Using Event file

I don't understand how to use an Event with a separated file with the function

for example

App/Config/Events.php

Events::on('pre_system', ['compress', 'compresstest']);

System/Events/compress.php

<?php
function compresstest()
{
console.log("Test");
}

I always have this error: ErrorException call_user_func() expects parameter 1 to be a valid callback, class 'compress' not found