I had this idea that, as soon as you made a closure, all the variables were neatly packaged up and were immutable outside of the closure itself. Then I saw some weird behaviour in Javascript, and investigated further.

Turns out that my assumption was just plain wrong. For instance: 


    test.pl:
    ----------------------------------------------
    sub say_hello {
        my $name = shift;

        my $text = 'Hello '.$name;
        my $sayAlert = sub { print "$text\n" };

        $text = "Not in Perl you don't";
        
        $sayAlert->();
    }

    say_hello('Bob');
    ----------------------------------------------

    > perl test.pl
    Not in Perl you don't

[download]
Now, reading that again, it makes sense that I would still have access to the variable in the closure from the same scope in which it was declared. So understandably, this version will work as I originally expected: 


    test.pl:
    ----------------------------------------------
    sub say_hello {
        my $name = shift;
        my $sayAlert;
        {
            my $text = 'Hello '.$name;
            $sayAlert = sub { print "$text\n" };
        }    
        my $text = "Not in Perl you don't";
        
        $sayAlert->();
    }
    say_hello('Bob');
    ----------------------------------------------

    > perl test.pl
    Hello Bob

[download]

However, try the same thing in Javascript, and it doesn't work: 


    function sayHello(name) {
      var sayAlert;
      {
          var text = 'Hello ' + name;
          sayAlert = function() { alert(text); }
      }
      var text='How confused am I?';
      sayAlert();
    }

    sayHello('Bob');

    ---> "How confused am I?"

[download]

It turns out that, unlike Perl, creating a new (nested) block in JS does NOT create a new scope. Instead a new scope is created when a function is declared. Oh, and it doesn't give you any "variable text redefined" warnings either.

In reply to "When closures capture their context" and "scope gotchas in Javascript" by clinton

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