Developer Tools: 10 Free Online Tools
This group collects tools developers reach for during everyday work: formatting and validating JSON, encoding and decoding data, querying CSV or JSON with SQL, trying HTML, CSS and JavaScript, generating test data and preparing a Progressive Web App.
Most tools work on text you paste or files you open, and run locally so you can use realistic data without sending it to a third-party server.
Who it is for: Front-end and back-end developers, QA testers, data analysts and students learning web technologies.
Tools in this category
Encoder & Decoder: Base64, Hex, Binary, JWT, AES
Encode and decode Base64, Hex, Binary and ASCII, inspect JWT tokens, create hashes and HMACs, and use AES-256 encryption. Processed locally in your browser.
Good for: developers, security learners.
Code to Image Converter
Turn source code into a shareable image with syntax highlighting, macOS or Windows window frames, gradient backgrounds and optional 3D tilt.
Good for: developers, bloggers, educators.
Netscape Cookie ↔ JSON Converter
Convert Netscape cookies.txt files to JSON and back for Selenium, Puppeteer, cURL and Postman. The conversion happens in your browser.
Good for: QA engineers, automation developers.
Internet Speed Test
Measure download and upload speed, ping and jitter from your browser using Cloudflare’s speed endpoints. Results depend on your connection and device.
Good for: home users, remote workers, gamers.
JSON Formatter & Validator
Format, validate, minify and repair JSON, browse it as a tree, run JSONPath queries and generate TypeScript or Dart types. Data stays in your browser.
Good for: developers, API testers.
Markdown Editor with Live Preview
Write Markdown with a live preview and export to PDF, HTML or a .md file. Your text is edited locally in the browser, with no account needed.
Good for: writers, developers, documentation authors.
Mock Data Generator for Testing
Generate fake JSON data, user profiles, addresses and Luhn-valid test card numbers for QA and development. Test data only, not real payment details.
Good for: QA testers, developers.
PWA Generator: Manifest & Service Worker
Generate a manifest.json, a service worker with offline caching and the app icon set to turn your website into an installable Progressive Web App.
Good for: web developers, site owners.
SQL Studio: Query CSV and JSON with SQLite
Import CSV or JSON into SQLite tables with WebAssembly and run SQL in your browser with syntax highlighting. Files are processed locally.
Good for: analysts, students, developers.
HTML, CSS & JavaScript Playground
Write HTML, CSS and JavaScript in a Monaco-based editor with live preview and a console. Code runs in your browser, with no sign-up.
Good for: front-end developers, students.
Privacy, processing and limits
- Formatting, encoding, SQL, cookie conversion, mock data and the playground run in your browser.
- The Internet Speed Test is the exception: it contacts Cloudflare’s speed servers and an IP-information service to measure your connection. In the Web Sandbox, any network request your own code makes goes where you point it.
See the privacy policy for how the site itself handles data.