is_front_page() variable not found

There are several issues with your code. is_front_page() is a PHP function, you need to call it within <?php ?>.

The script need to be printed in HTML in order to make it work. Try this..

function wpse_363853_run_on_front() {
    ?>
    <script>
        window.addEventListener("load", function() {
            <?php if( is_front_page() ){ ?>
            console.log("front page");
            <?php } ?>
        });
    </script>
    <?php
}
add_action( 'wp_head', 'wpse_363853_run_on_front', 99 );

Just a note, avoid using anonymous functions.