As long as the code is correctly in the import map, you can reference it with:

<script type="module">import "/assets/custom/alerts.js";</script>

Presuming the file is app/javascript/custom/alerts.js

Referenced in config/importmap.rb as:

pin_all_from "app/javascript/custom", under: "custom"

And imported into the application.js:

import "custom/alerts"

Edit: I now think this is an antipattern in Rails 7. It is very easy to use Stimulus controllers instead.

Stimulus Handbook For reference

For example, to dismiss an alert when a user clicks the "x"

// app/javascript/alerts_controller.js
import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
    dismiss () {
        this.element.style.display = 'none';
    }
}

<div data-controller="alerts">
    <h2> Alert! </h2>
    <span data-action="click->alerts#dismiss"><i class="fas fa-times"></i></span>
</div>