An object's constructor name as a string. Can definitely be determined?

[Peter Michaux]

Hi,

I want to know the name of an object's constructor function as a
string. Something like this

<script type="text/javascript">
function Foo(){};
var a = new Foo();
alert('"' +
a.constructor.t oString().match (/function\s*([a-zA-Z\$][\w\$]+)[^\w\$]/)[1]
+ '"');
</script>

Although this works in IE, Opera, Firefox, Safari I'm not sure that it
is guaranteed to work.

I looked in the ECMA-262 specs about the returned string for
Function.protot ype.toString() in section 15.2.4.2. It says that the
whitespace, line terminators and semi-colon use are implementation
dependent. I think the above regular expression can handle those
difference.

What I'm worried about is that he specs say the return value has the
form of a "function declaration". Does that mean the returned string
will definitely contain the name of the constructor function? Could it
instead return something like just "function () {}"?

Thank you,
Peter

[Richard Cornford]

VK wrote:

Richard Cornford wrote:

>Who said Opera was the only browser reporting the syntax error?

Do you have any other one besides Opera 9?

Yes. Not reporting this syntax error as a syntax error appears to be the
exception (unsurprisingly ).

I have no idea about Safari,
but I would like to dismiss it right away

Which brings us back to what I said earlier. You write code without
recognising a syntax error in the language you are using and then when
your scripts only work in a couple of browsers at most you blame the
browsers. That the fault here is yours rather than the browser's is
obvious as soon as you realise that people exist who do manage to write
effective code for Safari. You think they are doing that by putting
disproportionat e effort into the task, but it is more likely that they
are just presenting Safari with code devoid of syntax errors, mystical
incantations and your other nonsense, and the result just works.

(in case if): if you take a dring every time this
UA does something right with
scripting, you'll end up thirsty and angry :-)

The VK definition of what is "right" is too detached from reality for
that to be a criteria for judging anything.

Any other mainstream browser?

Earlier today it was "Any ECMAScript-compliant engine erroring out on the
below, please", but now that it should be obvious that
ECMAScript-compliant browsers are likely to generate syntax errors with
code that contains syntax errors you want to retreat to your 'safe'
ground of "mainstream browsers". Which presumably is "mainstream
browsers" as defined by VK, and so pretty much Windows IE and Firefox
only.

A good hint: if you're in dubts about something, check it
on Netscape 4.7

Idiot.

In 99% of cases whatever it does is what was really
intended,

Unlikely as Netscape 4.7 implemented ECMA 262 2nd Edition (with
extensions) and your "mainstream browsers" (and other current scriptable
browsers) implement ECMA 262, 3rd Edition.

if it contradicts to ECMAScript specs it simply means
that the free-lancers

Have you looked at the list of the people who wrote ECMA 262?

failed to describe properly what they've seen.

With the contributors to ECMA 262 having direct access to the source code
for both JavaScript(tm) and JScript it is unlikely that observation
determined their behaviour.

One more time: there is not syntax error here,

Only when "here" is restricted to the contents of your deranged mind.

but there is Opera's parser laziness:

It is not lazy to report a syntax error when one is seen.

instead of further interprete the source and to
understand that this is an expression w/o assignment:

There is nothing in the language's syntax that allows such an
interpretation, and you cannot assume that any browser that does not
report the syntax error is making such an interpretation. As far as you
know they are just recognising the futility of the construct and just
filtering it out. Indeed experimentation suggests that the syntax error
is being filtered out rather than being interpreted as a Function
Expression. Try:-

<html>
<head>
<title></title>
<script type="text/javascript">
var a;

function(){aler t('I was recognised 1');}

(a);

var b = function(){aler t('I was recognised 2');}

(a);

</script>
</head>
<body>
</body>
</html>

- in Firefox and IE 6 and you see "I was recognised 2" alerted by the
second function expression but no sign of 'I was recognised 1', which
would be the consequences of the - function(){aler t('I was recognised
1');} - line being interpreted as a function expression, as can be
illustrated by wrapping that line in parenthesise to give:-

<html>
<head>
<title></title>
<script type="text/javascript">
var a;

(function(){ale rt('I was recognised 1');})

(a);

var b = function(){aler t('I was recognised 2');}

(a);

</script>
</head>
<body>
</body>
</html>

- where the absence of any syntax error allows both 'I was recognised 1'
and 'I was recognised 2' to be alerted.

So " interprete the source and to understand that this is an expression
w/o assignment" is not what Firefox or IE are doing with the syntax
error. Instead they appear to be silently ignoring it. And silently
ignoring syntax errors is less useful than reporting them.

instead of that
it just errors out on the first opportunity.

Which tells the programmer at the earliest point possible that they have
made an error.

Obviously you would not appreciate that, as you don't like being told
that you have made an error, and prefer not to recognise them when you
have. But then that is one of the things makes you by far the worst
programmer I have ever encountered.

Richard.

[John G Harris]

In article <11************ **********@m73g 2000cwd.googleg roups.com>, VK
<sc**********@y ahoo.comwrites

>1) Found "function" keyword left hand side, searching for identifier.
2) If an identifier found then this is FunctionDeclara tion; if not then
go to step 3
3) No identifier found, left hand side anonymous function.

<snip>

>Obviously the programmer has forgotten to type something like
this.a =
at the beginning of the line, or has forgotten to type the function name
in the middle of the line. A halfway decent compiler will say
!!!ERROR!!!
so the programmer sees and corrects the mistake straight away.

Obviously I will never hire you for an application development :-(
because you have a holy mix of that a parser/engine is and what a
tidy-like suggestive checker is.

You obviously don't know what a parser does. It's a pattern matcher. A
match tells the compiler which syntax rule the code matches. Then the
compiler knows exactly what the programmer wants the program to do.

For instance, if the parser is given
var x = 42;
the parser finds that the code matches the syntax rule for creating a
variable named x with the initial value 42.

On the other hand, if the parser is given
x var 42 =;
the parser doesn't find a match with any of the hundreds of syntax rules
so the compiler doesn't know what the programmer was thinking.

In VK's universe the browser's AI plugin would come to life at this
point and say that it believes the programmer meant ... well, what?

>The engine's job is not to enforce some "preferred style" of
programming or some "best practice" onto developers. It has much more
humble but much more technically difficult task: from each segment of
the source code try to produce an execution code non-violating given
border criterias. That is not damn parser business what is this code
for and why did one use it this way.
Tidy-like checkers are free to declare ++i or --i harmful or declare
void function(){};
as an error (an opinion shared by no one real engine). Parser has not
such freedom: it has to eat whatever fed until chokes.

VK is confused once again. The parser's job is to see which syntax rule
the code matches. It's a style guide or lint program that's expected to
say it's dangerous to use 'with'. Thus the parser must declare that
; ++i; --i; void function() {};
are all legal since each matches a syntax rule. A style guide might say
that i++ is preferred to ++i, or that void is rarely needed, but it
doesn't have to say that.

Does javascript have syntax rules? Yes : they're written in ECMA 262.
Does every javascript programmer have to be able to read them? No, but
if they can't they mustn't tell people what the rules are or aren't.

John

Postscript :
No syntax rule matches
; function() {};
but there is one that matches
; void function() {};
Strange, but true.
--
John Harris