Connect event emitter with instance of each ES6 class in node

Question

I'm trying to associate each instance of a class with an event emitter. I'm trying the following:

const events = require("events");
const eventEmitter = new events.EventEmitter();

class Camera {
    constructor(ip) {
        this.ip = ip;

        eventEmitter.on("recordVideo", function() {
            this.recordClip();
        });
    }

    recordClip() {
        console.log("record " + this.ip);
    }
}

var cam = new Camera("0.0.0.0");
eventEmitter.emit("recordVideo");

But I get back:

TypeError: this.recordClip is not a function

How can I have each instance of my class listen for the event?

Answer 1

Your issue here is that this is in the context of the event emitter, not the class. So, eventEmitter does not have recordClip as a method. You need to either lexically bind your callback with an arrow function:

(Personally, I think this is the best, and most modern/readable way to do this)

eventEmitter.on("recordVideo", () => {
    this.recordClip();
});

Or, you need to bind the proper scope:

eventEmitter.on("recordVideo", function() {
    this.recordClip();
}).bind(this);

Or you can make a reference to this via the self methodology:

class Camera {
    constructor(ip) {
        this.ip = ip;
        const self = this; //assign this to self
        eventEmitter.on("recordVideo", function() {
            self.recordClip(); //use self, not this here
        });
    }

    recordClip() {
        console.log("record " + this.ip);
    }
}