wikipedia/en/3/34/Button_hide_comment.png',
            speedTip: 'Comment visible only for editors',
            tagOpen: '<!-- ',
            tagClose: ' -->',
            sampleText: 'Insert comment here',
            imageId: 'button-comment'
        } );
    } );
} );