I think you have to be a little careful because it depends on what you are trying to do. If you are using a child theme get_template_directory(); will still go to the parent theme. However get_stylesheet_directory(); will go to the current theme, child or parent. Also, both these functions return absolute server paths.
If you wanted a fully formed URI, for links or images, you should use
get_stylesheet_directory_uri(); using the correct one for the reasons stated.
get_stylesheet_directory(): file path to current Theme directory
get_stylesheet_directory_uri(): url path to current Theme directory
get_template_directory(): file path to parent Theme directory
get_template_directory_uri(): url path to parent Theme directory
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