What is the difference between this and var within a JavaScript class? [duplicate]

The difference is in the instances you create from this "class" . The instance only exposes what is in this of it or has been inherited :

var objeto = new Func(); 
objeto.fn_this();    // "this"
objeto.fn_func();    // ERRO - objeto.fn_func is not a function
objeto.fn_var();     // ERRO - objeto.fn_var is not a function

So basically you should put in this what you want it to be exposed (but often better put in the prototype). The other two forms you use for what is "private", which can only be accessed at scope of the builder.

It's worth to also understand The difference between function fn(){} and var fn = function(){} , and still why fn_func and fn_var can be called from within fn_this . See also: What is e How does the Javascript context work?

The this operator will always get the parent operator. If the parent is not defined by default it will be the window object.

The var , as the name already indicates, is a variable. However, it can receive objects and functions.

When you declare a function in Javascript, you can do it normally with function or use var fn = function() {} . In this way we say that the fn variable receives a function, and to be able to use it it would be fn(); .

When it is inside an object, the reference will be to the object itself, however we have to be careful about the scope. If it is in a function the reference will be the function. So you can find by then codes using var self = this; for the reference still to continue in the class.