tools.joostvanderlaan.nl

Miscellaneous HTML+JavaScript tools built mostly with the help of LLMs. See also /python/ for tools written using Python.

This collection is an experiment in prompt-driven development with very low stakes.

The colophon lists commit messages and transcripts for every tool.

The code lives in javdl/tools.

Image and media

• HEIC to JPG convert Apple HEIC/HEIF images to JPG
• Social media cropper crop images to 2×1 for social sharing
• Image resize and quality comparison compare JPEG quality settings
• Image to JPEG convert PNG or WebP files to JPEG
• Image to SVG trace bitmap images to SVG paths
• SVG to JPEG/PNG render an SVG to a raster image
• SVG sandbox display decoded SVG files safely
• SVG progressive render watch an SVG draw itself
• BBox cropper draw bounding boxes and read the coordinates
• Mask visualizer inspect JSON masks with bounding boxes
• FFmpeg crop helper generate FFmpeg commands for cropped videos
• TIFF orientation viewer inspect EXIF orientation metadata
• Avatar web component upload and crop avatars in place
• YouTube Thumbnails list thumbnail URLs for a video

Text and document

• OCR recognize text from images and PDFs in your browser
• PDF OCR run optical character recognition on uploaded PDFs
• Compare PDFs visualize differences between two PDFs
• Render Markdown convert Markdown to HTML using the GitHub API
• HTML preview type HTML on the left and see it rendered on the right
• RTF to HTML inspect RTF clipboard data and convert it to HTML
• Markdown math live preview of Markdown with LaTeX equations
• Footnotes experiment demo linking footnotes to popups
• Reading time calculator estimate how long text will take to read
• Word counter count words across multiple text blocks
• Text wrap balance nav explore the text-wrap: balance property
• Navigation for headings generate an ID-based table of contents
• Paste rich text inspect HTML and plain text on your clipboard
• Paste HTML subset see which tags survive HTML sanitization
• Clipboard viewer debug everything stored in your clipboard
• Extract URLs pull a list of links from pasted HTML
• JSON to Markdown transcript convert transcript JSON to Markdown
• JSON to YAML convert between JSON and YAML formats
• YAML Explorer browse YAML documents in a collapsible tree
• JSON schema builder visually design a JSON schema
• Incomplete JSON printer pretty print partial JSON documents
• PHP Deserializer turn serialized PHP into JSON
• SQL Pretty Printer reformat SQL queries for readability
• Pipfile.lock parser extract dependency versions from Pipfile.lock

Data and time utilities

• Timestamp Converter convert Unix timestamps to readable dates
• Timezones compare times across multiple time zones
• Date calculator count days between dates or only weekdays
• Transfer time estimator work out how long file transfers will take
• Token usage calculator summarize LLM token logs by model
• LLM prices redirect quick link to the latest model pricing site
• CSV marker map plot markers on a map from a CSV file
• Species observation map browse recent iNaturalist sightings

GitHub and development

• GitHub API write upload text or images directly to a repo
• GitHub issue viewer fetch GitHub issues and comments
• GitHub issue to Markdown turn an issue thread into Markdown
• Zip/Wheel explorer view the contents of Python wheels and zips
• Ares phonetic alphabet convert text to the ARES emergency phonetic code
• Code with Claude 2025 prototype workflow for Claude coding
• Side panel dialog demo experiment with the HTML dialog element
• Broadcast channel chat chat across tabs using BroadcastChannel

Bluesky and social tools

• Bluesky WebSocket Firehose watch real-time activity on Bluesky
• Bluesky resolve DID convert a handle like simonwillison.net into a DID
• Bluesky timeline view a user’s recent posts and replies
• Bluesky thread export save a Bluesky thread to Markdown
• Bluesky quote finder find all quotes of a Bluesky post
• Event planner rough schedule planner stored in localStorage
• Passkeys demo experiment with browser-based passkey authentication

LLM playgrounds and debuggers

• Source to Prompt convert local code projects into LLM-ready prompts
• Haiku generate haikus using Claude Haiku and your webcam
• Chrome Prompt Playground run prompts on Chrome’s Gemini Nano
• Gemini bounding box visualizer visualize bounding boxes returned by Gemini
• Gemini chat client simple chat UI for the Gemini API
• Gemini mask visualizer overlay segmentation masks from Gemini
• Gemini image JSON renderer display images from Gemini JSON output
• Claude Token Counter count tokens for Claude prompts
• OpenAI audio input record and send audio to OpenAI models
• OpenAI audio output generate speech with OpenAI voices
• OpenAI WebRTC demo interact with OpenAI’s real-time audio API
• GPT-4o Gist audio player play audio responses stored on GitHub Gist
• JSON schema builder build JSON schemas with a visual editor

Generative art

• Lowland Chiaroscuro seeded generative Dutch nocturne with wind fields, layered glazes, and p5.js
• Hartendief interactive Valentine's heart galaxy animation
• Floris de Dino orbiting dinosaur galaxy animation
• Mats de Racer orbiting racing car galaxy animation
• Julian's Ballen orbiting toy ball galaxy animation

Education

• Letters Oefenen three activities for kids to practice letter recognition, tracing, and word puzzles (Dutch)
• Patronen Spel pattern recognition coloring game for young children (Dutch)

Miscellaneous

• Arena animated animated chart of the LMSYS Chatbot Arena
• Amsterdam Clock Change see when daylight saving time changes in Amsterdam
• California Clock Change see when daylight saving time changes
• Open Sauce 2025 schedule browse the upcoming conference sessions
• OpenFreeMap demo MapLibre demo with random points in San Francisco
• Progress of the US presidency track days elapsed in the current term
• User Agent display show your browser’s user agent string
• Encrypt / decrypt message share short encrypted messages
• ARIA live regions demo of dynamic page announcements
• Prompts.js small library for nicer JavaScript prompts
• APSW SQLite query explainer explain SQLite queries using APSW

On Observable

On Observable:

• Blog to newsletter helps turn blog posts into a newsletter
• Convert Claude JSON to Markdown for sharing Claude transcripts
• Hacker News homepage with links to comments ordered by most recent first