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JE - Pure-Perl ECMAScript (JavaScript) Engine


Version 0.066 (alpha release)

The API is still subject to change. If you have the time and the interest, please experiment with this module (or even lend a hand :-). If you have any ideas for the API, or would like to help with development, please e-mail the author.


  use JE;

  $j = new JE; # create a new global object

  $j->eval('({"this": "that", "the": "other"}["this"])');
  # returns "that"

  $parsed = $j->parse('new Array(1,2,3)');
  $rv = $parsed->execute; # returns a JE::Object::Array
  $rv->value;             # returns a Perl array ref

  $obj = $j->eval('new Object');
  # create a new object

  $foo = $j->{document}; # get property
  $j->{document} = $obj; # set property
  $j->{document} = {};   # gets converted to a JE::Object
  $j->{document}{location}{href}; # autovivification

  $j->method(alert => "text"); # invoke a method

  # create global function from a Perl subroutine:
  $j->new_function(print => sub { print @_, "\n" } );

          function correct(s) {
                  s = s.replace(/[EA]/g, function(s){
                          return ['E','A'][+(s=='E')]
                  return s.charAt(0) +
                         s.substring(1,4).toLowerCase() +
          print(correct("ECMAScript")) // :-)


JE, short for JavaScript::Engine (imaginative, isn't it?), is a pure-Perl JavaScript engine. Here are some of its strengths:


Easy to install (no C compiler necessary*)


The parser can be extended/customised to support extra (or fewer) language features (not yet complete)


All JavaScript datatypes can be manipulated directly from Perl (they all have overloaded operators)


The JavaScript datatypes provide TO_JSON methods for compatibility with

JE's greatest weakness is that it's slow (well, what did you expect?). It also uses and leaks lots of memory. (There is an experimental JE::Destroyer (q.v.) module that solves this if you load it first and then call JE::Destroyer::destroy($j) on the JE object when you have finished with it.)

* If you are using perl 5.9.3 or lower, then Tie::RefHash::Weak is required. Recent versions of it require Variable::Magic, an XS module (which requires a compiler of course), but version 0.02 of the former is just pure Perl with no XS dependencies.

There is currently an experimental version of the run-time engine, which is supposed to be faster, although it currently makes compilation slower. (If you serialise the compiled code and use that, you should notice a speed-up.) It will eventually replace the current one when it is complete. (It does not yet respect tainting or max_ops, or report line numbers correctly.) You can activate it by setting to 1 the ridiculously named YES_I_WANT_JE_TO_OPTIMISE environment variable, which is just a temporary hack that will later be removed.


Simple Use

If you simply need to run a few JS functions from Perl, create a new JS environment like this:

  my $je = new JE;

If necessary, make Perl subroutines available to JavaScript:

  $je->new_function(warn => sub { warn @_ });
  $je->new_function(ok => \&Test::More::ok);

Then pass the JavaScript functions to eval:


  function foo() {
      return 42
  // etc.

  # or perhaps:
  use File::Slurp;
  $je->eval(scalar read_file 'functions.js');

Then you can access those function from Perl like this:

  $return_val = $je->{foo}->();
  $return_val = $je->eval('foo()');

The return value will be a special object that, when converted to a string, boolean or number, will behave exactly as in JavaScript. You can also use it as a hash, to access or modify its properties. (Array objects can be used as arrays, too.) To call one of its JS methods, you should use the method method: $return_val->method('foo'). See JE::Types for more information.

Custom Global Objects

To create a custom global object, you have to subclass JE. For instance, if all you need to do is add a self property that refers to the global object, then override the new method like this:

  package JEx::WithSelf;
  @ISA = 'JE';
  sub new {
      my $self = shift->SUPER::new(@_);
      $self->{self} = $self;
      return $self;

Using Perl Objects from JS

See bind_class, below.

Writing Custom Data Types

See JE::Types.


See also JE::Object, which this class inherits from, and JE::Types.

$j = JE->new( %opts )

This class method constructs and returns a new JavaScript environment, the JE object itself being the global object.

The (optional) options it can take are max_ops and html_mode, which correspond to the methods listed below.

$j->parse( $code, $filename, $first_line_no )

parse parses the code contained in $code and returns a parse tree (a JE::Code object).

If the syntax is not valid, undef will be returned and $@ will contain an error message. Otherwise $@ will be a null string.

The JE::Code class provides the method execute for executing the pre-compiled syntax tree.

$filename and $first_line_no, which are both optional, will be stored inside the JE::Code object and used for JS error messages. (See also add_line_number in the JE::Code man page.)

$j->compile( STRING )

Just an alias for parse.

$j->eval( $code, $filename, $lineno )

eval evaluates the JavaScript code contained in $code. E.g.:

  $j->eval('[1,2,3]') # returns a JE::Object::Array which can be used as
                      # an array ref

If $filename and $lineno are specified, they will be used in error messages. $lineno is the number of the first line; it defaults to 1.

If an error occurs, undef will be returned and $@ will contain the error message. If no error occurs, $@ will be a null string.

This is actually just a wrapper around parse and the execute method of the JE::Code class.

If the JavaScript code evaluates to an lvalue, a JE::LValue object will be returned. You can use this like any other return value (e.g., as an array ref if it points to a JS array). In addition, you can use the set and get methods to set/get the value of the property to which the lvalue refers. (See also JE::LValue.) E.g., this will create a new object named document:


Note that I used this.document rather than just document, since the latter would throw an error if the variable did not exist.

$j->new_function($name, sub { ... })
$j->new_function(sub { ... })

This creates and returns a new function object. If $name is given, it will become a property of the global object.

Use this to make a Perl subroutine accessible from JavaScript.

For more ways to create functions, see JE::Object::Function.

This is actually a method of JE::Object, so you can use it on any object:

  $j->{Math}->new_function(double => sub { 2 * shift });
$j->new_method($name, sub { ... })

This is just like new_function, except that, when the function is called, the subroutine's first argument (number 0) will be the object with which the function is called. E.g.:

          reverse => sub { scalar reverse shift }
  # ... then later ...
  $j->eval(q[ 'a string'.reverse() ]); # returns 'gnirts a'
$j->max_ops( $new_value )

Use this to set the maximum number of operations that eval (or JE::Code's execute) will run before terminating. (You can use this for runaway scripts.) The exact method of counting operations is consistent from one run to another, but is not guaranteed to be consistent between versions of JE. In the current implementation, an operation means an expression or sub-expression, so a simple return statement with no arguments is not counted.

With no arguments, this method returns the current value.

As shorthand, you can pass max_ops => $foo to the constructor.

If the number of operations is exceeded, then eval will return undef and set $@ to a 'max_ops (xxx) exceeded.

$j->html_mode( $new_value )

Use this to turn on 'HTML mode', in which HTML comment delimiters are treated much like //. new_value is a boolean. Since this violates ECMAScript, it is off by default.

With no arguments, this method returns the current value.

As shorthand, you can pass html_mode => 1 to the constructor.

$j->upgrade( @values )

This method upgrades the value or values given to it. See "UPGRADING VALUES" in JE::Types for more detail.

If you pass it more than one argument in scalar context, it returns the number of arguments--but that is subject to change, so don't do that.


Returns the JavaScript undefined value.


Returns the JavaScript null value.


Returns the JavaScript true value.


Returns the JavaScript false value.

$j->bind_class( LIST )

(This method can create a potential security hole. Please see "BUGS", below.)


     package => 'Net::FTP',
     name    => 'FTP', # if different from package
     constructor => 'new', # or sub { Net::FTP->new(@_) }

     methods => [ 'login','get','put' ],
     # OR:
     methods => {
         log_me_in => 'login', # or sub { shift->login(@_) }
         chicken_out => 'quit',
     static_methods => {
         # etc. etc. etc.
     to_primitive => \&to_primitive # or a method name
     to_number    => \&to_number
     to_string    => \&to_string

     props => [ 'status' ],
     # OR:
     props => {
         status => {
             fetch => sub { 'this var never changes' }
             store => sub { system 'say -vHysterical hah hah' }
         # OR:
         status => \&fetch_store # or method name
     static_props => { ... }

     hash  => 1, # Perl obj can be used as a hash
     array => 1, # or as an array
     # OR (not yet implemented):
     hash  => 'namedItem', # method name or code ref
     array => 'item',       # likewise
     # OR (not yet implemented):
     hash => {
         fetch => 'namedItem',
         store => sub { shift->{+shift} = shift },
     array => {
         fetch => 'item',
         store => sub { shift->[shift] = shift },

     isa => 'Object',
     # OR:
     isa => $j->{Object}{prototype},
 # OR:
     package => 'Net::FTP',
     wrapper => sub { new JE_Proxy_for_Net_FTP @_ }


(Some of this is random order, and probably needs to be rearranged.)

This method binds a Perl class to JavaScript. LIST is a hash-style list of key/value pairs. The keys, listed below, are all optional except for package or name--you must specify at least one of the two.

Whenever it says you can pass a method name to a particular option, and that method is expected to return a value (i.e., this does not apply to props => { property_name => { store => 'method' } }), you may append a colon and a data type (such as ':String') to the method name, to indicate to what JavaScript type to convert the return value. Actually, this is the name of a JS function to which the return value will be passed, so 'String' has to be capitalised. This also means than you can use 'method:eval' to evaluate the return value of 'method' as JavaScript code. One exception to this is that the special string ':null' indicates that Perl's undef should become JS's null, but other values will be converted the default way. This is useful, for instance, if a method should return an object or null, from JavaScript's point of view. This ':' feature does not stop you from using double colons in method names, so you can write 'Package::method:null' if you like, and rest assured that it will split on the last colon. Furthermore, just 'Package::method' will also work. It won't split it at all.


The name of the Perl class. If this is omitted, name will be used instead.


The name the class will have in JavaScript. This is used by Object.prototype.toString and as the name of the constructor. If omitted, package will be used.

constructor => 'method_name'
constructor => sub { ... }

If constructor is given a string, the constructor will treat it as the name of a class method of package.

If it is a coderef, it will be used as the constructor.

If this is omitted, the constructor will raise an error when called. If there is already a constructor with the same name, however, it will be left as it is (though methods will still be added to its prototype object). This allows two Perl classes to be bound to a single JavaScript class:

 $j->bind_class( name => 'Foo', package => 'Class::One', methods => ... );
 $j->bind_class( name => 'Foo', package => 'Class::Two' );
methods => [ ... ]
methods => { ... }

If an array ref is supplied, the named methods will be bound to JavaScript functions of the same names.

If a hash ref is used, the keys will be the names of the methods from JavaScript's point of view. The values can be either the names of the Perl methods, or code references.


Like methods but they will become methods of the constructor itself, not of its prototype property.

to_primitive => sub { ... }
to_primitive => 'method_name'

When the object is converted to a primitive value in JavaScript, this coderef or method will be called. The first argument passed will, of course, be the object. The second argument will be the hint ('number' or 'string') or will be omitted.

If to_primitive is omitted, the usual valueOf and toString methods will be tried as with built-in JS objects, if the object does not have overloaded string/boolean/number conversions. If the object has even one of those three, then conversion to a primitive will be the same as in Perl.

If to_primitive => undef is specified, primitivisation without a hint (which happens with < and ==) will throw a TypeError.


If this is omitted, to_primitive($obj, 'number') will be used. If set to undef, a TypeError will be thrown whenever the object is numified.


If this is omitted, to_primitive($obj, 'string') will be used. If set to undef, a TypeError will be thrown whenever the object is strung.

props => [ ... ]
props => { ... }

Use this to add properties that will trigger the provided methods or subroutines when accessed. These property definitions can also be inherited by subclasses, as long as, when the subclass is registered with bind_class, the superclass is specified as a string (via isa, below).

If this is an array ref, its elements will be the names of the properties. When a property is retrieved, a method of the same name is called. When a property is set, the same method is called, with the new value as the argument.

If a hash ref is given, for each element, if the value is a simple scalar, the property named by the key will trigger the method named by the value. If the value is a coderef, it will be called with the object as its argument when the variable is read, and with the object and the new value as its two arguments when the variable is set. If the value is a hash ref, the fetch and store keys will be expected to be either coderefs or method names. If only fetch is given, the property will be read-only. If only store is given, the property will be write-only and will appear undefined when accessed. (If neither is given, it will be a read-only undefined property--really useful.)


Like props but they will become properties of the constructor itself, not of its prototype property.


If this option is present, then this indicates that the Perl object can be used as a hash. An attempt to access a property not defined by props or methods will result in the retrieval of a hash element instead (unless the property name is a number and array is specified as well).

The value you give this option should be one of the strings '1-way' and '2-way' (also 1 and 2 for short).

If you specify '1-way', only properties corresponding to existing hash elements will be linked to those elements; properties added to the object from JavaScript will be JavaScript's own, and will not affect the wrapped object. (Consider how node lists and collections work in web browsers.)

If you specify '2-way', an attempt to create a property in JavaScript will be reflected in the underlying object.

To do: Make this accept '1-way:String', etc.


This is just like hash, but for arrays. This will also create a property named 'length'.

To do: Make this accept '1-way:String', etc.

unwrap => 1

If you specify this and it's true, objects passed as arguments to the methods or code refs specified above are 'unwrapped' if they are proxies for Perl objects (see below). And null and undefined are converted to undef.

This is experimental right now. I might actually make this the default. Maybe this should provide more options for fine-tuning, or maybe what is currently the default behaviour should be removed. If anyone has any opinions on this, please e-mail the author.

isa => 'ClassName'
isa => $prototype_object

(Maybe this should be renamed 'super'.)

The name of the superclass. 'Object' is the default. To make this new class's prototype object have no prototype, specify undef. Instead of specifying the name of the superclass, you can provide the superclass's prototype object.

If you specify a name, a constructor function by that name must already exist, or an exception will be thrown. (I supposed I could make JE smart enough to defer retrieving the prototype object until the superclass is registered. Well, maybe later.)

wrapper => sub { ... }

If wrapper is specified, all other arguments will be ignored except for package (or name if package is not present).

When an object of the Perl class in question is 'upgraded,' this subroutine will be called with the global object as its first argument and the object to be 'wrapped' as the second. The subroutine is expected to return an object compatible with the interface described in JE::Types.

If wrapper is supplied, no constructor will be created.

After a class has been bound, objects of the Perl class will, when passed to JavaScript (or the upgrade method), appear as instances of the corresponding JS class. Actually, they are 'wrapped up' in a proxy object (a JE::Object::Proxy object), that provides the interface that JS operators require (see JE::Types). If the object is passed back to Perl, it is the proxy, not the original object that is returned. The proxy's value method will return the original object. But, if the unwrap option above is used when a class is bound, the original Perl object will be passed to any methods or properties belonging to that class. This behaviour is still subject to change. See "unwrap", above.

Note that, if you pass a Perl object to JavaScript before binding its class, JavaScript's reference to it (if any) will remain as it is, and will not be wrapped up inside a proxy object.

To use Perl's overloading within JavaScript,, you don't have to do anything. If the object has "", 0+ or bool overloading, that will automatically be detected and used.


This returns a parser object (see JE::Parser) which allows you to customise the way statements are parsed and executed (only partially implemented).

$j->prototype_for( $class_name )
$j->prototype_for( $class_name, $new_val )

Mostly for internal use, this method is used to store/retrieve the prototype objects used by JS's built-in data types. The class name should be 'String', 'Number', etc., but you can actually store anything you like in here. :-)


If a piece of JS code is tainted, you can still run it, but any strings or numbers returned, assigned or passed as arguments by the tainted code will be tainted (even if it did not originated from within the code). E.g.,

  use Taint::Util;
  taint($code = "String.length");
  $foo = 0 + new JE  ->eval($code);  # $foo is now tainted

This does not apply to string or number objects, but, if the code created the object, then its internal value will be tainted, because it created the object by passing a simple string or number argument to a constructor.


Apart from items listed under "BUGS", below, JE follows the ECMAScript v3 specification. There are cases in which ECMAScript leaves the precise semantics to the discretion of the implementation. Here is the behaviour in such cases:

  • The global parseInt can interpret its first argument either as decimal or octal if it begins with a 0 not followed by 'x', and the second argument is omitted. JE uses decimal.

  • Array.prototype.toLocaleString uses ',' as the separator.

The spec. states that, whenever it (the spec.), say to throw a SyntaxError, an implementation may provide other behaviour instead. Here are some instances of this:

  • return may be used outside a function. It's like an 'exit' statement, but it can return a value:

      var thing = eval('return "foo"; this = statement(is,not) + executed')
  • break and continue may be used outside of loops. In which case they act like return without arguments.

  • Reserved words (except case and break) can be used as identifiers when there is no ambiguity.

  • Regular expression syntax that is not valid ECMAScript in general follows Perl's behaviour. (See JE::Object::RegExp for the exceptions.)

JE also supports the escape and unescape global functions (not part of ECMAScript proper, but in the appendix).


To report bugs, please e-mail the author.

Bona Fide Bugs

  • bind_class has a security hole: An object method’s corresponding Function object can be applied to any Perl object or class from within JS. (E.g., if you have allowed a Foo object's wibbleton method to be called from JS, then a Bar object's method of the same name can be, too.)

    Fixing this is a bit complicated. If anyone would like to help, please let me know. (The problem is that the same code would be repeated a dozen times in bind_class's closures--a maintenance nightmare likely to result in more security bugs. Is there any way to eliminate all those closures?)

  • The JE::Scope class, which has an AUTOLOAD sub that delegates methods to the global object, does not yet implement the can method, so if you call $scope->can('to_string') you will get a false return value, even though scope objects can to_string.

  • hasOwnProperty does not work properly with arrays and arguments objects.

  • Sometimes line numbers reported in error messages are off. E.g., in the following code--


    --, if foo is not a function, line 2 will be reported instead of line 1.

  • Currently, [:blahblahblah:]-style character classes don’t work if followed by a character class escape (\s, \d, etc.) within the class. /[[:alpha:]\d]/ is interpreted as /[\[:alph]\d\]/.

  • If, in perl 5.8.x, you call the value method of a JE::Object that has a custom fetch subroutine for one of its enumerable properties that throws an exception, you'll get an 'Attempt to free unreferenced scalar' warning.

  • On Solaris in perl 5.10.0, the Date class can cause an 'Out of memory' error which I find totally inexplicable. Patches welcome. (I don't have Solaris, so I can't experiment with it.)

  • Case-tolerant regular expressions allow a single character to match multiple characters, and vice versa, in those cases where a character's uppercase equivalent is more than one character; e.g., /ss/ can match the double S ligature. This is contrary to the ECMAScript spec. See the source code of JE::Object::RegExp for more details.

  • Currently any assignment that causes an error will result in the 'Cannot assign to a non-lvalue' error message, even if it was for a different cause. For instance, a custom fetch routine might die.

  • The parser doesn’t currently support Unicode escape sequences in a regular expression literal’s flags. It currently passes them through verbatim to the RegExp constructor, which then croaks.

  • Under perl 5.8.8, the following produces a double free; something I need to look into:

      "".new JE  ->eval(q| Function('foo','return[a]')() | )
  • The var statement currently evaluates the rhs before the lhs, which is wrong. This affects the following, which should return 5, but returns undefined:

      with(o={x:1})var x = (delete x,5); return o.x
  • Currently if a try-(catch)-finally statement’s try and catch blocks don't return anything, the return value is taken from the finally block. This is incorrect. There should be no return value. In other words, this should return 3:

      eval(' 3; try{}finally{5} ')
  • Compound assignment operators (+=, etc.) currently get the value of the rhs first, which is wrong. The following should produce "1b", but gives "2b":

      a = 1;  a += (a=2,"b")
  • Serialisation of RegExp objects with Data::Dump::Streamer is currently broken (and has been since 0.022).


  • JE is not necessarily IEEE 754-compliant. It depends on the OS. For this reason the Number.MIN_VALUE and Number.MAX_VALUE properties may not have the same values as ECMAScript, and sometimes rounding (via toPrecision, etc.) goes the wrong way.

  • A Perl subroutine called from JavaScript can sneak past a finally block and avoid triggering it:

      $j = new JE;
      $j->new_function(outta_here => sub { last outta });
      outta: {
              try { x = 1; outta_here() }
              finally { x = 2 }
      print $j->{x}, "\n";
  • NaN and Infinity do not work properly on some Windows compilers. 32-bit ActivePerl seems not to work, but I have been told 64-bit is OK. Strawberry Perl works fine, which is what most people are using.

Incompatibilities with ECMAScript...

...that are probably due to typos in the spec.

  • In a try-catch-finally statement, if the 'try' block throws an error and the 'catch' and 'finally' blocks exit normally--i.e., not as a result of throw/return/continue/break--, the error originally thrown within the 'try' block is supposed to be propagated, according to the spec. JE does not re-throw the error. (This is consistent with other ECMAScript implementations.)

    I believe there is a typo in the spec. in clause 12.14, in the 'TryStatement : try Block Catch Finally' algorithm. Step 5 should probably read 'Let C = Result(4),' rather than 'If Result(4).type is not normal, Let C = Result(4).'

  • If the expression between the two colons in a for(;;) loop header is omitted, the expression before the first colon is not supposed to be evaluated. JE does evaluate it, regardless of whether the expression between the two colons is present.

    I think this is also a typo in the spec. In the first algorithm in clause 12.6.3, step 1 should probably read 'If ExpressionNoIn is not present, go to step 4,' rather than 'If the first Expression is not present, go to step 4.'

  • The setTime method of a Date object does what one would expect (it sets the number of milliseconds stored in the Date object and returns that number). According to the obfuscated definition in the ECMAScript specification, it should always set it to NaN and return NaN.

    I think I've found yet another typo in the spec. In clause, 'Result(1)' and and 'Result(2)' are probably supposed to be 'Result(2)' and 'Result(3)', respectively.


perl 5.8.4 or higher

Scalar::Util 1.14 or higher

Exporter 5.57 or higher

Tie::RefHash::Weak, for perl versions earlier than 5.9.4

The TimeDate distribution (more precisely, Time::Zone and Date::Parse)

Encode 2.08 or higher

Note: JE will probably end up with Unicode::Collate in the list of dependencies.


Copyright (C) 2007-14 Father Chrysostomos <sprout [at] cpan [dot] org>

This program is free software; you may redistribute it and/or modify it under the same terms as perl.

Some of the code was derived from Data::Float, which is copyrighted (C) 2006, 2007, 2008 by Andrew Main (Zefram).


Some of the

Thanks to Max Maischein, Kevin Cameron, Chia-liang Kao and Damyan Ivanov for their contributions,

to Andy Armstrong, Yair Lenga, Alex Robinson, Christian Forster, Imre Rad, Craig Mackenna and Toby Inkster for their suggestions,

and to the CPAN Testers for their helpful reports.


The other JE man pages, especially the following (the rest are listed on the JE::Types page):


ECMAScript Language Specification (ECMA-262), JavaScript::SpiderMonkey and JavaScript::Lite--all interfaces to Mozilla's open-source SpiderMonkey JavaScript engine.