Make sure the ImageMagick directory is in your path. Add it to your system or user PATH environment variable.
Test by opening a new command window and running the following:
cd C:\PATH\TO\NODE_PROJECT\
echo %PATH%
imdisplay.exe
The last step should launch a viewer which is in the imagemagick path.
If you set and verifed the path is correct then you should not get any error when your run:
node app.js
If that fails try removing everything except imagemagick from your path in case your edits have resulted in a corrupt path.
set PATH=C:\PATH\TO\IMAGEMAGICK
C:\PATH\TO\NODE\node.exe app.js