Listening to events with a provided callback param in node.js

Question

My node.js server is connected to a websocket. It continuosly sends the Events.emit('ws-price', data):

From another part of the js file, we can start and stop listening for those events. To be able to removeListener - same callback function should be used for both Events.on and Events.off.

How would I access a provided "bot" param in "run" and "stop" functions within the callback method ?

const Events = require('../services/events')

module.exports = {
  run: async (bot) => {
    Events.on('ws-price', callback(event, bot)) // <--- pass "bot" variable
  },

  stop: async (bot) => {
    Events.off('ws-price', callback(event, bot)) // <--- pass "bot" variable
  }
}

const callback = (event, bot) => {
  console.log(bot?.id, event) // How to access "bot" here ?
}

I get an error: ReferenceError: event is not defined

Answer 1

You'll have to create a wrapper function that closes over bot that, when it's called, calls callback with both the bot and the event it receives. Then remember that function so you can remove it when stop is called (a Map is good for that):

// A map of the current callbacks (have been `run` and not `stop`ped)
const callbacks = new Map();
module.exports = {
    run: async (bot) => {
        const cb = (event) => callback(event, bot);
        callbacks.set(bot, cb);
        Events.on("ws-price", cb);
    },

    stop: async (bot) => {
        const cb = callbacks.get(bot);
        if (cb) {
            callbacks.delete(bot);
            Events.off("ws-price", cb);
        }
    },
};