无法通过回调设置类成员变量

Question

I'm trying to set a class member variable from a callback of a function I'm calling from the class constructor.

To be a bit more specific: I need to set the connection ID in the Connection class constructor based on the Redis INCR result (each client has a 'global' connection ID so I can have multiple nodes).

Here's the code.

class Connection {
  constructor() {
    client.incr('conn_id', (err, reply) => {
      this.connID = reply;
    });
  }
}

var lovely = new Connection();
console.log(`lovely connID is ${ lovely.connID }`);

This is the result: lovely connID is undefined

Answer 1

It seems that client.incr('conn_id' ....) is async , which means the callback will be invoked after your code run .

So

console.log( lovely connID is ${ lovely.connID } ); will be called before the callback

(err, reply) => { self.connID = reply; }

which is similar to this :

class Connection{
  constructor(){
    self=this;
    setTimeout( function(){self.client='somevalue';
                          console.log('value1');}, 10)

  }
}

var a = new Connection();

console.log(a.client); 

running this will result

undefined value1

Answer 2

As others here have mentioned, the issue seems to be that client.incr is asynchronous and your code does not wait for it to resolve before accessing the properties. To remedy this issue, you could try passing in an onReady callback to Connection to enssure the properties will be there before trying to access them. Something along these lines:

 'use strict'; // mock client.incr var client = { incr: (id, callback) => { setTimeout(() => callback(null, 'Hello World'), 0) }}; class Connection { // receive an "onReady" function constructor(onReady) { client.incr('conn_id', (err, reply) => { this.connID = reply; // call "onReady" function, and pass it the class if (typeof onReady === 'function') onReady(this) }); } } new Connection(lovely => { console.log(lovely.connID) }) 

I hope that helps!

Answer 3

In general putting heavy initialization logic in a constructor, especially if it's asynchronous, is not a good idea. As you've found, the constructor has no way to return the information about when the initialization is finished. An alternative is to create a promise for when the connection is ready. Then, in your outside code, you can hang a then off the property to specify the code you want to trun when it's ready.

class Connection {
  constructor() {
    this.connId = new Promise((resolve, reject) =>
      client.incr('conn_id', (err, reply) => {
        if (err) return reject(err);
        resolve(reply);
    });
  }
}

var lovely = new Connection();
lovely.connId . then(connId => console.log(`lovely connID is ${ connID }`);

Answer 4

You are using this inside the callback which might not refer to the same object. If you can set the context object as a parameter of client.incr() , try to do so. Otherwise using self is one of the common way to go around:

class Connection {
  constructor() {
    var self = this // Keep a reference to the current "this"
    client.incr('conn_id', (err, reply) => {
      self.connID = reply;
    });
  }
}