Asked  7 Months ago    Answers:  5   Viewed   27 times

I have a JavaScript file which is loaded by require.

// loaded by require()

var a = this; // "this" is an empty object
this.anObject = {name:"An object"};

var aFunction = function() {
    var innerThis = this; // "this" is node global object
};

aFunction();

(function(anyParameter){
    console.log(anyParameter.anObject);
})(
    this // "this" is same having anObject. Not "global"
);

My question is: this in var a = this; is an empty object whereas this statements in functions are shadows of node.js global object. I know this keyword is different in functions but I could not understand why first this is not equal to global and this in functions equals to global.

How does node.js inject global to this in function scopes, and why it does not inject it to the module scope?

 Answers

64

Here's a few fundamental facts you must understand to clarify the situation:

  • In the top-level code in a Node module, this is equivalent to module.exports. That's the empty object you see.

  • When you use this inside of a function, the value of this is determined anew before each and every execution of the function, and its value is determined by how the function is executed. This means that two invocations of the exact same function object could have different this values if the invocation mechanisms are different (e.g. aFunction() vs. aFunction.call(newThis) vs. emitter.addEventListener("someEvent", aFunction);, etc.) In your case, aFunction() in non-strict mode runs the function with this set to the global object.

  • When JavaScript files are required as Node modules, the Node engine runs the module code inside of a wrapper function. That module-wrapping function is invoked with a this set to module.exports. (Recall, above, a function may be run with an abitrary this value.)

Thus, you get different this values because each this resides inside a different function: the first is inside of the Node-created module-wrapper function and the second is inside of aFunction.

Tuesday, June 1, 2021
 
jeremyharris
answered 7 Months ago
74

You need to manually create a symlink /usr/bin/node. Shortcut for bash compatible shells:

sudo ln -s `which nodejs` /usr/bin/node

Or if you use non-standard shells, just hardcode the path you find with which nodejs:

sudo ln -s /usr/bin/nodejs /usr/bin/node

Later edit

I found this explanation in the link you posted

There is a naming conflict with the node package (Amateur Packet Radio Node Program), and the nodejs binary has been renamed from node to nodejs. You'll need to symlink /usr/bin/node to /usr/bin/nodejs or you could uninstall the Amateur Packet Radio Node Program to avoid that conflict.

Later later edit

It's been a while since I answered this. Although the solution I posted up here worked for me several times, users have reported a few more solutions within the comments:

From @user229115

sudo update-alternatives --install /usr/bin/node node /usr/bin/nodejs 10

From AskUbuntu (user leftium)

sudo apt-get --purge remove node
sudo apt-get --purge remove nodejs
sudo apt-get install nodejs
Tuesday, June 1, 2021
 
WooDzu
answered 7 Months ago
36

R.java is a class (with inner classes, like layout or string) generated during the build process with references to your app's resources. Every resource you create (or which is provided by Android) is referenced by an integer in R, called a resource id.

R.layout.* references any layout resource you have created, usually in /res/layout. So if you created an activity layout called activity_main.xml, you can then use the reference in R.layout.activity_main to access it. Many built-in functionality readily accepts such a resource id, for example setContentView(int layoutResid) which you use during the creation of your activity and where you probably encountered this particular example.

If you create a string resource (in strings.xml) like this:

<string name="app_name">Application name</string>

it will get a new reference in R.string.app_name. You can then use this everywhere where a string resource is accepted, for example the android:label for your application in AndroidManifest.xml, or on a TextView; either in the xml:

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/app_name"
    />

or in code: textview.setText(R.string.app_name).

You can access resources programmatically using the Resources class, to which you can get a reference by calling getResources on any context (like your activity). So for example you can get your app name described above in your activity by calling this.getResources().getString(R.string.app_name).

You can also supply different resources for different device properties/settings (like screen size or language), which you can access using the same references in R. The easiest example here, imho, is strings: if you add a new values folder in /res with a language specifier (so /res/values-nl for Dutch) and you add strings with the same identifier but a different translation and the resource management system cleverly figures out which one to provide for you based on your user's device.

I hope this helps a bit. For more information on resources see the documentation.

Tuesday, August 10, 2021
 
user2725742
answered 4 Months ago
84

$this refers to the class you want to use. for instance if you see $this->Post->find('all'), you're trying to access the class Post that extends AppModel. Through conventions, the Post Model uses the posts table in your database. $this->Post->find('all') is used because the AppModel has the find() method and the Post model extends AppModel.

http://api.cakephp.org/class/app-model http://book.cakephp.org/view/22/CakePHP-Conventions

Thursday, August 19, 2021
 
user336786
answered 4 Months ago
100

You're not storing the connections on the server side. You're just setting them up on the server to communicate directly back and forth to the server. If you want messages going to the server to be sent back out to everyone, you need to set up the .on('message', ...) function for each connection on the server to have that behavior. To do this, you'll need to store the connections as they are created. Try this:

var WebSocketServer = require('websocket').server;
var http = require('http');

var server = http.createServer(function(request, response) {
    console.log((new Date()) + ' Received request for ' + request.url);
    response.writeHead(404);
    response.end();
});
server.listen(5050, function() {
    console.log((new Date()) + ' Server is listening on port 5050');
});

wsServer = new WebSocketServer({
    httpServer: server,
    // You should not use autoAcceptConnections for production 
    // applications, as it defeats all standard cross-origin protection 
    // facilities built into the protocol and the browser.  You should 
    // *always* verify the connection's origin and decide whether or not 
    // to accept it. 
    autoAcceptConnections: false
});

function originIsAllowed(origin) {
  // put logic here to detect whether the specified origin is allowed. 
  return true;
}

//create an array to hold your connections
var connections = [];

wsServer.on('request', function(request) {
    if (!originIsAllowed(request.origin)) {
      // Make sure we only accept requests from an allowed origin 
      request.reject();
      console.log((new Date()) + ' Connection from origin ' + request.origin + ' rejected.');
      return;
    }
    var connection = request.accept('echo-protocol', request.origin);

    //store the new connection in your array of connections
    connections.push(connection);
    
    console.log((new Date()) + ' Connection accepted.');
    connection.on('message', function(message) {
        if (message.type === 'utf8') {
            console.log('Received Message: ' + message.utf8Data);

            //send the received message to all of the 
            //connections in the connection array
            for(var i = 0; i < connections.length; i++) {
                connections[i].sendUTF(message.utf8Data);
            }
        }
        else if (message.type === 'binary') {
            console.log('Received Binary Message of ' + message.binaryData.length + ' bytes');
            connection.sendBytes(message.binaryData);
        }
    });
    connection.on('close', function(reasonCode, description) {
        console.log((new Date()) + ' Peer ' + connection.remoteAddress + ' disconnected.');
    });
});
Monday, October 25, 2021
 
Nasenbaer
answered 1 Month ago
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