Hi,
> my question is simple : isn't there a way do define the class member
> functions in such a way
> that the scope (this) will refer to the object instance ?
`this` is never scope in Javascript (unless you use `with`, which
simulates scope to an extent). It's just an object reference. If you
want to refer to a property on the instance, you have to be explicit
(either by typing "this." or using "with").
So the answer to your question is no, there's no reasonable way (that
I know of) to define a member function such that unqualified symbols
resolve to properties on `this`, other than to wrap the code of every
member function inside a `with` statement, which I wouldn't recommend
for several reasons. (I say "reasonable" because it's probably
possible using `eval` and functions generated on the fly.)
Your best bet, in my book, is to simply use `this.` when referring to
a property on `this`. I think it's clearer anyway (in fact, I always
use `this.` in Java and C#, even though the compiler doesn't need me
to -- in my view, *I* need me to, as do any code maintainers coming
after me.) If the length bothers you, you can put `var t = this;` at
the top of every function and use `t.` instead of `this.`. (Doing that
also has the advantage that the meaning to `t.` will survive
transition into closures, whereas `this.` will not.)
HTH,
--
T.J. Crowder
Independent Software Consultant
tj / crowder software / com
www.crowdersoftware.com
On Apr 30, 1:30 pm, Ran Berenfeld <berenfeld...@gmail.com> wrote:
> Thanks a lot for your reply T.J
> I was partially familiar with this mechanism, and now I know more :)
> anyway, let me re-phrase my question
> we all know that an event received on a DOM element (i.e button onclick
> event for example)
> will have the "this" equals to the DOM element
>
> my question is simple : isn't there a way do define the class member
> functions in such a way
> that the scope (this) will refer to the object instance ?
>
> Thanks
> Ran B
>
> On Fri, Apr 30, 2010 at 9:31 AM, T.J. Crowder <t...@crowdersoftware.com>wrote:
>
>
>
>
>
> > Hi,
>
> > > is there a way that I can get rid of all these usages of "this"
>
> > No. `this` is different in Javascript than in some other languages
> > with the same keyword, in several ways. One of these is that
> > unqualified references like your _div (without anything in front of
> > it) are not automatically resolved against property names on `this`;
> > all identifier resolution in Javascript is lexically-based, using the
> > concept of a "scope chain" (more below if you're interested). So you
> > have to explicitly type `this.` when accessing the properties on
> > `this` (just as you have to type `foo.` to access the properties on
> > `foo`), or as you've found, you can use `with`. Be aware that using
> > `with` has some "gotchas" that are not obvious. Douglas Crockford
> > lists many of these in his "with Statement Considered Harmful"[1]
> > article (I'm not a fan of "Considered Harmful" articles[2], but
> > Crockford lays out the issues fairly well). My point isn't "don't use
> > it", it's "be sure you understand the gotchas when using it".
>
> > About the scope chain and identifier resolution: One of the things I
> > love about Javascript is that basically *everything* is an object,
> > even if some of the objects are hidden behind the scenes. One of these
> > behind-the-scenes objects is the "variable object" (the "binding
> > object of the variable environment" in the latest spec -- yikes). When
> > you declare a variable using `var`, you're adding a *property* to the
> > variable object for the current scope. The variable object has
> > properties for all `var`s, all functions declared in the scope, and
> > (if the scope relates to a function) all named arguments to the
> > function. When the step-by-step code within the scope is executing,
> > that scope's variable object is at the beginning of the "scope chain."
> > All unqualified references are first checked against that variable
> > object to see if they match a property on it. If they do, that
> > property is used; if not, the next variable object in the chain is
> > checked; etc.
>
> > Example:
>
> > var x = 1;
>
> > function foo() {
> > var y = 2;
>
> > function bar() {
> > var z = 3;
>
> > alert(x);
> > alert(y);
> > alert(z);
> > }
> > }
>
> > We have three "scopes" there: Global scope, `foo`'s scope, and `bar`'s
> > scope. Consequently, we have three variable objects. When code within
> > `bar` is being executed, the scope chain consists of `bar`'s variable
> > object, followed by `foo`'s, followed by the global variable object.
> > When `bar` references `z`, the `z` property is found on the first
> > variable object and used. When `bar` references `y`, it's not on the
> > first variable object but it is on the second one, so it's used from
> > there. Accessing `x` involves going back two variable objects to the
> > global one.
>
> > As you can see, `this` doesn't come into it -- it's nowhere on the
> > scope chain unless you insert it, which is exactly what `with` does:
> > Puts the object you specify at the beginning of the scope chain.
>
> > Now, I called the variable object a behind-the-scenes object. That's
> > usually true, but there's an exception: The variable object for the
> > global scope is the global object, which on browsers is `window` (more
> > accurately, on browsers the global object has a property, `window`, it
> > points to itself with). That's why when you declare variables and
> > functions at global scope, they magically show up as properties on
> > `window`.
>
> > [1]http://yuiblog.com/blog/2006/04/11/with-statement-considered-harmful/
> > [2]http://meyerweb.com/eric/comment/chech.html
>
> > HTH,
> > --
> > T.J. Crowder
> > Independent Software Consultant
> > tj / crowder software / com
> >www.crowdersoftware.com
>
> > On Apr 29, 9:17 am, Ran Berenfeld <berenfeld...@gmail.com> wrote:
> > > Hi all
> > > I have written a class using prototype Class object
> > > and after playing it for a while, I saw that I need to use the "with"
> > > keyword in the class members,
> > > so I can access its members
>
> > > here is the relevant part of the class : (the member is "_div")
>
> > > var JSClass = Class.create ({
> > > initialize: function(initParams) {
>
> > > if (initParams.div)
> > > {
> > > *this*._div = $(initParams.div);
> > > }
> > > else
> > > {
> > > *this*._div = new Element(div);
> > > }
> > > },
> > > // an example of a member function
> > > setComment: function(msg)
> > > {
> > > *with(this)*
> > > * {*
> > > * *_div.innerHTML = msg;
> > > * }*
> > > }
>
> > > });
>
> > > is there a way that I can get rid of all these usages of "this"
>
> > > Thanks
> > > Ran
>
> > > --
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