I run a Flask web app where I have a domain model (this example Thing) which properties (e.g. rating) could be updated via a HTTP PATCH request. In my development setup I hard coded the API URL where the request must be made to, but this becomes an open source self hosted app so the URL must be generated & set by Flask.

<script type="module">
import {Thing} from './things.js';

// This string must be available inside the Thing class
// The {{ }} syntax with url_for is Flask/Jinja template style (server side)
const API_URL = '{{ url_for('thing_update', id='{0}' }}';

let stars = document.querySelectorAll('.rating .star')
stars.addEventListener('click', function(el){
  // To create an object with object properties, like its id
  let thing = new Thing({{ thing.id }});
  thing.rating = el.getAttribute('value');
})
</script>

My module looks like:

class Entity {
    _API_URL = '';

    async update (data) {
        let response = await fetch(this._API_URL, {
            method: 'PATCH',
            headers: {'Content-Type': 'application/json;charset=utf-8'},
            body: JSON.stringify(data)
        })

        if (!response.ok) {
            throw new Error(`Server response ${response.status}: `)
        } else if (response.status == 204) {
            console.debug('Response success, no content')
            return true
        } else {
            let json = await response.json()
            console.debug('Response success, body content: ', json)
            return json
        }
    }
}

class Thing extends Entity {
    _id = null;
    _rating = null;

    constructor(id) {
        super()

        this._id = id
        // How to get the proper API URL here???
        this._API_URL = super._API_URL.format(id)
    }

    set rating(value) {
        this._rating = value
        this.update()
    }

    async update() {
        console.debug('Update rating to ' + this._rating)
        super.update({rating: this._rating})
    }
}

export {Thing}

I have experience in server side coding (php/python etc) and it just feels "wrong" to set the API URL via the constructor or setter method on the object instance. This should be something as constant on the class level.

However, because I use Flask and don't use javascript for a SPA or something, I struggle how to get this "right". Is there any best practice for this case?