SEN LLC

I build small, focused tools and ship them open-source. Based in Japan 🇯🇵 with SEN LLC, working through a 100+ portfolio at sen.ltd/portfolio. Posts about TypeScript, AWS, and bugs that taught me so

Tokyo, Japan Joined on Apr 10, 2026 https://sen.ltd

Work

Engineer at SEN LLC

SEN LLC

May 5

A Live KeyboardEvent Inspector — Settling the `event.key` vs `event.code` Question for JS Shortcuts

#javascript #webdev #frontend #dom

5 min read

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SEN LLC

May 3

Calling the Anthropic API Directly From the Browser — A 150-Line BYOK Comparison Tool for Opus / Sonnet / Haiku

#javascript #anthropic #ai #webdev

5 min read

SEN LLC

May 2

A 150-Line Rust CLI That Renders QR Codes in Your Terminal — Half-Block Characters Pack Two Modules per Cell

#rust #cli #productivity #devops

4 min read

SEN LLC

May 1

The Three Conditions for Seeing the ISS — A Browser-Only Visibility Calculator with Spherical Trigonometry

#javascript #space #math #webdev

5 min read

SEN LLC

Apr 30

Sample-Free Drum Synthesis in Web Audio — Building Kick, Snare, and Hi-Hat from Oscillators in 60 Lines

#javascript #webaudio #audio #music

6 min read

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Apr 29

A Per-URL TODO List Chrome Extension in 150 Lines — Single-Key chrome.storage Design, URL Normalization, and Three Sync Paths for the Toolbar Badge

#chrome #javascript #productivity #webdev

5 min read

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Apr 28

A 150-Line `whentime tokyo london ny` CLI in Rust — and Why You Need IANA tzdata, Not 'UTC + N'

#rust #cli #productivity #devops

5 min read

SEN LLC

Apr 28

Three Address Checksums, Three Engineering Philosophies — Verifying Base58Check, Bech32m, and EIP-55 in 250 Lines of Browser JS

#javascript #crypto #bitcoin #ethereum

6 min read

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Apr 27

A 230-Line Chrome MV3 Extension That Copies the Page Selection as Markdown — Without `<all_urls>`

#chrome #javascript #webdev #productivity

6 min read

SEN LLC

Apr 25

BIP39 in 180 Lines of Vanilla JS — Mnemonic Generation, Validation, Seed Derivation, and the Japanese Wordlist Trap

#javascript #crypto #security #webdev

6 min read

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Apr 24

Why Your Web Audio Tuner Sucks at Low Notes (and What to Use Instead of FFT)

#javascript #webaudio #frontend #audio

6 min read

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Apr 24

GitHub's Stargazers API Caps at 40,000. Here's How Star-History Tools Fake the Rest

#github #api #javascript #dataviz

6 min read

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Apr 23

A 3.7 KB Othello Engine: Bitboards in Rust, Raw WASM, No wasm-bindgen

#rust #webassembly #gamedev

6 min read

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Apr 22

I Built a Japanese Poetry Quiz and the Web Speech API Showed Me Its Teeth

#vue #typescript #webspeechapi #japanese

5 min read

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Apr 21

I Finally Understand the SVG d Attribute — Because I Built an Editor for It

#svg #svelte #typescript #webdev

8 min read

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Apr 20

I Put the Same Mandelbrot Kernel on the CPU and the GPU — and Watched float32 Crack

#webgl #javascript #graphics #performance

6 min read

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Apr 20

Why setTimeout Is a Bad Metronome — and What to Use Instead

#javascript #webaudio #frontend #performance

5 min read

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Apr 19

How Much Faster Is Rust/WASM Than JS for Image Filters? I Ran Both Side by Side and Measured

#webassembly #rust #javascript #performance

6 min read

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Apr 19

A Cross-OS Port Finder in Rust — One CLI, Three Completely Different Data Formats

#rust #cli #linux #networking

9 min read

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Apr 18

A 300-Line GitHub Actions Security Linter: Five Rules That Catch the CVE Patterns

#typescript #githubactions #security #cli

7 min read

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Apr 16

A TLS Certificate CLI in Rust — No OpenSSL, No Ceremony, Just a Fail-the-Build Exit Code

#rust #cli #tls #monitoring

7 min read

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Apr 16

CSV Sort, Done Right: Why `sort -t,` Is a Trap and What a 300-Line PHP CLI Does Instead

#php #cli #csv #tutorial

9 min read

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Apr 16

Reimplementing fd in 500 lines of Rust (and what I learned about the crates it stands on)

#rust #cli #filesystem #tutorial

10 min read

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Apr 16

I Built a .env Diff Tool That Refuses to Print Values

#typescript #env #cli #tutorial

10 min read

SEN LLC

Apr 16

Writing a Regex Playground in 200 Lines of Rust

#rust #regex #cli #tutorial

9 min read

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Apr 16

A 10 MB Markdown-to-HTML Service in Rust with pulldown-cmark

#rust #axum #markdown #tutorial

9 min read

SEN LLC

Apr 16

git log --pretty is a tiny DSL: building a zero-dep CHANGELOG generator

#typescript #git #cli #tutorial

8 min read

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Apr 16

Seed Data Without a Dependency Tree: A Tiny PHP CLI Instead of Faker

#php #cli #database #tutorial

10 min read

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Apr 16

sharp on Alpine Without Tears: a 260 MB Image Resize API in Hono

#typescript #hono #sharp #tutorial

9 min read

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Apr 16

Writing a tar Inspector in Rust with Three Dependencies

#rust #tar #cli #tutorial

9 min read

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Apr 16

A Tiny PHP Invoice-PDF API: Slim 4, dompdf, and the JPY Rounding Footgun

#php #slim #pdf #tutorial

9 min read

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Apr 16

Writing dig in 500 Lines of Rust (with hickory-resolver, the trust-dns successor)

#rust #dns #cli #tutorial

10 min read

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Apr 16

Writing Cron Semantics in ~100 Lines of TypeScript

#typescript #cron #cli #tutorial

8 min read

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Apr 16

PHPStan's JSON Output Is Noise. I Wrote a Formatter with a Baseline Diff.

#php #phpstan #cli #tutorial

9 min read

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Apr 16

I Got Tired of Pasting EXPLAIN Plans Into Web Tools, So I Wrote pg-explain-viz

#rust #postgres #cli #tutorial

10 min read

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Apr 16

Discord Slash Commands Without an SDK: ed25519 in 250 Lines of TypeScript

#typescript #hono #discord #tutorial

10 min read

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Apr 16

Writing an HTTP Load Tester That Doesn't Lie About p99

#rust #benchmark #http #tutorial

8 min read

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Apr 16

A Portfolio-Sized URL Shortener in 300 Lines of PHP

#php #slim #urlshortener #tutorial

9 min read

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Apr 16

Describing Cron Expressions: Why Templates Don't Work, and What Does

#typescript #cron #cli #tutorial

8 min read

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Apr 16

Writing a YAML Linter in Rust Because YAML Is Secretly Terrible

#rust #yaml #cli #tutorial

11 min read

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Apr 16

Parsing Prisma Schema By Hand: A 300-Line CLI That Emits Mermaid ER Diagrams

#typescript #prisma #mermaid #tutorial

10 min read

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Apr 16

Debugging TCP by Building Your Own Logging Proxy in 400 Lines of Rust

#rust #tokio #networking #tutorial

8 min read

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Apr 16

Building an XPath Query Tool in PHP — Slice XML Like a Pro

#php #cli #xml #xpath

8 min read

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Apr 16

Building a Changelog Generator in Rust — From Conventional Commits to Keep a Changelog

#rust #cli #git #changelog

8 min read

SEN LLC

Apr 16

Building a .gitignore Generator CLI in Python — 51 Templates, Zero Dependencies

#python #cli #git #devtools

9 min read

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Apr 16

Building a MIME Type Lookup CLI in TypeScript — 800+ Types, Magic Bytes, and Zero Dependencies

#typescript #cli #mime #fileformat

8 min read

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Apr 16

Building a TOML Formatter in Rust — Sorted Keys, Aligned Equals, and CI-Friendly Check Mode

#rust #cli #toml #formatter

8 min read

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Apr 16

Building an HTTP Status Code Reference CLI in PHP — No Dependencies Required

#php #cli #http #tutorial

8 min read

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Apr 16

I Wrote a .env Linter from Scratch — Here Are the 9 Rules That Actually Matter

#python #cli #dotenv #tutorial

8 min read

SEN LLC

Apr 16

Building a Configurable TCP Echo Server in Rust with Tokio

#rust #networking #tcp #cli

8 min read

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Apr 16

Implementing semver 2.0.0 from scratch in TypeScript — no semver package allowed

#typescript #cli #semver #devtools

9 min read

SEN LLC

Apr 16

Building a strict INI parser from scratch in Python — no configparser allowed

#python #cli #config #parser

8 min read

SEN LLC

Apr 16

Building a Concurrent TCP Port Scanner in Rust with Tokio Semaphores

#rust #networking #async #cli

9 min read

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Apr 16

phpcron: a tiny PHP CLI for poking at cron expressions

#php #cli #cron #scheduling

8 min read

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Apr 16

Writing a compliant RFC 6902 JSON Patch CLI in one evening, zero runtime deps

#typescript #cli #json #rfc6902

9 min read

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Apr 16

A 600-line Rust log rotator, or: what logrotate would look like if it had no config file

#rust #cli #logs #ops

10 min read

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Apr 15

Micro-benchmarking TypeScript Without Lying to Yourself

#typescript #benchmark #cli #tutorial

8 min read

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Apr 15

The Four Rate-Limiting Algorithms You Should Know (I Built a PHP Service That Runs All Five)

#php #slim #ratelimiting #tutorial

10 min read

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Apr 15

A Polite Port Scanner in 400 Lines of Tokio

#rust #cli #networking #tutorial

9 min read

SEN LLC

Apr 15

Counting TypeScript Escape Hatches — A Zero-Dependency CLI with a Baseline Gate

#typescript #cli #migration #tutorial

9 min read

Writing Debut

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