gistfile1.js

final_width = 400
final_height = 400

for (var i = 0; i < app.documents.length; i++) {
  crop_width_by = 0
  crop_height_by = 0
  doc = app.documents[i]
  app.activeDocument = doc
  landscape = (doc.height < doc.width)
  if (landscape == true) {
    doc.resizeImage(undefined, final_height)
    crop_width_by = (doc.width - final_width) / 2
  } else {
    doc.resizeImage(final_width)
    crop_height_by = (doc.height - final_height) / 2
  }
  doc.crop([crop_width_by, crop_height_by, doc.width - crop_width_by, doc.height - crop_height_by])
}