Is my code sent to a server to run it?

No. The code runs in a Web Worker inside the tab you already have open, the same JavaScript engine that renders this page. There is no compile step and no backend, so nothing you type is transmitted anywhere. Your last draft is kept in this browser's local storage so a refresh doesn't lose it, which also means it stays on this device and this browser.

What happens if I write an infinite loop?

Nothing bad. Because the code runs in a Web Worker rather than on the page's main thread, a busy loop can't freeze the UI, the page stays responsive and the Stop button still works. Every run is also capped at 5 seconds of wall-clock time, after which the worker is terminated and the output says so. A setInterval that is still ticking counts toward that cap.

Can I use document, window or alert?

No, and that is deliberate. A Web Worker has no DOM, so document, window and alert are not defined and calling them raises a ReferenceError. What you do get is the language itself plus the worker globals: timers, crypto, TextEncoder, structuredClone, URL and the rest. fetch is there too, though the site's Content-Security-Policy confines it to this origin. This is a console for logic, not a page sandbox, use it for algorithms, parsing, data shaping and quick language questions.

Does the Format button reformat like Prettier?

Partly. It re-indents by block depth, normalizes spacing around operators, commas and keywords, splits statements onto their own lines, and adds trailing commas to multi-line object and array literals. It does not reflow long lines to a fixed width, so a line you wrote long stays long. Strings, template literals, regular expressions and comments are treated as untouchable regions, so formatting can never change what the code does. If the braces don't balance, it leaves the source exactly as it is and tells you rather than guessing.

Is it safe to paste code I found online?

It is safer than pasting it into your browser's DevTools console, for three reasons: there is no DOM to touch, the worker is destroyed when the run ends, and this site's Content-Security-Policy limits fetch to this origin, so a snippet cannot quietly post your code, or anything it reads, to a server of its choosing. It is still not a security sandbox. Code you run gets its five seconds of CPU and can read the timing information any script can read, so treat anything you paste with the same suspicion you would anywhere else.

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Target: sumTo() · 2 cases

#1: [5,2,4] → [2,4,5]

#2: [10,-1,3] → [-1,3,10]

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