Node.JS Callback Scoping Issues

Question

I'm using JavaScript classes to write a small tool for work. I don't like callbacks nesting and nesting, so I like to break them out into separate functions. I'm trying to run requests, which take a callback, from within a for...of loop. The problem is, when I break the callback out into a separate function, it looses the scope of the for...in loop and it doesn't know what "service.url" is.

Here is my code:

const DATA = require("../resources/data.json");
const Request = require("request");

class Net {
    constructor(){}

    _handleResponses(e, r, b) {
        console.log("--------------------------------------------------------------");

        if(r.statusCode == 404) {
            console.log("\x1b[31m"+`[RESPONSE](Status  ${r.statusCode})`, "\x1b[0m");
            console.log("\x1b[31m"+`Server: ${service.server}`, "\x1b[0m");
            console.log("\x1b[31m"+`Service: ${service.service}`, "\x1b[0m");
            console.log("\x1b[31m"+`Body: ${b}`, "\x1b[0m");
        } else {
            console.log(`[RESPONSE](Status  ${r.statusCode})`);
            console.log(`Server: ${service.server}`);
            console.log(`Service: ${service.service}`);
            console.log(`Body: ${b}`);
        }
    }

    pollServices() {
        for(let service of DATA) {
            Request(service.url, this._handleResponses);
        }
    }
}

module.exports = Net;

Here is the error I get when the "pollServices" function/method runs:

C:\Users\payton.juneau\Desktop\Me\Projects\Node\com.etouchmenu.tools.crystalBall\utilities\net.js:17 console.log(Server: ${service.server}); ^

ReferenceError: service is not defined at Request._handleResponses [as _callback] (C:\Users\payton.juneau\Desktop\Me\Projects\Node\com.etouchmenu.tools.crystalBall\utilities\net.js:17:36) at Request.self.callback (C:\Users\payton.juneau\Desktop\Me\Projects\Node\com.etouchmenu.tools.crystalBall\node_modules\request\request.js:186:22) at Request.emit (events.js:159:13) at Request. (C:\Users\payton.juneau\Desktop\Me\Projects\Node\com.etouchmenu.tools.crystalBall\node_modules\request\request.js:1163:10) at Request.emit (events.js:159:13) at IncomingMessage. (C:\Users\payton.juneau\Desktop\Me\Projects\Node\com.etouchmenu.tools.crystalBall\node_modules\request\request.js:1085:12) at Object.onceWrapper (events.js:254:19) at IncomingMessage.emit (events.js:164:20) at endReadableNT (_stream_readable.js:1062:12) at process._tickCallback (internal/process/next_tick.js:152:19)

Any help would be much appreciated, kind of new to all of this type of stuff..

Why did you remove the content from your question leaving an absolutely worthless reference behind? This is NOT how you use stack overflow. — jfriend00

nem035 nem035 · Accepted Answer · 2018-01-25T22:22:50

When you define 2 methods on a class, they each create their own independent scope:

class Example {
  method1() {
    // scope of method 1 only visible to method 1
    let hiddenInM1 = 'example'; // <-- only available here
  }
  method2() {
    // scope of method 2 only visible to method 2
    // I don't know what m1 is
  }
}

There are 3 ways for these methods to share values with each other.

1. Using a variable available in an outer scope in which both of these methods belong to:

let sharedGlobal = 'example';
class Example {
  method1() {
    // I can see sharedGlobal
  }
  method2() {
    // I also can see sharedGlobal
  }
}

This is usually ill-advised because global state is prone to bugs.

2. Through the context of the class those methods belong to (via this)

class Example {
  constructor() {
    this.sharedClassVar = 'example'
  }
  method1() {
    // I can see this.sharedClassVar
  }
  method2() {
    // I also can see this.sharedClassVar
  }
}

3. By passing arguments to each other.

class Example {
  method1(fromMethod2) {
    // I can receive stuff from method2
  }
  method2() {
    this.method1('example')
  }
}

If you look at your code, none of these patterns are present and thus _handleResponses doesn't have access to the service defined in pollServices.

The easiest change you can make is to pass the service yourself:

_handleResponses(service, e, r, b) {
  //             ^^^^^^^ receive the service here
}

pollServices() {
  for (let service of DATA) {
    Request(service.url, (...args) => this._handleResponses(service, ...args))
    //                                                      ^^^^^^^ pass the service with the args
  }
}

Node.JS Callback Scoping Issues

1 Answers