node-red/test/resources/plugin/test-plugin/test-editor-plugin.html

<script type="text/javascript">
    console.log("Loaded test-plugin/test-editor-plugin")
    RED.plugins.registerPlugin("my-test-editor-only-plugin", {
        type: "bar",
        onadd: function() {
            console.log("my-test-editor-only-plugin.onadd called")
        }
    })
</script>