why quotes shown in WordPress?

I am not sure about the extra quotes. The best way to include a java script file is with wp_enqueue_script() as indicated here in the codex.

The safe and recommended method of adding JavaScript to a WordPress
generated page and WordPress Theme or Plugin is by using
wp_enqueue_script(). This function includes the script if it hasn’t
already been included, and safely handles dependencies.

add_action( 'admin_enqueue_scripts', 'wpse8170_enqueue_scripts' );
function wpse8170_enqueue_scripts() {
    wp_enqueue_script( 'wpse8170-admin-js', get_template_directory_uri() . '/admin/admin.js', null, null, true );
}

When in doubt use the function designed to do what you want to do. Reinventing the wheel almost always creates new problems.

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