Run Python, JavaScript, TypeScript and Java in any page

A code editor that actually runs the code it shows, as one custom element. No backend, no iframe, no sign-up — it all happens in the reader's own browser. Type in the box and press Run.

Python · JavaScript · TypeScript · Java Real CPython 3.14 via Pyodide — full standard library, input(), tracebacks, numpy on demand — plus the browser's own JavaScript engine, TypeScript with its types stripped, and a documented subset of Java 25 compiled right in the page. Each language loads only on the pages that use it.

name = input("what's your name? ") for i in range(1, 4): print(f"{i}. hello {name}") sum(n * n for n in range(1, 11))

That snippet blocks on input() — the program waits for you, the way a terminal does. No cross-origin isolation required.

One runtime per page

Every widget on a page shares a single Web Worker and a single Python runtime. Ten editors cost one ~6MB download, not ten.

See two editors share a worker →

Real modules and packages

A widget can hold a whole small project — flat modules, packages with __init__.py, data files you open(). Tracebacks name the real file and line.

See a package in a widget →

Runaway code can't freeze the page

Python runs in a Worker, so while True: is a stoppable mistake rather than a dead tab.

Try stopping an infinite loop →

Java, compiled in the browser

A subset of Java 25 — compact source files, methods, arrays, strings — with javac-style errors and Java stack traces. No JVM, no server, one small chunk.

Run some Java →

Lessons that keep their place

Edits persist in IndexedDB, and soft navigation keeps the runtime warm between pages — the first run on a new lesson takes milliseconds, not seconds.

Walk between two lessons →

What a page downloads

Each language loads only when a widget needs it. These are the downloads once a reader presses Run, with the CDN build: one-widget pages loaded in a browser, each file gzipped, and CDN files as the CDN sent them.

Page uses	code-playpad (gzip)	Also fetched from a CDN at run time	0.3.0 (gzip)
Python	166.3 KB	Pyodide, about 6.0 MB (a 3.3 MB wasm file and a 2.4 MB standard library)	165.8 KB
JavaScript	180.4 KB	nothing	179.9 KB
TypeScript	227.3 KB (with the 46.9 KB transform, since 0.5.2)	nothing	179.9 KB
Java	277.4 KB	nothing	216.4 KB
Java, with collections	372.5 KB	nothing	233.2 KB
All four	476.9 KB	Pyodide	273.9 KB

A shared base of 144.5 KB is in every row: the editor (CodeMirror) and the widget, the same as in 0.3.0.
Each language adds: Python 21.7 KB; JavaScript/TypeScript 35.9 KB; Java 132.8 KB, which is the 113.8 KB engine plus 19 KB of Java syntax highlighting. The first program that uses collections adds the 95.1 KB Java library, once per page.
Java grew from 0.3.0 because the subset did, from basic Java to most of what practice programs use.
Languages load lazily. A Java page never fetches the Python or JavaScript runtimes, and Python and JavaScript pages never fetch Java, so Python-only and JavaScript-only pages are the same size as in 0.3.0.
Java needs no CDN: it is self-contained, while Python's first run is dominated by the roughly 6 MB Pyodide download (cached after that).
With npm and a bundler you already ship CodeMirror yourself. code-playpad's own code then costs about 9 KB, plus the runtime chunk for each language used; Java's is 104 KB (74 KB more for its library, when a program uses it), plus @codemirror/lang-java if you install it.

Add it to a page

One script tag. CodeMirror is bundled; Python is fetched at runtime.

<script type="module"
  src="https://cdn.jsdelivr.net/npm/code-playpad@0.8.0/dist/standalone.js"></script>

<code-playpad height="180">
  print("hello from a CDN")
</code-playpad>

Or install it and use the modular build, with CodeMirror as a peer dependency so you never end up with two copies:

npm install code-playpad

React wrapper included — see it here. Works in plain HTML, Astro, Next.js, and anything else that can render an element.