How to assign anonymous function variables to class method variables?

Question

I'm trying to implement communication with coap server by using coap package . My goal is to get response (in coap.request() response event handler) and then pass it to other variables and functions.

Event: 'response'

 function (response) { } 

Emitted when a response is received. response is an instance of IncomingMessage .

If the observe flag is specified, the 'response' event will return an instance of ObserveReadStream . Which represent the updates coming from the server, according to the observe spec.

I've created class someClass , which contains serverRequest() method.

Method serverRequest() takes mandatory options argument (which sets request options for coap.request() ) and optional argument which sets message body if needed. Purpose of this method is to send a request to server and return response , which is instance of coap.IncomingMessage .

const coap = require('coap');

class someClass {
  constructor(someOption) {
    this.someOption = someOption;
  }

  serverRequest(options, messageBody=undefined) {
    const request = coap.request(options);
    let response;
    request.on('response', res => {
      console.log(response); // undefined
      response = res;
      console.log(response);  // valid response
    });
    if (messageBody !== undefined) {
      request.write(messageBody);
    }
    request.end();
    console.log(response);  // undefined
    return response;
  }
}

I send message and obtain response successfully, however response variable inside anonymous function seems to be unique and different from response variable inside serverRequest method.

Question: How can I pass variables from anonymous functions to other scopes?

Answer 1

You can call your method inside the body of an anonymous function:

class Test 
{
  constructor(someOption) {
    this.someOption = someOption;
  }

  myMethod ( data ) {
    //.. do something
  }

  anotherMethod() {
    var data = {answer:42};
    var that = this;
    functionWithCallback( function(differentOption) {
      that.myMethod(data);
      that.someOption = differentOption;
    });
  }       
}

Edit based on comments: Using var that = this; is a common way to cheat the scope. this will always belong to the scope of a function, method or anonymous, both are still functions.

Therefore to keep a reference to the class scope, I assign the this from the class method's scope to var that so that when this changes to the anonymous' function scope, we still have access to it.

In ECMAscript 6 and above when using arrow functions, the this keyword is not re-bound and we don't have to cheat the scope.

class Test 
{
  constructor(someOption) {
    this.someOption = someOption;
  }

  myMethod ( data ) {
    //.. do something
  }

  anotherMethod() {
    var data = {answer:42};
    functionWithCallback( (differentOption) => {
      this.myMethod(data);
      this.someOption = differentOption;
    });
  }        
}

Edit made by question author:

I didn't have problems by using this keyword, script produced same results.