Accessing a variable outside of Websocket.onmessage()

Question

This question concerns the WebSocket API(i.e. var webSocketServer = require('websocket').server;) in NodeJS.

function Game(canvas) {
    this.wArray;
    this.runConnection();
    // I want to be able to see changes in variables at this point
    console.log(this.wArray[1][2]); // is out of scope or something
}

_p = Game.prototype;

_p.runConnection = function() {
    this.connection = new WebSocket('ws://localhost:1337');
    this.connection.onmessage = function (message) {
           this.wArray = JSON.parse(message.data);
    };
    // code here runs before code inside onmessage, it must be asychronous
};

So I when I receive a message from the server, I should be able to take that message and update some variables and such in my code. At the moment, it seems all I can do is update things that are inside of the onmessage function. All of the examples online simply show people using console.log() inside of onmessage. I want the server to be able to send my client information, and then use that information to update certain aspects of my game as it is running. I think that there is some level of asychronousness about onmessage().

Please show me how to take the data that is passed to me via WebSocket.onmessage() and store it in variables that can be accessed throughout my game.

Carl K Carl K · Accepted Answer · 2015-07-09T02:42:57

The onMessage is fired asynchronously as a callback function. Therefore, you have to be mindful of the variable scope to which you are working. There are a number of possibilities to use: proxies, altered scope, functions, bind(), etc that you can search prior answers for. (There are many)

As a quick example, you could potentially use a self variable to access this in other places; however, it obviously depends on the purpose of this script overall.

function Game(canvas) {
    this.wArray = [];
    this.runConnection();
    console.log(this.wArray[1][2]); 
   //log()  will likely not work as you should wait for [1][2] to be filled
}

_p = new Game();

_p.runConnection = function() {
    this.connection = new WebSocket('ws://localhost:1337');
    var self = this;
    this.connection.onmessage = function (message) {
           self.wArray.push(JSON.parse(message.data));
        };
};

Accessing a variable outside of Websocket.onmessage()

2 Answers