I built 60+ developer tools that run 100% in your browser — here's how the "network" ones work

김상현 (Developk) — Sun, 18 Oct 2026 13:26:00 +0000

Every time I paste a JWT into some random online decoder, a small voice in my head asks: where is this token going? Most online tools POST your input to a server. For a JSON formatter that's mildly annoying. For a JWT decoder or a password generator, it's genuinely bad practice.

So I built developk.kr — a collection of 60+ developer utilities where nothing you type ever leaves your browser. No installs, no sign-ups, no uploads. Formatting, hashing, encoding, diffing — all client-side JavaScript.

The easy part was the tools that are pure computation. The interesting part was the "network" tools — DNS lookup, WHOIS, SSL checks — which need data from the outside world. Here's how I kept those server-free too.

The trick for network tools: public CORS-friendly APIs

A DNS lookup tool can't just open a socket in the browser. Instead of running my own backend, I used existing public resolvers that speak HTTP:

DNS / MX lookups → DNS over HTTPS via cloudflare-dns.com, which returns JSON and allows CORS:

const res = await fetch(
  "https://cloudflare-dns.com/dns-query?name=example.com&type=A",
  { headers: { accept: "application/dns-json" } }
);
const { Answer } = await res.json();

WHOIS → RDAP (rdap.org) instead of port-43 WHOIS. RDAP is the modern JSON replacement for WHOIS and works fine over HTTPS.
IP geolocation → ip-api.com / ipwho.is free tiers.
HTTP header / OGP checks → direct fetch to the target with mode: "cors", and the tool explains when a target blocks it.

Result: the "server" for these tools is the public internet itself. My site ships only static files.

What stays client-side

Everything else never touches a network at all:

JSON formatter / encoder / diff — pure JS parsing and diffing
Hash generator — crypto.subtle.digest() (the Web Crypto API, not a JS sha lib)
Base64 / URL / HTML entity encoders — btoa/atob plus manual UTF-8 handling for non-ASCII
CIDR calculator, timestamp converter, JWT decoder — bit math and Date

The JWT decoder is the one I'm happiest about: it decodes the header and payload segments and formats claims (exp, iat, nbf) into human-readable timestamps — all offline. Decoding a JWT is just Base64URL parsing; it never needed a server, yet most online decoders still send your tokens to theirs.

Why "no server" is more than a privacy slogan

Beyond not seeing your data, it changes the whole cost and trust model:

Zero upload latency — a 5MB JSON file formats instantly because nothing goes over the wire
Nothing to breach — there is no server-side storage to leak
Static hosting — the whole site is HTML/CSS/JS on Cloudflare Pages, so it's cheap and fast anywhere

Reference content as a bonus

Each tool also ships with a written guide (60+ of them so far) — e.g. a JWT explained guide with the registered claims table, or a Base64 alphabet reference. These are meant to be the pages you bookmark, with the tool one tab away.

The tools are all free at developk.kr. There's also a sister set for PDF work (pdf.developk.kr) and images (image-tools.developk.kr) with the same browser-only rule — files are processed locally and never uploaded.

If you've got a network tool idea that seems impossible without a backend, I'd bet there's a public HTTPS API that makes it work. Curious what others have used for this pattern.

I built a Remotion kit that renders vertical shorts from a single content file

김상현 (Developk) — Fri, 09 Oct 2026 14:42:24 +0000

Faceless short-form channels live or die on volume — you have to ship daily. But every tool in that space wants a subscription, a timeline editor, or an API key you babysit.

So I built a kit that turns a small TypeScript content file into a finished vertical MP4, using Remotion — React, but the output is a video file.

The promo above is one continuous 3D camera shot — pure CSS transforms, no WebGL.

The workflow

One file = one video:

// src/content/my-quote.ts
export const props: QuoteVideoProps = {
  theme: "paper",
  quote: "You have power over your mind, not outside events...",
  author: "Marcus Aurelius",
  authorTitle: "Meditations",
  cta: "Daily stoic wisdom — follow",
};

npx remotion render QuoteVideo out/quote.mp4

Zod schemas validate the props, which has a nice side effect: AI coding agents (Claude Code, Cursor, Devin) can generate content files safely — the schema catches their mistakes before render.

What's inside

7 formats × 5 themes:

RedditStory — narration over drifting captions (the AITA/horror genre)
TextStory — fake-text conversations
QuoteVideo — cinematic quotes
Listicle — top-N countdowns
Quiz — trivia engagement bait
CaptionOverlay — word-level captions over your own footage
DataStory — animated bar-chart races

Video length derives from the content — no durationInFrames to babysit.

Try one template free

I open-sourced the QuoteVideo template (MIT). Clone, npm install, edit one file, render:

👉 https://github.com/developk/remotion-quote-shorts

The full kit is $19 on Gumroad and just got listed on the official Remotion resources page.

Happy to answer questions — what would you build with it?

DEV Community: 김상현 (Developk)