Add JavaScript to a specific WordPress page or post

gistfile1.txt

function mm_js_in_wp_head($pid){
       global $post;
        // only for post Id = 720
       $mm_id_of_page="720";
       if($post->ID==$mm_id_of_page){
            echo "<script>alert('hello world');</script>";
       }
}
add_action( 'wp_head', 'mm_js_in_wp_head' );