Just A Summary : Javascript scoping makes my head hurt

Comment on Javascript scoping makes my head hurt by Brendan

Thu, 20 Mar 2008 13:31:29 -0500

Here’s how I would have done this sort of thing in Javascript:

var subs = [];
for (var i in [0,1,2,3,4]) {
  subs[i] = {
    execute: function() {return this.i;}
  };
  subs[i].i = i;
}
alert(subs[0].execute());

You’re right, though: Javascript’s scoping is very often contrary to expectations.

Comment on Javascript scoping makes my head hurt by anon

Thu, 20 Mar 2008 16:02:40 -0500

I think it’s perl that’s being dodgy here, for a ‘scripting’ language. The equivalent in python behaves the same as js and ruby:

[lambda: i for i in range(5)][0]() # 4

Note though, if you use the value of i in the list comprehension scope rather than keeping the reference alive with the lambda, it acts as you want it to:

[lambda n=i: n for i in range(5)][0]() # 0

A bit of a cheat, but a short way to spell it.

Comment on Javascript scoping makes my head hurt by Piers Cawley

Thu, 20 Mar 2008 17:12:28 -0500

@Brendan: I’ve always disliked that style – what’s the point of manually unpicking a perfectly serviceable closure?

Comment on Javascript scoping makes my head hurt by Piers Cawley

Thu, 20 Mar 2008 17:17:07 -0500

@anon: Ah, one more reason to maintain my dislike for Python.

Comment on Javascript scoping makes my head hurt by James Duncan

Thu, 20 Mar 2008 19:19:10 -0500

In JavaScript 1.7 you get “let” which does what you’re looking for. Of course that doesn’t help you in the majority of browsers, but it is a recognized problem.

Comment on Javascript scoping makes my head hurt by Aristotle Pagaltzis

Thu, 20 Mar 2008 19:59:50 -0500

Yeah, scoping is insufficiently thought out in Javascript and downright braindead in Python. At least in Javascript, you can fix this particular example by reformulating it in the same way as the reformulated Ruby example:

var subs = [0,1,2,3,4].map( function(i) {
    return function() { i };
} );

Of course I’d write this in Perl just like you did in “real Ruby”:

my @subs = map { my $i = $_; sub { $i } } 0 .. 4;

And gosh, here I was annoyed by the verbosity of having to write “sub” in front of every block snippet. Welcome to Javascript which makes you write “function()”!

Comment on Javascript scoping makes my head hurt by Piers Cawley

Fri, 21 Mar 2008 14:56:20 -0500

Heh. Javascript continues in its mission to turn people onto Lisp (but with extra syntax) eh? Are any of the various Javascript implementations doing tail call optimization yet? It’d certainly make continuation passing style a good deal easier to manage.

Comment on Javascript scoping makes my head hurt by Piers Cawley

Fri, 21 Mar 2008 15:06:59 -0500

Aristotle: And no way to alias @function either. Giles Bowkett came up with a neat ruby hack:

alias :L :lambda

Which I find rather delightful.

Comment on Javascript scoping makes my head hurt by Aristotle Pagaltzis

Fri, 21 Mar 2008 18:53:53 -0500

Ah, then you’ll enjoy lambda. :-)

Comment on Javascript scoping makes my head hurt by Daniel Berger

Mon, 24 Mar 2008 11:05:01 -0500

Pssh, Ruby doesn’t require a third party library:

http://www.oreillynet.com/ruby/blog/2007/10/fun_with_unicode_1.html

Comment on Javascript scoping makes my head hurt by Piers Cawley

Mon, 24 Mar 2008 14:13:37 -0500

Daniel: You obviously haven’t looked at lambda.pm very closely. It’s not substantially longer than the ruby implementation, but it has some very neat tricks for lexically scoping the λ.

Perl’s conventions for module inclusion and pragma-like modules knock Ruby’s into a cocked hat, frankly.

Comment on Javascript scoping makes my head hurt by David Cantrell

Tue, 25 Mar 2008 04:00:57 -0500

The loop counter is declared outside the for{} block, so to me it makes sense that it could be scoped to the containing block. Perl’s the odd one out here. I like the way it’s odd, but it’s still odd.

And, digging into the dusty recesses of my memory, isn’t that the same as how a for-loop counter would be scoped in C?

Comment on Javascript scoping makes my head hurt by Piers Cawley

Tue, 25 Mar 2008 05:26:33 -0500

Who cares how it’s scoped in C? Frankly, the difference in scope only really becomes important/noticeable when your language gets closures. The slight weirdness of the Perl 5 scoping’s been addressed in Perl 6: It’s now:

foreach @array -> $each {
  ...
}

Where -> $whatever { ... } is the new way of declaring parameterized anonymous blocks. I’m sure it won’t be long before someone writes a library to let them spell that as λ $foo { ... }