Why are class fields not initialized?

Question

Node.js

Hello! There is the following code:

import { Client } from 'service'; 
import Redis from 'ioredis';

import config from './config.js';

class ServiceFactory
{
    #client = null;
    #redis = null;

    #qr = null;
    #ready = false;
    #error = null;

    constructor()
    {
        this.#redis = new Redis(config.redis);
        this.init();
    }

    async init()
    {
        this.#client = new Client(await this.#loadSession());

        this.#client.on('auth_failure', this.#onFail);
        this.#client.on('ready', this.#onReady);
        this.#client.on('disconnected', this.#onFail);
        this.#client.on('qr', this.#onQr);

        this.#client.initialize();
    }

    #onFail(error_msg)
    {
        this.#ready = false;
        this.#error = error_msg;
    }

    #onReady()
    {
        this.#ready = true;
    }

    #onQr(qr_code)
    {
        this.#qr = qr_code;
    }

    async #loadSession()
    {
        const session = await this.#redis.get(config.service.redisKey);
        return JSON.parse(session);
    }

    async #saveSession(session)
    {
        return await this.#redis.set(config.service.redisKey,  JSON.stringify(session));
    }
}

export default ServiceFactory;

An error occurs:

this.#qr = qr_code;
                 ^

TypeError: Cannot write private member #qr to an object whose class did not declare it

It turns out because of the asynchronous init () class fields do not have time to initialize? How can this be fixed?

Answer 1

this in the handler refers to the Client class , not the ServiceFactory . To fix it, you can do this or make the #onQr method an arrow function
this.#client.on('qr', this.#onQr.bind(this));