passing private variables

[jeffg]

The answer probably is staring me in the face and I don't see it, so
I'll just ask.

I try to limit the scope of variables and avoid global variables. But
sometimes I get stumped by the following situation and need some help:

A function sets the value of a variable that is limited to the scope of
the function. Later, another function needs the variable but does not
need to call the first function and trigger everything else in that
function. What's the best practice way of getting at that variable
without making it global?

[Michael Winter]

Re: passing private variables

On 7 Dec 2004 06:55:12 -0800, jeffg <jeffgutsell@fu se.net> wrote:

[snip]
[color=blue]
> I try to limit the scope of variables and avoid global variables.[/color]

Excellent! :D

[snip]
[color=blue]
> A function sets the value of a variable that is limited to the scope of
> the function. Later, another function needs the variable but does not
> need to call the first function and trigger everything else in that
> function. What's the best practice way of getting at that variable
> without making it global?[/color]

It may depend on your actual code. A URL would be nice. That said, the
general approach would be to have the two functions share the same
"private" scope. For example:

/* Global variables that will soon hold
* references to your functions.
*/
var myA, myB;

/* Define and call an anonymous function. When this occurs,
* a new scope will be created where you can place your
* common, local variables.
*/
(function() {
var myCommon;

/* These inner functions for a closure which keep the
* local variable, myCommon, in memory once the anonymous
* function returns.
*/
myA = function() {
/* ... */
};
myB = function() {
/* ... */
};
/* Note the call here... */
})();

Both of the functions, myA and myB, can now access myCommon. The pattern
is similar if myA and myB were methods of an object.

Take a look at <URL:http://www.crockford.c om/#javascript> and
<URL:http://www.jibbering.c om/faq/faq_notes/closures.html> for articles
that delve into this sort of thing more deeply.

Hope that helps,
Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.

[VK]

Re: passing private variables

JavaScript doesn't have a fine-grained scope settings (like main - local -
my in Perl, public - protected - private in Java etc.)
Actually, if you need to keep the variable value between function calls,
there is nothing non-academical (if you are concerned about that) in making
such variable global.

If you cannot determine in advance how many global variable to declare, you
can use an object as a "variable holder". I would suggest do not trash up
the window, just make a new holder with a unusual name (to do not override
it by mistake):

var $_glob = new Object();
....
$_glob[variableName] = variableValue;
....
someFunction($_ glob[variableName]);
....

[Michael Winter]

Re: passing private variables

On Tue, 7 Dec 2004 16:23:14 +0100, VK <schools_ring@y ahoo.com> wrote:
[color=blue]
> JavaScript doesn't have a fine-grained scope settings (like main - local
> - my in Perl, public - protected - private in Java etc.)[/color]

Not through the use of keywords, no, but private and public is still
simple to achieve. Protected is more tricky, though. See my other post and
the links there.
[color=blue]
> Actually, if you need to keep the variable value between function calls,
> there is nothing non-academical (if you are concerned about that) in
> making such variable global.[/color]

If that code is going to be reused, I disagree. Library-like code should
be completely self-contained and expose as little as necessary to make
sure clashes don't occur.
[color=blue]
> If you cannot determine in advance how many global variable to declare,
> you can use an object as a "variable holder".[/color]

That's effectively the same thing. The only difference is that the
variable/activation object of a function is used, rather than a property
of the global object.

[snip]

Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.