I studied the links James posted earlier.
There is something I couldn't understand:
"face.evaluateNormal( uv )" function returns the surface normal on given "uv" value, right?
According to :
https://moi3d.com/forum/lmessages.php?webtag=MOI&msg=9547.25
There is no function that returns the surface "uv" value,
So I don't know what "uv" value should look like.
what is "uv" parameter type?
I tried to set the "uv" value with array numbers [0.5,0.5],also I tried to set points as "uv", but that didn't work either.

With "face.planarFrame" property I managed to get normals of planar surfaces
This property uses the "untrimmed" version of the surface
And normal appears on the corner of the "untrimmed" surface.