How to access variable from module in the global scope with JS

I have this code in my index.html:

<script type='module'>

import { Synthetizer } from "xxx.js";

window.test = () => {

    const synth = new Synthetizer(context.destination,soundFontBuffer);

};

window.customfunction = () => {

    //Here I need to use the variable `synth` created when `window.test` was executed. 
    synth.setProperty();

};

</script>
    

I have a div element on my page and the onclick event of that element triggers the function customfunction(); however I cant find a way to access the variable synth. I tried setting the synth variable as global with var however for some reason the library stops working, it only allows me to use const when using that variable synth.

Any idea how I can solve this?