Strange Scoping Problem - comp.lang.javascript

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Strange Scoping Problem

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shaneal

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More options Feb 17 2008, 3:29 am

Newsgroups: comp.lang.javascript

From: shaneal <sma...@gmail.com>

Date: Sun, 17 Feb 2008 00:29:58 -0800 (PST)

Local: Sun, Feb 17 2008 3:29 am

Subject: Strange Scoping Problem

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Hello all,

I've been trying to learn some javascript and I ran into a strange
scoping problem I was hoping someone here could help with. I have a
fair amount of experience with functional programming, but very little
with javascript, so please excuse the unidiomatic code.

When I do:

var functions = new Array();

function printNum(x) {
alert(x);

}

for(var i=0;i<5;i++) {
functions[i] = function() { printNum(i);}

}

Now, I'd not expect
functions[0]();
to alert with 0, but instead it alerts with 5. Why is that, and how
can I work around it?

I was banging my head on a strange problem for a bit, until I finally
managed to track down the problem to something isomorphic to this bit
of code.

PS: functions[1-4](); all alert with 5 too ... in case that is
relevant

Thanks for the help, and I apologize in advance for any breaches of
protocol I've made here (first time on comp.lang.javascript).

David Mark

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More options Feb 17 2008, 3:46 am

Newsgroups: comp.lang.javascript

From: David Mark <dmark.cins...@gmail.com>

Date: Sun, 17 Feb 2008 00:46:58 -0800 (PST)

Local: Sun, Feb 17 2008 3:46 am

Subject: Re: Strange Scoping Problem

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On Feb 17, 3:29 am, shaneal <sma...@gmail.com> wrote:

- Show quoted text -

Because i *is* 5 after the loop is finished. All of the functions
created are referencing the same i variable. If the for loop is run
in the global context, then i is a global variable. If not, i is a
local variable and part of a closure formed by assigning references to
the functions to the global functions array.

Something like this will produce the desired results:

functions[i] = (function(j) { return function() { printNum(j); }; })
(i);

That pattern creates a distinct closure for each of the five
functions.

> I was banging my head on a strange problem for a bit, until I finally

Scope-related issues (particularly closures) are a common source of
confusion for beginning JS programmers.

[snip]