12 free browser-only dev tools (JSON, YAML, JWT, regex, hashes) that don't upload your data

Here are 12 free browser-based developer tools that keep your data private:

1. JSON Formatter: Convert messy JSON blobs into readable, indented code with optional sorted keys for easier comparison. The parsing and formatting occurs in your browser, so sensitive data never leaves your machine. Sorting keys beforehand can reveal half the differences between two payloads.

2. JSON Compare: Easily spot what changed between two JSON objects. It compares structure, not just text, and lists added/removed/changed values with their key paths. Perfect for identifying why an API response changed after a recent update.

3. JSONPath Tester: Quickly extract specific values from large JSON responses using powerful path expressions. Supports wildcards, recursive descent, slices, and filters. Test your expressions in the tool before using them in code or scripts like jq.

4. JSON to TypeScript Converter: Paste a JSON sample and get TypeScript interfaces back, complete with nested types. It saves time compared to manually writing types. Refine the converter's suggestions as needed.

5. JSON ↔ YAML Converters: Switch between JSON and YAML formats effortlessly. The JSON→YAML converter handles JSON5-style comments and trailing commas, so you don't have to clean them up later. Both converters work entirely in the browser.

6. YAML Validator: Quickly find indentation errors that break YAML parsing. It provides the exact line and column of the error along with a summary of the document structure, saving you time compared to waiting for CI to fail.

7. JWT Decoder: Decode JWTs to view the header and payload before regenerating the token. Shows standard claims like exp, aud, and iss in readable UTC times. The tool only inspects the token and never verifies the signature.

8. Regex Tester & Explainer: Test JavaScript regex patterns safely in your browser. The tester uses the same engine as your front end, so you won't get "works in PCRE, fails in JS" surprises. The explainer breaks down patterns token by token for better understanding.

9. Hash Generator: Compute MD5, SHA-1, SHA-256, and SHA-512 hashes locally using the open-source @noble/hashes library. Useful for verifying downloads or debugging signature mismatches. Remember, fast hashes aren't suitable for password hashing – use Argon2 or bcrypt instead.

10. Base64 Decode & Encode: Decode Base64-encoded data and encode data to Base64. The site also offers a ready-made chain for encoding Base64 data then URL-encoding the result, which is handy for certain auth headers and query strings.

11. Unix Timestamp Converter: Convert between Unix timestamps and human-readable UTC dates, handling seconds and milliseconds. The tool confirms that your data never leaves your device when performing conversions. This is particularly useful when reading log files.

12. Cron Expression Explainer: Make sense of complex cron schedules (e.g., "0 */6 * * 1-5"). The tool provides a plain-English description and shows the next run times. It's a helpful reference when reviewing someone else's scheduler PR without counting asterisks.

All of these tools keep your data private by performing the necessary operations directly in your browser, without uploading your information to any external servers.