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JavaScript

tayjohn
PLUS
tayjohn
Courses Plus Student 1,595 Points

Calling a defined function from a callback function

I have a function defined like this:

this.notifyObservers = function(obj) {
    for (var i = 0; i < this.observer.length; i++){
        this.observer[i](this, obj); //observer array is defined earlier in the code
    }
}

Now I have an ajax call in another function that takes a callback function as one of it's parameters:

this.getDish = function (id, cb) {
    $.ajax( {
      ...
        success: function(data) {
            cb(data);
        }
      ...
  )
}

Now when I call this.getDish, I would like to call this.notifyObservers like this:

    this.getDish(id, function(data) {
        ...
        this.notifyObservers(this);
        ...
    });

However I keep getting a typeError that notifyObservers is not a function:

 Uncaught TypeError: this.notifyObservers is not a function

My assumption is that it has something to do with the scope, but could someone explain it further? And how could I go to solve this?

(The "..." notation is meant to mean omitted code)

2 Answers

Thomas Nilsen
Thomas Nilsen
14,957 Points

Your mistake is here;

 this.getDish(id, function(data) {
        ...
        this.notifyObservers(this); //the first 'this' on the line doesn't refer to the 'this' you think
        ...
    });

Here is a small example to demonstrate the difference;

var myObject = {
    whatsThis: function() {
        console.log(this); //here - 'this' refers to 'myObject'
        function whatsThisThen() {
            console.log(this); //here - 'this' does NOT refer to 'myObject', but the global object
         }
        whatsThisThen();
    }
}

myObject.whatsThis();

So having looked very fast through your code, you should be able to do something like this;

this.getDish(id, function(data) {
        ...
        this.notifyObservers(this);
        ...
    }.bind(this)); //Sets it to the correct 'this'

Working with this - is always fun.

Also - this is a very good article when it comes to understanding 'this' as well as a few other native javascript-methods.

tayjohn
tayjohn
Courses Plus Student 1,595 Points

Thank you for your detailed response.

After further research, another trick I found out was declaring a variable outside the function that points to "this" as in the global object.

Regarding the example you provided:

var myObject = {
    whatsThis: function() {
        console.log(this); //here - 'this' refers to 'myObject'
        function whatsThisThen() {
            console.log(this); // How come this does not refer to myObject anymore?
         }
        whatsThisThen();
    }
}

myObject.whatsThis();

How come the "this" in the whatsThisThen() doesn't refer to myObject anymore? myObject contains a property whatsThis that contains a function. The first "this" refers to myObject which makes sense.

But the second "this" in whatsThisThen is still part of the property "whatsThis" that is part of myObject.. So why wouldn't it refer back to myObject but instead back to the global object?