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If you are in the loop then the other answers will help you. If not (for example you are hooking on init or plugins_loaded) you can resort to a PHP primitive like parse_url().

Here's a function that works in both cases:

function get_the_slug() {
    global $post;
    $slug = $post->post_name ?? '';

    if ( ! $slug ) {
        $slug = basename( parse_url( $_SERVER['REQUEST_URI'] ?? '', PHP_URL_PATH ) );
    }

    return $slug;
}

Please note that this approach only works on posts/pages at root level, because of how basename() works.