you can add a static folder on your custom module directory, and create a js file

custom-module

    --> static

            --> js

                --> filename.js

after that don't forget to add it in backend assets.

eg:

<!-- add css / js -->
<template id="assets_backend" name="js assets" inherit_id="web.assets_backend">
    <xpath expr="." position="inside">
        <script 
            type="text/javascript" 
            src="/path/filename.js">
         </script>
         <link
            href="/custom_module_name/static/src/css/filenmae.css" 
            rel="stylesheet" 
            type="text/css"/>
    </xpath>
</template>