About JavaScript.ac
Learn JavaScript by writing JavaScript β directly in your browser.
Our Story
We started JavaScript.ac in early 2024 because every developer in our team had at some point watched a friend or coworker bounce off learning JavaScript β not because the language is hard, but because the path to "running real code" was hidden behind installers, package managers, build tools, and accidentally-broken Node versions. We wanted a place where you could open a tab, see a problem, write a solution, hit Run, and learn something β in the same minute. That single design constraint shaped everything that came after.
The first version of the site was a single problem and a Monaco editor wired to a tiny eval harness. Within a week we had ten more problems, and within a month we had what is roughly the structure you see today: a curated set of problems, tutorials, a code playground, a JavaScript reference, a Go reference for cross-trainers, a printable cheatsheet, an interview-prep section, learning roadmaps, and a multilingual surface that meets people in English, Korean, Japanese, and Chinese.
We are unapologetically a small team. We treat that as an advantage: every problem you read was written by a working JavaScript developer, every tutorial was reviewed line-by-line, and every translation passed through native-speaker eyes. Nothing on the site is drive-by content. The trade-off is that we move slower than venture-backed competitors β but we move with intent, and the content compounds.
We also chose, deliberately, to keep JavaScript.ac free. We monetise with display advertising, never with paywalls, never with content gates, never by selling user data. The first time we shipped an ad, our internal rule was simple: if the page becomes worse to use because of the ad, the ad goes. That rule still holds.
Our Mission
JavaScript.ac exists to lower the barrier to entry for self-taught developers worldwide. Whether you are a complete beginner taking your first steps in coding, a student preparing for technical interviews, an engineer brushing up on a fundamental, or a professional dropping in to grab a regex or a snippet β our goal is that you find what you came for in seconds and leave knowing something concrete.
We believe the best way to learn programming is by doing. Reading about closures is fine; running a closure, breaking it, and reading the error message is unforgettable. So we built the entire platform around a fast, browser-native execution loop: zero install, zero login, immediate feedback. The editor is the same one that powers VS Code (Monaco), the JavaScript engine is your browser's V8 / SpiderMonkey / JavaScriptCore, and the page renders before most marketing sites have finished bouncing their hero animation.
Long-term, our mission is broader than JavaScript itself. We want to be the place developers send a friend who is "thinking about getting into programming" β and have that friend three months later still using the site for daily reference, interview prep, or a quick browser-based playground.
Who We Are
JavaScript.ac is operated by SDK Co., Ltd., a Korean software company founded in 2024 that builds developer-focused tools and free online utilities. Our team has been writing JavaScript professionally since the Node.js 0.x era and has shipped production code across SvelteKit, React, Vue, Svelte, Astro, and vanilla browser environments. We have built and operated APIs serving millions of monthly requests, and we have debugged enough memory leaks, hydration mismatches, and async race conditions to know which lessons new developers benefit from most.
Beyond JavaScript.ac, the same team operates SDK.is (developer utilities), SDK.bz (financial and crypto calculators), and SD.gy (everyday calculators). The shared infrastructure means lessons learned on one site β accessibility patterns, performance budgets, ad-density thresholds β get applied across all of them.
- Company
- SDK Co., Ltd.
- Founded
- 2024
- Focus
- Developer tools, learning platforms, and free online utilities
- Business Registration
- 871-81-03242
- E-Commerce Registration
- 2024-κ³ μμΌμ°μ-1820
- Address
- 4F, 127 Namseong-ro, Jeju-si, Jeju Special Self-Governing Province, Republic of Korea
What You'll Find on the Site
Everything below is free, immediate, and runs in your browser.
Instant Code Playground
A Monaco-based editor (the same one that ships in VS Code) wired to your browser's native JavaScript engine. Write code, hit Run, see output. No installs, no signup, no waiting on a sandbox to spin up. Console output is captured and displayed inline. Ideal for quick experiments, snippet testing, and teaching demos.
45+ Curated Practice Problems
A growing collection of problems organised by category (basics, arrays, strings, objects, functions, algorithms, async) and difficulty (beginner, easy, medium, hard). Each problem ships with the prompt, examples, hidden test cases, and (after you complete it) a discussion of the canonical solution and common pitfalls.
Step-by-Step Tutorials
Long-form tutorials covering closures, prototypes, the event loop, async/await, modules, ES2020+ features, regular expressions, and more. Each tutorial is structured so you can read it linearly or jump straight to a section. Code samples are copy-paste-runnable in the playground.
JavaScript Reference
Quick-access reference for built-in objects, methods, syntax, and patterns. Designed for the moment when you remember the shape of an API but not the exact arguments β so you don't have to leave your editor to look up a method signature.
Go Reference
A growing Go-language reference for developers cross-training between JavaScript and Go β typically backend engineers picking up Go, or Go engineers occasionally writing frontend code. Same fast-lookup philosophy as the JavaScript reference.
Printable Cheatsheet
A one-page, printable cheatsheet of must-know JavaScript snippets, idioms, and gotchas. Designed for the wall above your desk or for the bottom of an interview-prep folder.
Interview Prep Track
A curated path through the topics that come up most often in JavaScript-heavy interviews β closures, async patterns, prototype chain, Big-O on arrays and strings, system-design fundamentals for frontend engineers.
Learning Roadmaps
Opinionated roadmaps that tell you, given where you are right now, what to learn next and in what order. Designed for self-taught developers who don't have a curriculum handed to them.
JavaScript News
A curated feed of meaningful JavaScript ecosystem news β language proposals, runtime releases, framework milestones, browser changes β without the daily-noise of social timelines.
Multilingual Support
First-class English, Korean, Japanese, and Chinese versions. Translations are reviewed by native speakers, not produced by a one-shot machine pass and forgotten.
Our Principles
Free Forever
Core learning content is free for everyone. Ads keep the lights on, never paywalls.
Privacy-First
Code runs in your browser. We do not record your keystrokes, we do not store your solutions on our servers, we do not log your code anywhere.
Fast by Default
Deployed on Cloudflare's global edge for sub-100ms response times worldwide. Pages render before most analytics scripts have loaded.
Open Access
No login required to read, learn, or run code. Sign-in is optional and only used for cross-device progress tracking, where offered.
Beginner-Friendly
Every concept is explained from first principles. We assume curiosity, not prior knowledge.
No Dark Patterns
No fake countdown timers, no roach-motel cookie banners, no manufactured scarcity. The only thing we want you to do is learn.
Technology Stack
We chose every piece of the stack to optimise for reading speed, write speed, and long-term maintainability with a small team.
Our frontend is built on SvelteKit and TypeScript, styled with Tailwind CSS, and deployed to Cloudflare Pages. The code editor is Monaco, customised with a project-specific theme. We use no traditional backend: pages are statically pre-rendered or generated at request time at the edge, code execution happens entirely in the user's browser, and dynamic features that need persistence use the user's own localStorage.
This architecture has practical consequences for you. First, time-to-interactive is measured in tens of milliseconds, not seconds. Second, your code never leaves your machine, so there is no server log to leak. Third, our infrastructure cost is low enough that we can afford to keep the platform free indefinitely without ever introducing a "Pro" tier.
Privacy Promise
All code execution and problem solving happens client-side in your browser. We do not transmit your code to any server. The only data we collect is standard, aggregated analytics (page views, country, device type) and AdSense-related cookies as described in our Privacy Policy. We never sell or share personal data, we never read what you type into the editor, and we never use your activity to train machine-learning models.
If our ad provider changes its consent rules, we will update the consent banner immediately and post a clear changelog entry on the Privacy Policy page. We treat consent as a permanent design choice, not as a compliance afterthought.
Contact Us
Have questions, suggestions, or feedback? We'd love to hear from you.
- General questions and feedback: contact@javascript.ac
- Bug reports: please include reproduction steps, browser, and OS.
- Content suggestions: tell us what you wish existed on the site β we keep a public-ish backlog and prioritise from it.
- Partnership and content collaboration: include "Partnership" in your subject line and we'll route it accordingly.
- Press and media: same e-mail address, with "Press" in the subject line.
Legal & Operator Information
JavaScript.ac is operated by SDK Co., Ltd., a Korean corporation registered with the Republic of Korea's Ministry of the Interior and Safety and the Korean Fair Trade Commission.