About LocalDevTools

LocalDevTools is a small collection of everyday developer utilities built around one rule: your data stays on your device.

Why this site exists

Formatting a JSON response, decoding a JWT or checking a regular expression are tasks developers do many times a week. The quickest option is usually an online tool, but many of them send whatever you paste to a server. That input is often sensitive: access tokens, customer data in an API payload, internal hostnames, production logs.

Every tool here does its work with JavaScript in your browser. There is no backend that receives your input, no account and no upload. Once a page has loaded, it keeps working even if you go offline.

What you'll find

There are currently 12 tools, each with a short guide, worked examples and answers to common questions. The goal is for each page to be the one you keep open: fast, accurate, and clear about edge cases like UTF-8 in Base64, the cron day-of-week rule or why a JWT is readable without its key.

How the tools are built

• Plain HTML, CSS and small, dependency-free scripts, so pages load quickly on any connection.
• Standard browser APIs where possible: JSON.parse, the Web Crypto API for SHA hashes and HMAC, crypto.getRandomValues for UUIDs, and Intl for time zones.
• Open-source libraries where needed, such as the MIT-licensed gpt-tokenizer for exact token counts.

Accuracy and feedback

Each tool is tested against known reference values (for example, published SHA-256 and MD5 test vectors). If you find a bug, an inaccurate explanation or a tool you wish existed, please get in touch.