Question

In C++, the language I’m most comfortable with, usually one declares an object like this:

class foo
{
public:
    int bar;
    int getBar() { return bar; }
}

Calling getBar() works fine (ignoring the fact that bar might be uninitialized). The variable bar within getBar() is in the scope of class foo, so I don’t need to say this->bar unless I really need to make it clear that I’m referring to the class’ bar instead of, say, a parameter.

Now, I’m trying to get started with OOP in Javascript. So, I look up how to define classes and try the same sort of thing:

function foo()
{
     this.bar = 0;
     this.getBar = function() { return bar; }
}

And it gives me bar is undefined. Changing the bar to this.bar fixes the issue, but doing that for every variable clutters up my code quite a bit. Is this necessary for every variable? Since I can’t find any questions relating to this, it makes me feel like I’m doing something fundamentally wrong.

---

EDIT: Right, so, from the comments what I’m getting is that this.bar, a property of an object, references something different than bar, a local variable. Can someone say why exactly this is, in terms of scoping and objects, and if there’s another way to define an object where this isn’t necessary?

Answer 1

JavaScript has no classes class-based object model. It uses the mightier prototypical inheritance, which can mimic classes, but is not suited well for it. Everything is an object, and objects [can] inherit from other objects.

A constructor is just a function that assigns properties to newly created objects. The object (created by a call with the new keyword) can be referenced trough the this keyword (which is local to the function).

A method also is just a function which is called on an object – again with this pointing to the object. At least when that function is invoked as a property of the object, using a member operator (dot, brackets). This causes lots of confusion to newbies, because if you pass around that function (e.g. to an event listener) it is “detached” from the object it was accessed on.

Now where is the inheritance? Instances of a “class” inherit from the same prototype object. Methods are defined as function properties on that object (instead of one function for each instance), the instance on which you call them just inherits that property.

Example:

function Foo() {
    this.bar = "foo"; // creating a property on the instance
}
Foo.prototype.foo = 0; // of course you also can define other values to inherit
Foo.prototype.getBar = function() {
    // quite useless
    return this.bar;
}

var foo = new Foo; // creates an object which inherits from Foo.prototype,
                   // applies the Foo constructor on it and assigns it to the var
foo.getBar(); // "foo" - the inherited function is applied on the object and
              // returns its "bar" property
foo.bar; // "foo" - we could have done this easier.
foo[foo.bar]; // 0 - access the "foo" property, which is inherited
foo.foo = 1;  // and now overwrite it by creating an own property of foo
foo[foo.getBar()]; // 1 - gets the overwritten property value. Notice that
(new Foo).foo;     // is still 0

So, we did only use properties of that object and are happy with it. But all of them are “public”, and can be overwritten/changed/deleted! If that doesn’t matter you, you’re lucky. You can indicate “privateness” of properties by prefixing their names with underscores, but that’s only a hint to other developers and may not be obeyed (especially in error).

So, clever minds have found a solution that uses the constructor function as a closure, allowing the creating of private “attributes”. Every execution of a javascript function creates a new variable environment for local variables, which may get garbage collected once the execution has finished. Every function that is declared inside that scope also has access to these variables, and as long as those functions could be called (e.g. by an event listener) the environment must persist. So, by exporting locally defined functions from your constructor you preserve that variable environment with local variables that can only be accessed by these functions.

Let’s see it in action:

function Foo() {
    var bar = "foo"; // a local variable
    this.getBar = function getter() {
        return bar; // accesses the local variable
    }; // the assignment to a property makes it available to outside
}

var foo = new Foo; // an object with one method, inheriting from a [currently] empty prototype
foo.getBar(); // "foo" - receives us the value of the "bar" variable in the constructor

This getter function, which is defined inside the constructor, is now called a “privileged method” as it has access to the “private” (local) “attributes” (variables). The value of bar will never change. You also could declare a setter function for it, of course, and with that you might add some validation etc.

Notice that the methods on the prototype object do not have access to the local variables of the constructor, yet they might use the privileged methods. Let’s add one:

Foo.prototype.getFooBar = function() {
    return this.getBar() + "bar"; // access the "getBar" function on "this" instance
}
// the inheritance is dynamic, so we can use it on our existing foo object
foo.getFooBar(); // "foobar" - concatenated the "bar" value with a custom suffix

So, you can combine both approaches. Notice that the privileged methods need more memory, as you create distinct function objects with different scope chains (yet the same code). If you are going to create incredibly huge amounts of instances, you should define methods only on the prototype.

It gets even a little more complicated when you are setting up inheritance from one “class” to another – basically you have to make the child prototype object inherit from the parent one, and apply the parent constructor on child instances to create the “private attributes”. Have a look at Correct javascript inheritance, Private variables in inherited prototypes, Define Private field Members and Inheritance in JAVASCRIPT module pattern and How to implement inheritance in JS Revealing prototype pattern?