Private methods called from public methods

R

Robert

Hi,
I have been reading the article at
http://www.crockford.com/javascript/private.html
and I was wondering if there also was some way to be able to have
private methods that can be called from public methods, but not from
outside.
Just throwing an exception if a private method was called from outside
would be sufficient, but I don't know how to determine if the method was
called from outside. Anyone else thought about doing this?

Robert.
 
V

VK

Robert said:
I was wondering if there also was some way to be able to have
private methods that can be called from public methods, but not from
outside.
Click to expand...

A bit strange question. The whole idea of private methods is that they
*cannot* be called outside. If you can call a method from outside, then
it is not private. ;-)

Could you post some code or an object structure you're trying to
achieve?
 
R

Robert

VK said:
A bit strange question. The whole idea of private methods is that they
*cannot* be called outside. If you can call a method from outside, then
it is not private. ;-)
Click to expand...

Of course I would prefer that the private method could not be called,
but I would be satisfied with an error.
Could you post some code or an object structure you're trying to
achieve?
Click to expand...

The code I am thinking of usual public method for black-box
functionality and private methods for internal support functions.

For example:

function List()
{
this.array = new Array();
}
//public
List.prototype.insert = insert;
//private
List.prototype.shiftRight = shiftRight;

function insert(obj, index)
{
this.shiftRight(index);
this.array[index] = obj;
}

function shiftRight(index)
{
// move array elements starting at index one to the right
}

The "solution" at http://www.crockford.com/javascript/private.html would
mean that I have to move everything inside the List constructor.
Insert must become a privileged method and shiftRight a private method.
I prefer not having to move everything inside to keep the code more
legible. If only a method could detect if the call came from in or
outside then I would be satisfied.

Robert.
 
V

VK

Robert said:
The code I am thinking of usual public method for black-box
functionality and private methods for internal support functions.
Click to expand...

This is the way I'm using (please not I'm not a real OOP expert of any
kind):

<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type"
content="text/html; charset=iso-8859-1">
<script>
function List(arr) {
this.array = arr || [];
this.count = List.count;
}
List.count = function() {
if (this instanceof List) {
return this.array.length;
}
else {
throw new Error('Private method call ourside of scope');
}
}

var obj = new List([1,2,3]);
try {
alert( obj.count() );
alert( List.count() );
}
catch(e) {
alert(e.message);
}
</script>
</head>

<body>

</body>
</html>


There are other ways different in approaches and needed amount of
different brackets. I'm using this one as - IMHO - the simpliest one as
it doesn't require to twist your brains every second by trying to
figure out what "this" is pointing to in this or that place.

The drawback of this way is that JScript.Net in quick mode doesn't
allow Function object augmentation. So if you plan to port some your
solutions on JScript.Net, you may better use something else. From the
other side JScript.Net has regular "public" and "private" modifiers, so
the problem as such is not applicable there.
 
R

Robert

VK said:
Robert said:
The code I am thinking of usual public method for black-box
functionality and private methods for internal support functions.
Click to expand...


This is the way I'm using (please not I'm not a real OOP expert of any
kind):

function List(arr) {
this.array = arr || [];
this.count = List.count;
}
List.count = function() {
if (this instanceof List) {
return this.array.length;
}
else {
throw new Error('Private method call ourside of scope');
}
}
Click to expand...

This checks if it is called AS an instance or static method.
It does not check if it is called FROM a list instance method.

In my example this in the shiftRight method would be an instance of
List, because it is called from the insert method.
 
R

Robert

Robert said:
VK said:
Robert said:
The code I am thinking of usual public method for black-box
functionality and private methods for internal support functions.
Click to expand...



This is the way I'm using (please not I'm not a real OOP expert of any
kind):

function List(arr) {
this.array = arr || [];
this.count = List.count;
}
List.count = function() {
if (this instanceof List) {
return this.array.length;
}
else {
throw new Error('Private method call ourside of scope');
}
}
Click to expand...


This checks if it is called AS an instance or static method.
It does not check if it is called FROM a list instance method.

In my example this in the shiftRight method would be an instance of
List, because it is called from the insert method.
Click to expand...

And if it's called from outside as list.shiftRight() then this would
still be and instance of List..

I forgot to add this.
 
V

VK

Robert said:
Robert said:
VK said:
Robert wrote:

The code I am thinking of usual public method for black-box
functionality and private methods for internal support functions.



This is the way I'm using (please not I'm not a real OOP expert of any
kind):

function List(arr) {
this.array = arr || [];
this.count = List.count;
}
List.count = function() {
if (this instanceof List) {
return this.array.length;
}
else {
throw new Error('Private method call ourside of scope');
}
}
Click to expand...


This checks if it is called AS an instance or static method.
It does not check if it is called FROM a list instance method.

In my example this in the shiftRight method would be an instance of
List, because it is called from the insert method.
Click to expand...

And if it's called from outside as list.shiftRight() then this would
still be and instance of List..

I forgot to add this.
Click to expand...
Program in C++ and use public/private/protected modifiers as descent
people do :)
Seriously: JavaScript doesn't have such modifiers, so it has to be
severely hacked to get anything close to what you get by a single
modifier in say Java.

Here is the most popular (AFAIK) way:
<http://www.litotes.demon.co.uk/js_info/private_static.html#mProt>

P.S. I keep promising to myself to eventually understand what and how
are they doing - but just losing it on the 2nd pair of brackets :-(
:)
 
R

Robert

VK said:
Program in C++ and use public/private/protected modifiers as descent
people do :)
Seriously: JavaScript doesn't have such modifiers, so it has to be
severely hacked to get anything close to what you get by a single
modifier in say Java.

Here is the most popular (AFAIK) way:
<http://www.litotes.demon.co.uk/js_info/private_static.html#mProt>
Click to expand...

Thanks. I'll try to read it soon.
And if all else fails I will just assume that 'private' methods are not
called by using a convention such as clearly stating they are private
and using a different syntax for the function names.
 
V

VK

Robert said:
And if all else fails I will just assume that 'private' methods are not
called by using a convention such as clearly stating they are private
and using a different syntax for the function names.
Click to expand...

Here's the voice of a Man Who Got the Life Right! :)
 
R

RobG

Robert said:
Hi,
I have been reading the article at
http://www.crockford.com/javascript/private.html
and I was wondering if there also was some way to be able to have
private methods that can be called from public methods, but not from
outside.
Click to expand...

A method that can call a private method is called 'privileged'. From
memory, the article provides an example of a private & privileged
functions like:


var aFn = (function(){
function privateMethod(x){
alert('privateMethod called with: ' + x);
};

return {
privilegedMethod : function(x){
privateMethod(x);
}
};
})();

aFn.privilegedMethod('blah');


JavaScript is an object-based language, it isn't fully OO like C++. You
can't declare a method as private, you can only infer it from the code
structure as above.

Just throwing an exception if a private method was called from outside
would be sufficient, but I don't know how to determine if the method was
called from outside. Anyone else thought about doing this?
Click to expand...

Make the method so it can't be called from outside and you're done.
 
M

Michael Winter

That's certainly a reasonable approach, as long as the convention is
clear, simple, and well-documented. However, I really don't see how

function List() {
var data = [];

this.insert = function(value, index) {
shiftRight(index);
data[index] = value;
};


function shiftRight(i) {
for (var j = data.length; j > i; j--) {
data[j] = data[j - 1];
}
}
}

can be considered illegible (with or without more members).
Here's the voice of a Man Who Got the Life Right! :)
Click to expand...
^^^^^^^^^^^^^^^^^^^^^^^^^^
You /really/ need to stop making things like that up.

Stick to well-known English idioms and phrases. If you don't know if one
qualifies as well-known, don't use it (all English-speaking posters
should adhere to that, particularly with regard to colloquialisms, not
just you).

Mike
 
R

Robert

Michael said:
That's certainly a reasonable approach, as long as the convention is
clear, simple, and well-documented. However, I really don't see how

function List() {
var data = [];

this.insert = function(value, index) {
shiftRight(index);
data[index] = value;
};


function shiftRight(i) {
for (var j = data.length; j > i; j--) {
data[j] = data[j - 1];
}
}
}

can be considered illegible (with or without more members).
Click to expand...

I guess there are two things about this that makes it more illegible for me.
1) No clear list of (public) methods.
2) Extra indentation

I don't know. Maybe I should go for this solution and put a commented
list of (public) methods outside the constructor.

What about the memory usage? Isn't it a lot less efficient to have all
those methods per instance? Or is that negligible?
 
M

Michael Winter

On 04/05/2006 15:43, Robert wrote:

[snip]
I guess there are two things about this that makes it more illegible for
me.
1) No clear list of (public) methods.
Click to expand...

That's what documentation is for. :)
2) Extra indentation
Click to expand...

Perhaps one becomes accustomed to that; I certainly can't relate to it.

[snip]
What about the memory usage?
Click to expand...

There is a cost to 'private' data, certainly. During execution of the
constructor function, function expressions must be reevaluated, and yes,
the resulting objects will take up more memory than an equivalent
property of the prototype object (which will only be created once).
Isn't it a lot less efficient to have all those methods per instance?
Or is that negligible?
Click to expand...

'A lot less efficient'? No. Most of the time, the difference should be
negligible.

Mike
 
M

Michael Winter

On 03/05/2006 12:26, Robert wrote:

[snip]
//public
List.prototype.insert = insert;
//private
List.prototype.shiftRight = shiftRight;

function insert(obj, index)
{
this.shiftRight(index);
this.array[index] = obj;
}

function shiftRight(index)
{
// move array elements starting at index one to the right
}
Click to expand...

Using function declarations in this way isn't sensible. There is no
point in creating a variable that won't be used beyond a one-time
assignment. Use function expressions instead.

List.prototype.insert = function(obj, index) {
/* ... */
};
List.prototype._shiftRight = function(index) {
/* ... */
};

[snip]

Mike
 
V

VK

Michael said:
That's certainly a reasonable approach, as long as the convention is
clear, simple, and well-documented.
Click to expand...

But certainly not the only one possible - I guess I was rather clear on
it in this thread and I did not try to hide other options.

However, I really don't see how

function List() {
var data = [];

this.insert = function(value, index) {
shiftRight(index);
data[index] = value;
};


function shiftRight(i) {
for (var j = data.length; j > i; j--) {
data[j] = data[j - 1];
}
}
}

can be considered illegible (with or without more members).
Here's the voice of a Man Who Got the Life Right! :)
Click to expand...
^^^^^^^^^^^^^^^^^^^^^^^^^^
You /really/ need to stop making things like that up.

Stick to well-known English idioms and phrases. If you don't know if one
qualifies as well-known, don't use it (all English-speaking posters
should adhere to that, particularly with regard to colloquialisms, not
just you).
Click to expand...

It is an English idiom though... You may not hear it in London, but
easy to pick up in West Oakland. :) If you mean to use only British
idioms used in modern American English then it will be a challenge, as
very few of them made it though the three centuries. Off my head I know
only of "What does it have to do with tea and China?" (means "It is
irrelevant to the current subject"). Amasingly enough it is still well
known at least in California - despite most of the speakers have no
clue what tea or China are about in this context.
 
R

Richard Cornford

VK said:
It is an English idiom though... You may not hear it in
London, but easy to pick up in West Oakland. :)
Click to expand...

While it is not uncommon for regional dialects of English to employ
grammar that is very distinct from normal English grammar the
questionable wisdom of presenting expressions that employ such grammar
to an international audience was the point of Michael's comment. No
matter how common that expression may be in West Oakland its grammar is
such that it would not qualify as a meaningful statement in English
throughout most of the English speaking word.

(Incidentally, I don't believe your assertion that the denizens of West
Oakland employ English that badly. If you listen you will find that the
expression that they actually use is distinct from your version.)
If you mean to use only British idioms used in modern
American English then it will be a challenge,
Click to expand...

As the United States has spend the last 70-odd years exporting talking
movies and, more recently, television globally, common US English idioms
are quite familiar to the entire English speaking world. Very few of the
attempts you make to employ such idioms, expressions or proverbs are
familiar from exported US popular culture or from the language spoken by
the many UK resident US citizens that I have know.
as
very few of them made it though the three centuries.
Click to expand...
Off my head
Absolutely.

I know only of "What does it have to do with tea and China?"
(means "It is irrelevant to the current subject").
Click to expand...
<snip>

With your talent for being staggeringly irrelevant I can see how you may
have become familiar with that expression.

Richard.
 
L

Lee

VK said:
Michael said:
That's certainly a reasonable approach, as long as the convention is
clear, simple, and well-documented.
Click to expand...

But certainly not the only one possible - I guess I was rather clear on
it in this thread and I did not try to hide other options.

However, I really don't see how

function List() {
var data = [];

this.insert = function(value, index) {
shiftRight(index);
data[index] = value;
};


function shiftRight(i) {
for (var j = data.length; j > i; j--) {
data[j] = data[j - 1];
}
}
}

can be considered illegible (with or without more members).
Here's the voice of a Man Who Got the Life Right! :)
Click to expand...
^^^^^^^^^^^^^^^^^^^^^^^^^^
You /really/ need to stop making things like that up.

Stick to well-known English idioms and phrases. If you don't know if one
qualifies as well-known, don't use it (all English-speaking posters
should adhere to that, particularly with regard to colloquialisms, not
just you).
Click to expand...

It is an English idiom though... You may not hear it in London, but
easy to pick up in West Oakland. :) If you mean to use only British
idioms used in modern American English then it will be a challenge, as
very few of them made it though the three centuries. Off my head I know
only of "What does it have to do with tea and China?" (means "It is
irrelevant to the current subject"). Amasingly enough it is still well
known at least in California - despite most of the speakers have no
clue what tea or China are about in this context.
Click to expand...

It may well be known in that form in California, but more of the
English-speaking world recognizes it as "What does it have to do
with the price of tea in China?"


--
 

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