Accessing instance variable from parent function

Question

I have an object that inherit from another one. In the child object, I initialize some instance variables. In the parent object, I have a method that call private functions that use these instance variables.

function objA() {
  this.funcA = function() {
    alert(this.var);
    funcB();
  };
  function funcB() {
    alert(this.var);
  };
}

objB.prototype = new objA();
objB.prototype.constructor = objB;
function objB() {
  this.var ="hello";
  this.funcA();
}

the first alert gives "hello", the second undefined...

I understant that 'this' in javascript is the identity of the object calling the function, not the instance itself... I assume that when calling funcB, i'm in some kind of parent context where the variable does not exists...

I'm a pure javascript newbie, and I can't figure out how to solve that. Of course, I could pass it as agument, but it's not very OOP, and in my project, there are a lot more variables to make availables...

Thanks

Answer 1

Please don't abuse JavaScript's ability to define functions after they're used. It's really difficult to read code like that.

As you noticed, the problem is not in your inheritance chain, but in how you want to access this from the funcB function. Calling a function by doing foo() is not the same as calling obj.foo() or this.foo(). The value of this depends on how you call the function. When you do obj.foo() this will be obj. But if you call foo() the value of this will be the window object.

You can change the value of this inside a function by using Function.prototype.call and Function.prototype.apply so writing foo.call(obj) (now this will be obj). With that in mind your code will look like this:

function objA() {
  function funcB() {
    alert(this.var);
  }
  this.funcA = function() {
    alert(this.var);
    funcB.call(this); // <- notice .call()
  };
}

function objB() {
  this.var ="hello";
  this.funcA();
}
objB.prototype = new objA();
objB.prototype.constructor = objB;

Also, keep in mind that property names can't be reserved words (for example, "var") in older browsers.