Re: Replacing a private function an keeping access to private variables

[Thomas 'PointedEars' Lahn]

Gregor Kofler wrote:

function foo() {
   var privateVar = 42;
 
   var privateFunc = function() {
     alert(privateVar);
   };
 
   return {
     replacePF: function(f) { privateFunc = f; },
     accessPF: function() { privateFunc(); }
   };
}
 
var obj = foo();

I am curious, why did you choose to use a factory instead of a constructor?

obj.accessPF(); // 42
 
obj.replacePF(function() {
   window.alert("The answer is "+privateVar);
});
obj.accessPF(); // privateVar is undefined

It's a closure and `privateVar' in privateFunc() is bound to the execution
context of foo(), in which `privateFunc' was declared a variable. However,
the function expression in the obj.replacePF() call creates a closure in
which `privateVar' is bound to the execution context in which the call is
executed and the anonymous Function object is created, and in which scope
chain no object has such a property.

ISTM the only way to do this is to have a public method that returns
`privateVar', which would become "protected" through this: Either

function foo()
{
var privateVar = 42;
 
var privateFunc = function() {
window.alert(privateVar);
};
 
return {
getPrivateVar: function() { return privateVar; },
replacePF: function(f) { privateFunc = f; },
accessPF: function() { privateFunc(); }
};
}
 
var obj = foo();
 

or

function Foo()
{
var privateVar = 42;
 
var privateFunc = function() {
window.alert(privateVar);
};
 
this.getPrivateVar = function() { return privateVar; };
this.replacePF = function(f) { privateFunc = f; };
this.accessPF = function() { privateFunc(); };
}
 
var obj = new Foo();
 
and then
 
// 42
obj.accessPF();
 
obj.replacePF(function() {
window.alert("The answer is " + obj.getPrivateVar());
});
 
// "The answer is 42"
obj.accessPF();

(It's the usual simple getter and setter implementation as it is.)


HTH

PointedEars
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[Gregor Kofler]

Thomas 'PointedEars' Lahn meinte:

I am curious, why did you choose to use a factory instead of a constructor?

I've started switching from constructor instantiation to a factory like
pattern after reading Crockfords new book, and realizing that I tend to
have objects with a meager amount of public properties, but lots of
private functions. On the other hand I do need a factory occassionally.
Since I try to avoid mixing the patterns, I stick to factories most of
the time now.

It's a closure and `privateVar' in privateFunc() is bound to the execution
context of foo(), in which `privateFunc' was declared a variable. However,
the function expression in the obj.replacePF() call creates a closure in
which `privateVar' is bound to the execution context in which the call is
executed and the anonymous Function object is created, and in which scope
chain no object has such a property.

Yep, I know the reason, why I can't access it this way, but I was rather
tired yesterday evening and had this feeling that missed something.

ISTM the only way to do this is to have a public method that returns
`privateVar', which would become "protected" through this

[snip]

Ok, that's the approach I already use.

HTH

Sort of.

Gregor


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