old_contents: #!/usr/bin/python
  import os
  
  if __name__ == '__main__':
  	js_file = open('cropList.js', 'w')
  	js_file.write('imageNames = [\n')
  	js_file.write(',\n'.join(['\t"%s"' % name for name in os.listdir('originals')
  		if name[-4:] in ('.JPG', '.jpg', '.PNG', '.png')]))
  	js_file.write('\n\t];\n')
  	js_file.close()
  
new_contents: #!/usr/bin/python
  import os
  
  if __name__ == '__main__':
  	js_file = open('cropList.js', 'w')
  	js_file.write('var imageNames = [\n')
  	js_file.write(',\n'.join(['\t"{}"'.format(name) for name in os.listdir('originals')
  		if name[-4:] in ('.JPG', '.jpg', '.PNG', '.png')]))
  	js_file.write('\n];\n')
  	js_file.close()
  
subject: Declare imageNames with var, use .format() instead of %, remove tab before closing bracket