ready() {
        // Return true if no ready promise exists
        if (this.#ready_promise === true) return Promise.resolve();

        // Create a Promise if one does not exist for ready event
        if (this.#ready_promise === undefined)
            this.#ready_promise = new Promise((resolve) => (this.#ready_resolve = resolve));

        return this.#ready_promise;
    }
Returns a promise which resolves once first reload is complete.

@returns {Promise}