Testers & Validators
Testers give you an instant feedback loop for the expressions and payloads that are hard to get right in your head: a regular expression against sample text, an XPath query against real HTML, a cron schedule's next run times, or whether a version satisfies an npm range.
Validators and diff tools answer the debugging questions — what exactly changed between two API responses, why a CORS request is blocked, whether a card number passes the Luhn check. Results update live as you type.
22 test tools
Regex Tester
Test and debug regular expressions with live highlighting
Diff Checker
Compare two texts and highlight differences
CRON Expression Tester
Parse cron expressions and preview upcoming run times
HTTP Request Builder
Build and send HTTP requests like a lightweight Postman
WebSocket Tester
Connect to any ws:// or wss:// endpoint, send messages, and watch a live, timestamped log
SemVer Calculator & Range Checker
Test versions against npm ranges like ^1.2.3 or ~2.0, compare versions, and preview version bumps
YAML / TOML / JSON Validator
Validate and convert between YAML, TOML, and JSON formats
Text Diff
Inline character-level diff between two short texts
XPath Tester
Test XPath expressions against XML and HTML with live results
CSS Selector ↔ XPath Converter
Convert between CSS selectors and XPath expressions for the common patterns both languages share
JSON Diff Checker
Compare two JSON objects and see exactly what changed, path by path
JSON Patch (RFC 6902) Generator & Applier
Diff two JSON documents into an applyable patch, or apply an existing patch to a document
IBAN Validator & Generator
Validate an IBAN's MOD-97 checksum, or generate a test IBAN for any supported country
Credit Card Test Number Generator
Generate Luhn-valid fake card numbers for testing payment forms, or validate a Luhn checksum
Unified Diff / Patch Generator
Generate a real, downloadable .patch file from two texts — usable with git apply or patch
UUID Parser
Decode any UUID or ULID into its version, variant, and embedded timestamp
TypeScript 7 Migration Checker
Paste a tsconfig.json and see exactly which compiler options TypeScript 7 removes
Node.js Type-Stripping Checker
Scan a TypeScript file for syntax Node's built-in type stripping can't run directly
HTTP Header Inspector
Paste raw response headers and get a plain-English explanation of every one
CORS Error Debugger
Walk through exactly why a cross-origin request would be blocked — or confirm it wouldn't be
JSONPath Tester
Test JSONPath expressions against JSON with live results and the exact path to each match
User-Agent String Parser
Break a User-Agent header into browser, engine, OS, and device — with bot detection
Guides
🧶 npm ERR! code ERESOLVE unable to resolve dependency tree — How to Fix It
What npm's ERESOLVE error actually means, how to read the Found / Could not resolve lines, and the right fix for each case — upgrading, overrides, or (carefully) --legacy-peer-deps.
💧 Error: Hydration failed because the server rendered HTML didn't match the client
React's hydration mismatch error explained — the six most common root causes (Date formatting, window/localStorage access, browser extensions, invalid HTML nesting) and the exact fix for each one.
🩹 How to Generate and Apply a JSON Patch (RFC 6902) — With Real Examples
A practical guide to JSON Patch: what the six operations actually do, how to write a real HTTP PATCH request body, and the mistakes that cause a patch to fail against a document that looks like it should accept it.
🟦 TypeScript 7 Migration Guide: What Actually Breaks, and How to Fix It
TypeScript 7's Go-based compiler shipped in mid-2026 turning years of 6.0 deprecation warnings into hard errors. Here's exactly which compilerOptions get removed, which tools break, and the migration order that avoids debugging blind.
🆔 UUID v7 vs v4 vs ULID: Which One Should Actually Be Your Database Primary Key
Random UUIDv4 primary keys quietly wreck your index performance as tables grow. Here's why, and how UUIDv7 and ULID fix it without giving up the properties that made you choose a UUID in the first place.
📐 YAML Error: 'Mapping Values Are Not Allowed in This Context' Explained
This cryptic YAML parser error almost always comes down to one of three things: an unquoted colon, a stray tab character, or inconsistent indentation. Here's how to spot which one you have.