Playgrounds have quietly become part of how a lot of people learn syntax, test an API, or check a hunch. The collection below came out of a Mastodon thread asking for recommendations, and it is heavily weighted toward browsers — the whole point being that you can poke at something without installing anything.
Compilers and bytecode
Matt Godbolt's godbolt compiler explorer is the obvious starting point for compiled output, and it has company for platform-specific bytecode: SharpLab by Andrey Shchekin for .NET and dis this by Pamela Fox for Python.
Regex, DNS and server configuration
Regular expressions get their own cluster, split by target language or dialect:
- many languages: regex101 by Firas Dib
- Ruby: Rubular by Michael Lovitt
- JS/PCRE: regexr by gskinner
- Python: Pythex by Gabriel Rodríguez
For infrastructure-adjacent work there's Mess With DNS by Julia Evans and Marie Flanagan, plus a separate DNS lookup tool, an nginx playground and Fastly's fastly fiddle for Varnish.
Databases
- SQLite: sqlime by Anton Zhiyanov, which lets you load arbitrary SQLite databases
- SQL: DB fiddle from Status200, and a separate sql playground
- Postgres: the postgres playground from Crunchydata, which runs postgres in the browser
- Git: oh my git by blinry and bleeptrack
Frontend, shaders and hardware
For the browser stack, CodePen and JSFiddle are the long-standing general options, while flems takes a different approach and saves all state in the URL. shadertoy covers shaders, and wokwi does the same job for Arduino and IoT work.
Data formats
- Floating point: Float Exposed by Bartosz Ciechanowski
- Unicode: the Unicode analyzer from fontspace and What unicode character is this? from babelstone
- ASN.1 certificates: ASN.1 JavaScript debugger by Lapo Luchini
- SVG: sssvg, an interactive SVG reference from fffuel, which also hosts a pile of other tools
- CBOR: the CBOR playground
- JSON: JSON editor online by Jos de Jong
- cron: crontab guru from cronitor
General-purpose language sandboxes
Several languages ship their own: Go, Rust, Ruby, Typescript, Haskell, C#, Elm, Kotlin, Scala, OCaml, Crystal and Svelte.
Community-run alternatives fill in the gaps, either for a single language or across many:
- PHP: 3v4l by Sjon Hortensius
- Python, JS, C, C++ and Java: Python Tutor by Philip Guo
- Javascript: JS Console by @rem
- many languages: riju by Radon Rosborough, and replit
- niche languages: jqplay for jq and tryapl for APL
