Running JavaScript within Ruby April 17, 2025 @hmsk 

.self @hmsk / Kengo Hamasaki Product Engineer at Persona withpersona.com RubyKaigi 2010-2019 Organizer/Sta ff /Volunteer Home-brewer Origin from Ehime 

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gem 'quickjs' github:hmsk/quickjs.rb 

irb⏎ 

quickjs https://bellard.org/quickjs/ “A small and embeddable JavaScript engine” By Fabrice Bellard Support ES2023 (Almost) C, standalone, light 

Just wrapped it…? Why I need to run JS in Ruby? What does building a native ext today look like today? How might we run JS in Ruby? Real world outcome 

raise 

withpersona.com Identity veri fi cation SaaS platform Monolithic Rails We’re hiring Rubyists :) 

Inquiry Veri fi cation Case Graph 

Work fl ows 

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“Custom Code” Code on No-code Temporal glue for products’ gap Arbitrary JS code by customers “Sandboxing” Run on Function as a Service 

Problem with FaaS Add points of failure and latency Host, network Maintenance cost Node, npm for non-Frontend purpose in Ruby company Implement “XYZ” again for the env…? e.g.) handling outgoing HTTP requests 

Run within Ruby/Rails? Prior arts mini_racer (v8), Node Duktape (ES5) quickjs looked minimal and modern C, standalone, very light, Support ES2023 (Almost) Some adoptions in the community 

‘kay, then build a native ext Somewhat I knew about C For micro-computers at Kosen in 2005 Have built one; github:hmsk/lzfx From a workshop by @sgwr_dts at Cookpad in 2010 JUST LINK AND CALL Started from a personal project in June 2024 

rescue 

New to native exts in 2024 Great modern resources Articles, implementations by other languages Incremental C-ing With powerful Ruby features Found the bless by CRuby authors 💖 AI helped sometimes 

Great modern resources “A Rubyist's Walk Along the C-side” https://blog.peterzhu.ca/ruby-c-ext/ By Peter Zhu; Ruby committer, Speaker at Day 3 ✨ Learn from quickjs bindings for other languages quickjs-go, rquickjs, quickjs-rust, and quickjs-python 

Incremental C-ing 1. Write what I need by Ruby 2. Just call it from C VALUE class = rb_const_get(rb_cClass, rb_intern("class name”)); rb_funcall(class, rb_intern(“method name”), 0); 3. Write tests in Ruby for it 4. Convert Ruby impl to C gradually 5. REPEAT! 

e.backtrace 

Not only just wrapping… Sandboxing Poly fi lls Interfacing for Ruby 

Sandboxing Disable all features touching OS resources directly Optionally allow it Provide a replacement Control timeout and stack size 

Poly fi lls “Intl” is not fully supported by quickjs new Date().toLocaleString("en-US", { month: "short", day: "2-digit", year: "numeric", }); Expected: "Apr 17, 2025" Quickjs: "04/17/2025, 11:25:00 AM" Provide poly fi lls by @formatjs/intl-* 

Poly fi lls Evaluate huge JS every time…?? Compile the JS to C, then compile within quickjs.rb’s build quickjs supports compiling JS to C or binary (WHAT!?) 

“How we wanna run JavaScript from Ruby…?” Import ESM from Ruby String Persisting logs from console.log…etc Call Ruby from JS Interfacing for Ruby 

import ESM Import from String of Ruby Import like how JavaScript does # JS: import aliasedDefault, { member } from './exports.esm.js'; vm.import({ default: 'aliasedDefault', member: 'member' }, from: File.read('exports.esm.js')) # JS: import { member, defaultMember } from './exports.esm.js'; vm.import(['member', 'defaultMember'], from: File.read('exports.esm.js')) # JS: import DefaultExport from './exports.esm.js'; vm.import('DefaultExport', from: File.read('exports.esm.js')) # JS: import * as all from './exports.esm.js'; vm.import('* as all', from: File.read('exports.esm.js')) 

console.log Record logs from console.(log|info|warn|error) irb⏎ 

de fi ne_methodfunction Ruby evaluates JavaScript which calls Ruby e.g.) global.fetch (in JS) -> net/http (in Ruby) Translate exceptions transparently raise in Ruby-> catch is JS raise in Ruby -> (no catch in JS) -> raise in Ruby irb⏎ 

ensure 

Use in real world Now processing over 400M/day, 2.5k/min (in peek) Save the cost of FaaS Obvious improvement in performance Found some problems per extending use-cases 😇 

“Single source” solution A coworker adopted for a di ff erent purpose while I was still experimenting 🤣 A parser for DSL for both the backend and frontend Massive tra ffi c; 200+M/day, 50k/min 

end github:hmsk/quickjs.rb Building native exts for CRuby is fun Thanks, RubyKaigi! withpersona.com is hiring I have some swag for the token of my referral