MaszToZadanieDomowe.com/assets/js/akordeon.js

class Akordeon extends HTMLElement {
    constructor() {
        super();
        this._internals = this.attachInternals();
    }
    
    get collapsed() {
        return this._internals.states.has("hidden");
    }

    set collapsed(flag) {
        if (flag) {
            // Existence of identifier corresponds to "true"
            this._internals.states.add("hidden");
        } else {
            // Absence of identifier corresponds to "false"
            this._internals.states.delete("hidden");
        }
    }

    connecterCallback() {
    }

    attributeChangedCallback(atrybut, poprzednio, obecnie) {

    }
}

customElements.define("akordeon", Akordeon);