Question

I wanted to know how can we read the value of a local variable in another class / instance of a class in javascript.

eg:

I have a method in a class :

myClass1.prototype.myMethod0 = function()

{
 this._myVar = null; //initialize this._myVar
}

myClass1.prototype.myMethod1 = function(list)
{
 this._myVar = msg.list;
}

and

myClass1.prototype.myMethod2 = function()
{
 //do something 
// and update the list like say:

list1 = this._myVar; //access the this_myVar.

}

and in my another calss say,

myClass2.prototype.myMethod = function()
{
 //call the class1's method here..

myClass1.prototype.myMethod2();

}

myMethod2 is a callback and i bind it in myClass2.

It means, actually, myMethod2 is being called like this: myClass1.callback();

But my problem is , when i call myClass1.prototype.myMethod2();, the list1 = this._myVar; is not getting updated and it is becoming undefined. I am not getting the fix for the same.

The problem is the variable, “this._myVar” is “undefined” in myMethod2 of myclass1

Answer 1

What do is possible but not the way you did it:

1. Without the var keyword, a variable is not local but global. So your code actually created a global variable this_myVar which is shared by all JavaScript code on the same page.

2. list1 will be undefined if myMethod1() was never called.

See [here for an introduction into JavaScript OO programming][1].

[EDIT] I see you updated your question and the variable is now accessed via this. The automatic variable this is only assigned when the function is called as a “method”.

myClass1.prototype.myMethod2() will call the original definition and this will be undefined.

You have to use code like this:

var inst = new myClass1();
inst.myMethod2();

Inside of myMethod2, the variable this will have the same value as inst if you call it like that.

[EDIT2] If you want to access instance properties, then you must call myMethod2() via an instance of myClass1 – JavaScript doesn’t try to read your mind what your code might mean.

So at the time when you want to call any method of myClass1, you must have an instance of it. Try to create one in myClass2:

var myClass2 = function() {
    this.inst1 = new myClass1();
}

myClass2.prototype.myMethod = function() {
    this.inst1.myMethod2();
}

Alternatively, you will have to pass the instance to the method:

myClass2.prototype.myMethod = function(inst1) {
    inst1.myMethod2();
}

But if you call myMethod2() like any other global method, this won’t have any useful value. There is way to say “When I call a function with a prefix of myClass1.prototype., then look for an instance of myClass1 and put that into this“. What should JavaScript do when there are 15 instances of this type? Select one by random?