Javascript subclass doesnt inherit parent variable

Question

I'm working with Javascript "classes", I have a parent "class" with a variable and a function, something like this

function WS() {
  this.auth = { client: '', password: '' };
  this.Execute = function(url, callback) {
    soap.createClient(url, function(err, client) {
      if (err) return callback(err);
      return callback(null, client);
    });
  }
}

The "subclass" uses this function and variable, like this

function Stats() {
  this.url = 'http://';

  this.emailsByDate = function(params, callback) {
    this.Execute(this.url, function(err, client) {
      if (err) return callback(err);
      client.Get(this.auth.concat(params), function(err, results) {
        if (err) return callback(err);
        return callback(results);
      });
    });
  }
}

Stats.prototype = new WS;

I'm getting through this.Execute() function, but this.auth variable is undefined, why is that

You might replace the Stats.prototype = new WS; with util.inherits(Stats, WS); and then adding your prototype functions after that line. — mscdex
– mscdex, Commented Aug 8, 2014 at 17:46
See How to access the correct this / context inside a callback? and Benefits of using Object.create for inheritance — Felix Kling
– Felix Kling, Commented Aug 8, 2014 at 17:59

Patrik Oldsberg · Accepted Answer · 2014-08-08 17:45:20Z

2

The context in which you are accessing this.auth isn't your Stats function, but instead your anonymous callback function.

You could either save the value of this.auth outside the callback function:

function Stats() {
  var auth = this.auth
  this.url = 'http://';

  this.emailsByDate = function(params, callback) {
    this.Execute(this.url, function(err, client) {
      if (err) return callback(err);
      client.Get(auth.concat(params), function(err, results) {
        if (err) return callback(err);
        return callback(results);
      });
    });
  }
}

Or you can bind the context of your callback function to that of your Stats function:

function Stats() {
  this.url = 'http://';

  this.emailsByDate = function(params, callback) {
    this.Execute(this.url, function(err, client) {
      if (err) return callback(err);
      client.Get(this.auth.concat(params), function(err, results) {
        if (err) return callback(err);
        return callback(results);
      });
    }.bind(this));
  }
}

answered Aug 8, 2014 at 17:45

Patrik Oldsberg

1,55012 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

cjav_dev Over a year ago

In addition to having the correct binding of this you might consider calling WS.call(this); in your Stats constructor.

Patrik Oldsberg Over a year ago

@w1zeman1p That depends, it might be intended that the auth property is assigned to the prototype, as all the instances might want to share credentials. In that case WS.call(this) would make auth and Execute properties of each instance of Stats, and that might not be the desired effect. I think the real issue is that Execute and -- if intended -- auth should be moved to the prototype of WS, and the Stats prototype should inherit from the WS prototype using Object.create. Then the WS.call(this) call would make a lot of sense.

Collectives™ on Stack Overflow

Javascript subclass doesnt inherit parent variable

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related