Return the name of the undo file that would be used for a file with name {name} when writing. This uses the 'undodir' option, finding directories that exist. It does not check if the undo file exists. {name} is always expanded to the full path, since that is what is used internally. If {name} is empty undofile() returns an empty string, since a buffer without a file name will not write an undo file. Useful in combination with :wundo and :rundo. When compiled without the +persistent_undo option this always returns an empty string.

Can also be used as a method:

GetFilename()->undofile()