Insert the directory of the current buffer in command line mode

Hi all,

I saw one line command map used to insert the directory of the current
buffer in command line mode in steve losh's vimrc:

cnoremap <expr> %% getcmdtype() == ':' ? expand('%:h').'/' : '%%'

I imagined it would insert directory of the current buffer when enter
%% in command mode. But it seemed not work as I imagine. So what I
missed?

Thanks.

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