Type JavaScript, press Run, and watch the console. The editor above executes your code in a sandboxed frame and reports console.log output, warnings, and errors — with line numbers that match the editor, so a mistake on line 12 says line 12. It is the quickest loop there is for trying an idea: no project folder, no node, no browser devtools to dig through.
Run JavaScript online, safely
Every run executes in a fresh, isolated frame: variables, timers, and console state from the previous run cannot bleed into the next one, so results are always reproducible. The frame is sandboxed to your own page — your code cannot touch the editor itself, the site around it, or anything on your disk. There is no server-side execution at all; what you run, your browser runs.
The status line shows how long your code took, and console.log, console.warn, and console.error keep their levels in the output panel — errors in red, warnings in amber. Objects and arrays you log are formatted as JSON, which is usually the fastest way to inspect a data structure you just built. Unhandled promise rejections are captured too, so an async function that throws does not fail silently.
Learning with the examples
The examples menu starts with the classics — array methods, objects and destructuring, functions and recursion, date formatting, and an async/await sequence with visible timing — each written to be read as well as run. Load one, change something, run again: the editor keeps your modified version, and pressing Reset brings back the original example when you want a clean slate.
If you are working through the JavaScript curriculum, this editor is the scratchpad for those lessons. The loop of read a concept → type a variation → see the output is exactly how the lesson runnables work, and having the same loop in a full-size editor means you can grow your experiments beyond the little boxes inside a lesson.
Timing, errors, and the console discipline
Because each run is isolated and timed, the editor is genuinely useful for micro-benchmarks — swap two implementations of a loop and compare the milliseconds in the status bar. When code throws, the error message arrives with its line and column: a typo on line 8 reports line 8, not an offset number you have to translate. Syntax errors are reported the same way, before any of your code runs.
One deliberate limit: there is no DOM to script against — the runtime is a blank frame, not a web page. For DOM practice, the Web Playground runs HTML, CSS, and JavaScript together with a live preview, and the HTML editor gives you a full document with a console. This tool is the pure-language scratchpad of the three.
Your code stays with you
Code is saved in your browser’s local storage — reload the page, close the tab, come back tomorrow: your last edit is still there. Share packs the current code into the link itself (the part after # never reaches a server), so you can send a working snippet to a colleague or keep one on a second machine. Import opens a .js file from your computer; Download saves the current code as script.js; Copy puts it on your clipboard. Reset clears local storage and restores the starter example — nothing to clean up anywhere else, because nothing was ever anywhere else.
Frequently asked questions
Which JavaScript is this? Whatever your browser implements — modern ECMAScript runs as written: optional chaining, ??, class fields, top-level await in async contexts, all of it.
Can I use fetch or setTimeout? Yes. Timers, promises, and network requests all work; the async example in the menu demonstrates a visible sequence of awaited delays.
Can I use npm packages? No — there is no bundler here, by design. Paste the code you want to try, or link a library from a CDN inside the Web Playground’s HTML file instead.
Is my code uploaded anywhere? No. Execution is local, storage is local, and share links carry the code in the URL fragment.
Does it work on a phone? Yes — the editor and console stack vertically on small screens, and Run stays within thumb reach in the header.