JavaScript::QuickJS - Run JavaScript via QuickJS in Perl


Quick and dirty …

    my $val = JavaScript::QuickJS->new()->eval( q<
        let foo = "bar";
        [ "The", "last", "value", "is", "returned." ];
    > );

… or load ES6 modules:

    my $js = JavaScript::QuickJS->new()->helpers();

    $js->eval_module( q/
        import * as coolStuff from 'cool/stuff';

        for (const [key, value] of Object.entries(coolStuff)) {
            console.log(key, value);
    / );


This library embeds Fabrice Bellard’s QuickJS engine into a Perl XS module. You can thus run JavaScript (ES2020 specification) directly in your Perl programs.

This distribution includes all needed C code; unlike with most XS modules that interface with C libraries, you don’t need QuickJS pre-installed on your system.


$obj = CLASS->new( %CONFIG_OPTS )

Instantiates CLASS. %CONFIG_OPTS have the same effect as in configure() below.

$obj = OBJ->configure( %OPTS )

Tunes the QuickJS interpreter. Returns OBJ.

%OPTS are any of:

  • max_stack_size

  • memory_limit

  • gc_threshold

For more information on these, see QuickJS itself.

$obj = OBJ->set_globals( NAME1 => VALUE1, .. )

Sets 1 or more globals in OBJ. See below for details on type conversions from Perl to JavaScript.

Returns OBJ.

$obj = OBJ->helpers()

Defines QuickJS’s “helpers”, e.g., console.log.

Returns OBJ.

$obj = OBJ->std()

Enables QuickJS’s std module and creates a global of the same name that’s usable from both script and module modes.

This resembles qjs’s --std flag except that it only enables std, not os.

Returns OBJ.

$obj = OBJ->os()

Like std() but enables QuickJS’s os module instead of std.

$VALUE = OBJ->eval( $JS_CODE )

Like running qjs -e '...'. Returns $JS_CODE’s last value; see below for details on type conversions from JavaScript to Perl.

Untrapped exceptions in JavaScript will be rethrown as Perl exceptions.

$JS_CODE is a character string.

$promise = OBJ->eval_module( $JS_CODE )

Runs $JS_CODE as a module, which enables ES6 module syntax. Note that no values can be returned directly in this mode of execution.

Returns a promise that resolves once the module is loaded.

$obj = OBJ->await()

Blocks until all of OBJ’s pending work (if any) is complete.

For example, if you eval() some code that creates a promise, call this to wait for that promise to complete.

Returns OBJ.

$obj = OBJ->set_module_base( $PATH )

Sets a base path (a byte string) for ES6 module imports.

Returns OBJ.

$obj = OBJ->unset_module_base()

Restores QuickJS’s default directory for ES6 module imports (as of this writing, it’s the process’s current directory).

Returns OBJ.


This module converts returned values from JavaScript thus:

  • JS string primitives become character strings in Perl.

  • JS number & boolean primitives become corresponding Perl values.

  • JS null & undefined become Perl undef.

  • JS objects …


Generally speaking, it’s the inverse of JS → Perl:

  • Perl strings, numbers, & booleans become corresponding JavaScript primitives.

    IMPORTANT: Perl versions before 5.36 don’t reliably distinguish “numeric strings” from “numbers”. If your perl predates 5.36, typecast accordingly to prevent your Perl “number” from becoming a JavaScript string. (Even in 5.36 and later it’s still a good idea.)

  • Perl undef becomes JS null.

  • Unblessed array & hash references become JavaScript arrays and “plain” objects.

  • Types::Serialiser booleans become JavaScript booleans.

  • Perl code references become JavaScript functions.

  • Perl JavaScript::QuickJS::Function, JavaScript::QuickJS::RegExp, and JavaScript::QuickJS::Date objects become their original JavaScript objects.

  • Anything else triggers an exception.


If any instance of a class of this distribution is DESTROY()ed at Perl’s global destruction, we assume that this is a memory leak, and a warning is thrown. To prevent this, avoid circular references, and clean up all global instances.

Callbacks make that tricky. When you give a JavaScript function to Perl, that Perl object holds a reference to the QuickJS context. Only once that object is DESTROY()ed do we release that QuickJS context reference.

Consider the following:

    my $return;

    $js->set_globals(  __return => sub { $return = shift; () } );

    $js->eval('__return( a => a )');

This sets $return to be a JavaScript::QuickJS::Function instance. That object holds a reference to $js. $js also stores __return(), which is a Perl code reference that closes around $return. Thus, we have a reference cycle: $return refers to $js, and $js refers to $return. Those two values will thus leak, and you’ll see a warning about it at Perl’s global destruction time.

To break the reference cycle, just do:

    undef $return;

… once you’re done with that variable.

You might have thought you could instead do:

    $js->set_globals( __return => undef )

… but that doesn’t work because $js holds a reference to all Perl code references it ever receives. This is because QuickJS, unlike Perl, doesn’t expose object destructors (DESTROY() in Perl), so there’s no good way to release that reference to the code reference.


QuickJS (like all JS engines) assumes its strings are text. Since Perl can’t distinguish text from bytes, though, it’s possible to convert Perl byte strings to JavaScript strings. It often yields a reasonable result, but not always.

One place where this falls over, though, is ES6 modules. QuickJS, when it loads an ES6 module, decodes that module’s string literals to characters. Thus, if you pass in byte strings from Perl, QuickJS will treat your Perl byte strings’ code points as character code points, and when you combine those code points with those from your ES6 module you may get mangled output.

Another place that may create trouble is if your argument to eval() or eval_module() (above) contains JSON. Perl’s popular JSON encoders output byte strings by default, but as noted above, eval() and eval_module() need character strings. So either configure your JSON encoder to output characters, or decode JSON bytes to characters before calling eval()/eval_module().

For best results, always interact with QuickJS via character strings, and double-check that you’re doing it that way consistently.


Note the following if you expect to deal with “large” numbers:

  • JavaScript’s numeric-precision limits apply. (cf. Number.MAX_SAFE_INTEGER.)

  • Perl’s stringification of numbers may be less precise than JavaScript’s storage of those numbers, or even than Perl’s own storage. For example, in Perl 5.34 print 1000000000000001.0 prints 1e+15.

    To counteract this loss of precision, add 0 to Perl’s numeric scalars (e.g., print 0 + 1000000000000001.0); this will encourage Perl to store numbers as integers when possible, which fixes this precision problem.

  • Long-double and quad-math perls may lose precision when converting numbers to/from JavaScript. To see if this affects your perl—which, if you’re unsure, it probably doesn’t—run perl -V, and see if that perl’s compile-time options mention long doubles or quad math.


QuickJS supports Linux, macOS, and Windows natively, so these work without issue.

FreeBSD, OpenBSD, & Cygwin work after a few patches that we apply when building this library. (Hopefully these will eventually merge into QuickJS.)


QuickJS uses C11 atomics. Most platforms implement that functionality in hardware, but others (e.g., arm32) don’t. To fill that void, we need to link to libatomic.

This library’s build logic detects whether libatomic is necessary and will only link to it if needed. If, for some reason, you need manual control over that linking, set JS_QUICKJS_LINK_LIBATOMIC in the environment to 1 or a falsy value.

If you don’t know what any of that means, you can probably ignore it.


Other JavaScript modules on CPAN include:


This library is copyright 2023 Gasper Software Consulting.

This library is licensed under the same terms as Perl itself. See perlartistic.

QuickJS is copyright Fabrice Bellard and Charlie Gordon. It is released under the MIT license.