Source

File: wp-includes/theme.php

function get_template_directory_uri() {
	$template         = str_replace( '%2F', '/', rawurlencode( get_template() ) );
	$theme_root_uri   = get_theme_root_uri( $template );
	$template_dir_uri = "$theme_root_uri/$template";

	/**
	 * Filters the current theme directory URI.
	 *
	 * @since 1.5.0
	 *
	 * @param string $template_dir_uri The URI of the current theme directory.
	 * @param string $template         Directory name of the current theme.
	 * @param string $theme_root_uri   The themes root URI.
	 */
	return apply_filters( 'template_directory_uri', $template_dir_uri, $template, $theme_root_uri );
}