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/* 
 * Will only work if called before 'replace'
 * because after 'replace' the instance of the editor 
 * would have already been created and this won't run
*/
CKEDITOR.on("instanceCreated", function(event) {
  alert('Editor instance created');  
});

CKEDITOR.replace( 'editor');

CKEDITOR.on("instanceReady", function(event) {
  alert('Editor instance ready');  
});

<textarea name="editor"></textarea>