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Explaining JS closures in the terms of perl

I understand without any problems how perl's closures works, like the next one

use 5.012;
use strict;
use warnings;

sub countdown {
        my $start = shift;
        return sub { $start-- }

my $c10 = countdown(3);
say while( $_ = $c10->() );

I'm trying to understand the next piece of Javascript:

var runInSandbox = (function(js, inputPath) {

  (function() {
    if ((!context.initialized__QUERY)) {
      return createContext();
  (function() {
    if (typeof(inputPath) !== 'undefined') {
      (process.argv)[1] = inputPath;;
      (context)["__dirname"] = path.dirname(inputPath);;
      return (module)["filename"] = inputPath;;
  return vm.runInContext(js, context, "sibilant");

NO CHANCE! :( PLEASE can someone rewrite the above to perl ? I know perl a bit - so for me will be extremely useful to understanding JS basics and the constructions like:

(...)() - more precisely

double (( in the if

    if ((!context.initialized__QUERY)) {

or the next

      (context)["__dirname"] = something ;;


       return (module)["filename"] = inputPath;; // why double ;;?

And if someone coul'd suggest me any resource something like: Learning Javascript for perl programmers - would be very nice ;)

Ps: the JS (shortened) is from here: https://github.com/jbr/sibilant/blob/master/lib/cli.js

like image 447
novacik Avatar asked Mar 24 '23 00:03


1 Answers

I'm not extremely well-versed with Perl closures, so I will at least try to demystify this for you.

The form:

(function(...) {

is a self-invoked anonymous function1. This means that you write out an anonymous function, and then invoke it immediately. This is usually done for encapsulation2. For example, if you end up creating a bunch of variables, but don't want it to pollute the global namespace, you can put it inside an anonymous, self-invoked function. However, in this case I don't see why the first invocation is necessary at all, since it's simply checking a flag or something. What is even stranger is the return inside those self-invoked functions. They aren't being assigned to anything. I would hazard a guess that createContext() initializes the context variable, but that return in there is effectively useless. The same goes for the following:

return (module)["filename"] = inputPath;;

As far as the double (( and )), they seem to be largely unnecessary and so I'm not sure why the author originally put it in there. For example:

if ((!context.initialized__QUERY)) 

Isn't any different from:

if (!context.initialized__QUERY) 

Also, the parentheses in the following are also unnecessary, as are the double semicolons:

(context)["__dirname"] = something ;;

Honestly, it just looks like poorly-written Javascript, or perhaps JavaScript that was autogenerated (this is most probably the case).

You could rewrite it like so:

var runInSandbox = function(js, inputPath) {

    if (!context.initialized__QUERY) {

    if (typeof inputPath !== 'undefined') {
       process.argv[1] = inputPath;
       context["__dirname"] = path.dirname(inputPath);
       module["filename"] = inputPath;

    return vm.runInContext(js, context, "sibilant");


  1. In Perl, that would be sub { ... }->().
  2. In Perl, one would use { my $var; ... } instead of sub { my $var; ... }->() and do { my $var; ...; EXPR } instead of sub { my $var; ...; return EXPR; }->().
like image 139
Vivin Paliath Avatar answered Mar 31 '23 18:03

Vivin Paliath