Question

var editor = CKEDITOR.instances['fullHTMLTextArea'];
if(editor)
{
    editor.destroy(true); 
}

Can anyone explain it properly?

Answer 1

According to CKEDITOR API doc

var editor = CKEDITOR.instances['fullHTMLTextArea'];

part tells

Holds references to all editor instances created. The name of the
properties in this object correspond to instance names, and their
values contains the CKEDITOR.editor object representing them.

and

if(editor){
    editor.destroy(true);---> must be
 //editor.destroy();

}

tells

Destroys the editor instance, releasing all resources used by it. If
the editor replaced an element, the element will be recovered.