Return value to custom function [duplicate]

Question

I'm writting a function to integrates NERDTree with Ack. The goal is to create a "Find in Directory".

I create this:

function! Fif_NERDTreeGetPath()
  let nodePath = g:NERDTreeFileNode.GetSelected().path.str()
  return nodePath
endfunction
:command! -nargs=+ Fif :Ack "<args>" Fif_NERDTreeGetPath()

The function Fif_NERDTreeGetPath() returns the correct path for the directory that cursor is on.

But, when I execute Fif word it return:

:ack -s -H --nopager --nocolor --nogroup --column "word" Fif_NERDTreeGetPath()

It's almost right. The problem is the path is not being returned. I'm expecting something like:

:ack -s -H --nopager --nocolor --nogroup --column "word" /path/to/directory/

This is my first vim function. I don't know how to solve. Tks!