Does this run in the browser or on a server?

You edit in the browser (Monaco). When you click Run, Code Arena sends the source to a sandboxed Node.js environment on the server and streams the result back.

Can I use npm packages?

Not in the default sandbox. Runs are offline and meant for self-contained snippets. Stick to built-in JavaScript / Node APIs available without installs.

Why is there an AST tab?

The AST view shows how the parser understands your program. Click a node to highlight the matching source range — useful for learning syntax and structure.

Treat pasted code as something processed to fulfill your run. Shared links encode source in the URL — anyone with the link can open it. Avoid secrets.

Can I use top-level await?

It depends on how the sandbox invokes your file. If await fails at the top level, wrap async work in an async function and call it, or use .then on a Promise.

Why did my program time out?

Infinite loops, busy waits, and huge nested work can exceed the sandbox limit. Add a break condition, shrink the input size, or move heavy work to a local machine.

Does dark mode affect execution?

No. Theme only changes the UI. Output and runtime behavior are the same in light or dark mode.

Free Javascript Online Compiler (No Signup)

Free JavaScript online compiler (no signup)

Write and run JavaScript in the browser with Code Arena. Code executes in an isolated Node.js sandbox so you can try ideas, debug small programs, and inspect the AST — no account, no install.

How to use this editor

Open the editor on this page (or start from the starter snippet).
Edit your code in main.js — use console.log to print values.
Press Run (or Ctrl/Cmd + Enter) to execute in the sandbox.
Check the Output tab for stdout/stderr, or open the AST tab to inspect the parse tree.
Use Share to copy a link that restores your language and source.
Scroll this guide for tutorials, FAQ, and links to deeper Learn articles.

Runtime notes

Runtime: Node.js 24 inside a Vercel Sandbox.
Network access is disabled during runs (deny-all policy).
Long-running or resource-heavy programs may time out.
AST visualization is available for JavaScript source on this page.
Runs are rate-limited to keep the playground fair for everyone.
Prefer self-contained snippets — npm installs are not available mid-run.

Your first useful snippet

Start with a function that returns a value and log the result. That pattern teaches you how Code Arena maps console output to the Output panel, and it gives you a clear pass/fail signal when you change the code.

Keep the program under ~30 lines while you learn the playground. Short scripts are easier to share, easier to reason about in the AST tab, and less likely to hit timeouts.

Write a named function (for example sum or greet)
Call it with sample inputs
console.log the return value
Run again after each small edit

Reading errors without panic

When Node throws, Code Arena surfaces the message in Output. Read the error name first (ReferenceError, TypeError, SyntaxError), then the line hint. Fix the first failure you see — later errors often cascade from the first one.

If the message mentions an unexpected token, you usually have a missing bracket, quote, or parenthesis. If it says something is undefined, you used a name before assigning it or misspelled a property.

Using the AST tab as a teacher

The AST tab is not just a novelty — it shows how the parser groups your code. Click CallExpression or BinaryExpression nodes and watch Monaco highlight the matching source. That connection is one of the fastest ways to understand nesting and precedence.

Try wrapping an expression in parentheses, adding a second argument, or turning a statement into a block. Re-open the AST view and notice which nodes appear or disappear.

When to leave the playground

Code Arena is ideal for experiments, interview warm-ups, teaching demos, and sharing a failing case. Move to a local project (or a fuller cloud IDE) when you need packages, files, tests, persistent state, or network access.

FAQ

Does this run in the browser or on a server?: You edit in the browser (Monaco). When you click Run, Code Arena sends the source to a sandboxed Node.js environment on the server and streams the result back.
Can I use npm packages?: Not in the default sandbox. Runs are offline and meant for self-contained snippets. Stick to built-in JavaScript / Node APIs available without installs.
Why is there an AST tab?: The AST view shows how the parser understands your program. Click a node to highlight the matching source range — useful for learning syntax and structure.
Is my code private?: Treat pasted code as something processed to fulfill your run. Shared links encode source in the URL — anyone with the link can open it. Avoid secrets.
Can I use top-level await?: It depends on how the sandbox invokes your file. If await fails at the top level, wrap async work in an async function and call it, or use .then on a Promise.
Why did my program time out?: Infinite loops, busy waits, and huge nested work can exceed the sandbox limit. Add a break condition, shrink the input size, or move heavy work to a local machine.
Does dark mode affect execution?: No. Theme only changes the UI. Output and runtime behavior are the same in light or dark mode.

Example programs

Open a ready-made snippet, then run it in this editor. Browse javascript examples.

Keep learning

main.jsjavascript·node24

⌘/Ctrl + Enter

Editor

Output

Ready

Run the code to see output here.

Tip: press Ctrl + Enter

Ln 1, Col 1|UTF-8|javascript