TypeScript in CodePen

Can you do TypeScript in CodePen?  

The question is valid as CodePen reveals three containers, one for HTML, one for CSS and one for JavaScript, in its default interface.

To enable TypeScript input, go to Settings, select JavaScript preprocessor and choose TypeScript. Other available preprocessors are LiveScript, CoffeeScript (billed as a "simple and elegant way" to write JavaScript) and Babel.

Note that CodePen will run TypeScript without type-checking errors blocking execution.

Bun - The JavaScript Runtime used by All (Cool Cats)

Bun is a fast JavaScript runtime. It's website is bun.sh (where the suffix sh denotes a St Helena domain). Bun is built from scratch to "serve the modern JavaScript ecosystem".

A major selling point of Bun is it starts fast and runs fast. It extends the performance-minded JS engine built for Safari known as JavaScriptCore. Fast start times leads to fast apps like Claude Code CLI.

Bun also boasts "cohesive DX" (developer experience) with a package manager, test runner and bundler all included.

Design-wise it has been designed to be a drop-in replacement for Node.js. Thousands of Node.js and Web APIs have been implemented in Bun like fs, path and Buffer.

Bun's ambition is to run most of the world's server-side JavaScript.

CodePen

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Inside the Claude Code CLI

On 31 March 2026, Anthropic's CLI tool Claude Code that lets you interact with Claude for software engineering tasks from the command line - edit files, search codebases, manage git workflows and more - had its src directory leaked revealing TypeScript code with UI written in React and Ink (React for interactive command-line applications). It uses the Bun runtime - a fast JavaScript, TypeScript and JSX toolkit.