Resolved: How to synchronize a method in javascript?

Question:

I am trying to synchronise a singleton. I would need to make this method like the equivalent of synchronized in java. What happens to me is that because the socket takes a while, if the first two requests are very close together I get two websockets created. (Then from the third one onwards it takes the instance correctly).

import io from 'socket.io-client';

export default class SocketIo {
  static socket = null;

  static instance = null;

  async initialize() {
    this.socket = await io(`http://${ip}:10300/`, {
      transports: ['websocket'],
    });
  }

  static async getInstance() {
    logger.info('socketIo.api.getInstance: BEGIN');
    if (!this.instance) {
      logger.info('socketIo.api.getInstance: creating new socket instance...');
      try {
        const o = new SocketIo();
        await o.initialize();
        this.instance = o;
        logger.info('socketIo.api.getInstance: socket instance created SUCCESSFULLY');
      } catch (e) {
        moaLog('socketIo.api.getInstance: ERROR: ', e);
        throw e;
      }
    } else {
      logger.info('socketIo.api.getInstance: a socket instance already exists, reusing that one');
    }
    logger.info('socketIo.api.getInstance: END');
    return this.instance;
  }
}

in main.js

const socket1 = SocketIo.getInstance();
const socket2 = SocketIo.getInstance();

// ... after a while
const socket3 = SocketIo.getInstance();

2022-06-16T17:53:40.658Z: socketIo.api.getInstance: BEGIN
2022-06-16T17:53:40.660Z: socketIo.api.getInstance: creating new socket instance...
2022-06-16T17:53:41.140Z: socketIo.api.getInstance: BEGIN
2022-06-16T17:53:41.141Z: socketIo.api.getInstance: creating new socket instance...
2022-06-16T17:53:41.379Z: socketIo.api.getInstance: socket instance created SUCCESSFULLY
2022-06-16T17:53:41.382Z: socketIo.api.getInstance: END
2022-06-16T17:53:41.411Z: socketIo.api.getInstance: socket instance created SUCCESSFULLY
2022-06-16T17:53:41.415Z: socketIo.api.getInstance: END
...
2022-06-16T17:56:13.076Z: socketIo.api.getInstance: BEGIN
2022-06-16T17:56:13.078Z: socketIo.api.getInstance: a socket instance already exists, reusing that one
2022-06-16T17:56:13.079Z: socketIo.api.getInstance: END

And from server view I see two websocket connections.
Any ideas?

Answer:

Below is an attempt to synchronize your singleton. The idea is to store the o.intitialize promise and check if it already has been acquired before.
I added an uid, a random value set in initialize to show that only single instance is created.

class SocketIo {
    static instance = null;
    static _lock = null;

    async initialize() {
        this.uid = Math.random();
    }

    static async getInstance() {
        console.log('socketIo.api.getInstance: BEGIN');
        if (!this.instance) {
            console.log('socketIo.api.getInstance: creating new socket instance...');

            const o = new SocketIo();
            if (!this._lock) {
                this._lock = o.initialize();
                await this._lock;
                this.instance = o;
                console.log('socketIo.api.getInstance: socket instance created SUCCESSFULLY');
                this._lock = null;
            } else {
                await this._lock;
            }
        }
        console.log('socketIo.api.getInstance: END');
        return this.instance;
    }
}

async function Main() {
    var socket1 = SocketIo.getInstance();
    var socket2 = SocketIo.getInstance();

    console.log((await socket1).uid);
    console.log((await socket2).uid);
}

Main()

Compare it to your version:

class SocketIo {
    static instance = null;

    async initialize() {
        this.uid = Math.random();
    }

    static async getInstance() {
        console.log('socketIo.api.getInstance: BEGIN');
        if (!this.instance) {
            console.log('socketIo.api.getInstance: creating new socket instance...');

            const o = new SocketIo();
            await o.initialize();
            this.instance = o;
            console.log('socketIo.api.getInstance: socket instance created SUCCESSFULLY');
        }
        console.log('socketIo.api.getInstance: END');
        return this.instance;
    }
}

async function Main() {
    var socket1 = SocketIo.getInstance();
    var socket2 = SocketIo.getInstance();

    console.log((await socket1).uid);
    console.log((await socket2).uid);
}

Main()

Hope this is what suits you.

If you have better answer, please add a comment about this, thank you!

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