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

Apr 12

A Data-Driven Portfolio Landing Page Where Adding an Entry Doesn't Rebuild the App

#react #typescript #vite #architecture

5 min read

SEN LLC

Apr 12

A Tiny CSS Gradient Designer in 200 Lines — And Why Sorting Happens at Output Time

#javascript #webdev #css #design

4 min read

SEN LLC

Apr 12

An Area Unit Converter With 15 Units — Including Four Different Sizes of Japanese Tatami

#javascript #webdev #japanese #history

4 min read

SEN LLC

Apr 12

Getting Leap Years Right, and Listing All 27 Leap Seconds Ever Inserted

#javascript #webdev #history #datetime

4 min read

SEN LLC

Apr 12

Writing a Hepburn Hiragana ↔ Katakana ↔ Rōmaji Converter (With All The Annoying Edge Cases)

#javascript #webdev #i18n #tutorial

4 min read

SEN LLC

Apr 12

A BMI Calculator That Shows Both JASSO and WHO Categories, Because They Disagree

#javascript #webdev #health #tutorial

4 min read

SEN LLC

Apr 12

A Keyboard-Driven HTML Entity Lookup That Lets You Copy All Three Forms

#javascript #webdev #html #reference

4 min read

SEN LLC

Apr 12

A Searchable HTTP Status Code Reference with 36 Codes, One-Line Descriptions, and Yes, 418

#javascript #webdev #http #reference

4 min read

SEN LLC

Apr 12

Why 1100 / 1.1 = 999.9999… Is a Tax Calculator Bug, and Two Ways to Fix It

#javascript #webdev #math #tutorial

4 min read

SEN LLC

Apr 11

A GUI Cron Builder — And Why Building Is Way Easier Than Parsing

#javascript #webdev #cron #devtools

4 min read

SEN LLC

Apr 11

A Bill Splitter Where Unit and Rounding Are Orthogonal (They Usually Aren't)

#javascript #webdev #ux #math

4 min read

SEN LLC

Apr 11

Converting Between Japanese Eras and Gregorian Dates — and Why the Boundary Days Matter

#javascript #webdev #i18n #tutorial

4 min read

SEN LLC

Apr 11

Writing Conway's Game of Life With Uint8Array and Canvas, Including a Gosper Glider Gun

#javascript #webdev #canvas #tutorial

4 min read

SEN LLC

Apr 11

50 Japanese Traditional Colors, Sorted By Hue — Because the Names Aren't Enough

#javascript #webdev #css #design

4 min read

SEN LLC

Apr 11

Every Time Format At Once: A Converter That Stops the Unix-Seconds-or-Milliseconds Dance

#javascript #webdev #datetime #beginners

4 min read

SEN LLC

Apr 11

A JWT Debugger That Never Sends Your Token Anywhere

#javascript #security #jwt #webdev

4 min read

SEN LLC

Apr 11

I Built a Side-by-Side Base64 / URL / HTML / Encoder and Finally Stopped Confusing Them

#javascript #webdev #encoding #beginners

5 min read

SEN LLC

Apr 11

UUID v7, ULID, KSUID — What's the Difference? I Implemented All Five

#javascript #uuid #webdev #tutorial

6 min read

SEN LLC

Apr 11

Writing a Regex Parser and Tree Visualizer in ~400 Lines of Vanilla JS

#javascript #regex #webdev #tutorial

5 min read

SEN LLC

Apr 11

Writing a TypeScript Type Inference Engine in 300 Lines of Vanilla JS

#typescript #javascript #webdev #tutorial

5 min read

SEN LLC

Apr 10

You Probably Don't Need a Timezone Library Anymore

#showdev #javascript #webdev #tutorial

5 min read

DEV Community

SEN LLC

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Skills/Languages

Currently hacking on

A Data-Driven Portfolio Landing Page Where Adding an Entry Doesn't Rebuild the App

A Tiny CSS Gradient Designer in 200 Lines — And Why Sorting Happens at Output Time

An Area Unit Converter With 15 Units — Including Four Different Sizes of Japanese Tatami

Getting Leap Years Right, and Listing All 27 Leap Seconds Ever Inserted

Writing a Hepburn Hiragana ↔ Katakana ↔ Rōmaji Converter (With All The Annoying Edge Cases)

A BMI Calculator That Shows Both JASSO and WHO Categories, Because They Disagree

A Keyboard-Driven HTML Entity Lookup That Lets You Copy All Three Forms

A Searchable HTTP Status Code Reference with 36 Codes, One-Line Descriptions, and Yes, 418

Why 1100 / 1.1 = 999.9999… Is a Tax Calculator Bug, and Two Ways to Fix It

A GUI Cron Builder — And Why Building Is Way Easier Than Parsing

A Bill Splitter Where Unit and Rounding Are Orthogonal (They Usually Aren't)

Converting Between Japanese Eras and Gregorian Dates — and Why the Boundary Days Matter

Writing Conway's Game of Life With Uint8Array and Canvas, Including a Gosper Glider Gun

50 Japanese Traditional Colors, Sorted By Hue — Because the Names Aren't Enough

Every Time Format At Once: A Converter That Stops the Unix-Seconds-or-Milliseconds Dance

A JWT Debugger That Never Sends Your Token Anywhere

I Built a Side-by-Side Base64 / URL / HTML / Encoder and Finally Stopped Confusing Them

UUID v7, ULID, KSUID — What's the Difference? I Implemented All Five

Writing a Regex Parser and Tree Visualizer in ~400 Lines of Vanilla JS

Writing a TypeScript Type Inference Engine in 300 Lines of Vanilla JS

You Probably Don't Need a Timezone Library Anymore