Creating names of variables at run-time

[Helmut Giese]

Hello out there,
I am new to JS so please bear with me.

I am tasked to investigate, whether it is feasible to port one of our
applications from Tcl to JS - the idea being to write a program to do
the conversion, mapping language constructs from Tcl to equivalent
constructs in JS.
If we pursue this project I will surely come here more often :) but
for a starter I have this question:
Tcl syntax:
set i 1 // equivalent to 'var i = 1;'
puts "$i" // prints '1' - $i returning the current value of 'i'

In Tcl one can create names of variables at run-time, like
set i 1
set a$i "some string"
the last statement creating a variable with the name 'a1' and the
value "some string".
Question: How do I achieve this in JS?

Any insight will be greatly appreciated.
Best regards
Helmut Giese

[Martin Honnen]

Re: Creating names of variables at run-time

Helmut Giese wrote:

In Tcl one can create names of variables at run-time, like
set i 1
set a$i "some string"
the last statement creating a variable with the name 'a1' and the
value "some string".
Question: How do I achieve this in JS?

Variables in JavaScript are properties of an object and properties can
be accessed with both dot and bracket notation where bracket notation
allows you to concatenate the name as an expression:
var g = this;
g['i'] = 1;
g['a' + g[i]] = "some string";
That would only work for global variables however as there the global
object is accessible.
Other than that there is eval:
eval('a' + i + ' = ' + '"some string"');



--

Martin Honnen

http://JavaScript.FAQTs.com/

[Henry]

Re: Creating names of variables at run-time

On Apr 1, 2:24 pm, Martin Honnen wrote:
<snip>

That would only work for global variables however as
there the global object is accessible.
Other than that there is eval:
eval('a' + i + ' = ' + '"some string"');

eval('var a' + i + ' = ' + '"some string"');

- for the runtime creation of function local variables. But in a
machine translation context it might be effective when these variables
had restricted scope to inset an object on the scope chain with a -
with - statement and add properties to that. The overheads would not
necessarily be different/worse than using - eval -.

[Henry]

Re: Creating names of variables at run-time

On Apr 1, 10:10 am, Helmut Giese wrote:

Hello out there,
I am new to JS so please bear with me.
>
I am tasked to investigate, whether it is feasible to port
one of our applications from Tcl to JS

<snip>

Feasible rather than possible because it must be possible by may be an
unrealistic effort. As it is possible to write a javascript engine in
javascript it is should also be possible to write a Tcl interpreter/
engine in javascript. I don't know Tcl which makes it impossible for
me to judge the effort involved, and that will be a problem when
asking questions here as that may also be true of a large proportion
of the contributors to this group. For example, when you ask about the
runtime creation of variables I wondered about Tcl's variable scoping
(and how it would relate to how javascript handles variables (lexical
scoping at function units)). So when asking questions in relation to
this it may be a good idea to go overboard on the expatiations of what
Tcl does (and how it behaves). Unfortunately it would also be a good
idea to express those explanations in relation to javascript, but if
you are new to javascript that may not be that practical.

[Helmut Giese]

Re: Creating names of variables at run-time

Hi Erwin,

>If I have to introduce objects in those situations which go beyond the
>simple example from above, things get - well - interesting. Currently
>I don't really see a way to handle this _without_ human intervention.
>>
>If you have any other idea don't hesitate to post it :)

>
>To be honest, I think you might get yourself in all kind of JavaScript
>problems since you don't know too much about JavaScript.

you are very polite. I know next to nothing about JS and I _will_ get
me into all kinds of trouble :) - but it is my job to find a realistic
answer to the question "to port or not to port".

[snip example of scope]

>This is just an example. You must know about such things I expect,
>before writing a TCL->JS translator.

Thanks for the hint, but I am aware of the differences between
different languages how to handle name lookup.

>I am inclined to advise you to write the JavaScript from scratch.
>Maybe hire a JavaScript guy/girl and learn him him/her how to read TCL.

Never, no way. We are talking about some 50.000 lines of code here,
maybe more.

>It depends on how much time you want to invest yourself to learn
>Javascript of course. ;-)

Couple o' days? :)) Might depend a bit on the kind of advice I can
expect from this newsgroup :) - but so far I am positively surprised.
Thanks again and keep up the good work - I might^H^H^H^H^H will be
back.
Best regards
Helmut Giese

[Helmut Giese]

Re: Creating names of variables at run-time

>Variables in JavaScript are properties of an object and properties can

>be accessed with both dot and bracket notation where bracket notation
>allows you to concatenate the name as an expression:
var g = this;
g['i'] = 1;
g['a' + g[i]] = "some string";
>That would only work for global variables however as there the global
>object is accessible.
>Other than that there is eval:
eval('a' + i + ' = ' + '"some string"');

Thanks Martin,
this news group seems to be really helpful. Combining the advice of
the 3 of you it looks like there is a way.
Best regards
Helmut Giese

[DS]

Re: Creating names of variables at run-time

On Tue, 01 Apr 2008 11:10:09 +0200, Helmut Giese wrote:

Hello out there,
I am new to JS so please bear with me.
>
I am tasked to investigate, whether it is feasible to port one of our
applications from Tcl to JS - the idea being to write a program to do
the conversion, mapping language constructs from Tcl to equivalent
constructs in JS.

Hi, why are you converting 50k lines of presumably fully-working bug-free
tcl code?
And why javascript in particular?
Why not ruby, python or lua etc?
You might be better served leaving the tcl code as-is and extending your
codebase with javascript objects as needed.