\installfontfeature[otf][ardi]

\definefontfeature
 [fea]
 [mode=node,language=dflt,script=arab,
  init=yes,medi=yes,fina=yes,isol=yes,
  liga=yes,dlig=yes,rlig=yes,frac=yes,
  calt=yes,ccmp=yes,kern=yes,mark=yes,
  mkmk=yes,ardi=yes,featurefile=digit.fea]

\font\ara=arabtype*fea at 20pt\ara
\textdir TRT\pardir TRT
بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيم

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\bye