JavaScript Tester

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Write and run JavaScript instantly in a safe sandbox — see console output, return values, errors and timing without opening DevTools. Free and runs in your browser.

Runs in a sandboxed iframe in your browser — nothing is sent to a server.

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Press Ctrl/⌘ + Enter to run

JavaScript

Console

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<iframe src="https://tweakee.com/studio/dev/javascript-tester?embed=1" width="100%" height="720" style="border:1px solid #e2e8f0;border-radius:12px;max-width:680px;" loading="lazy"></iframe>

', ].join('\n'); } function destroyFrame() { if (frame && frame.parentNode) frame.parentNode.removeChild(frame); frame = null; } function run() { const code = input.value; if (code.trim() === '') { clearConsole(); return; } // New token; any message from an older frame is now stale. runToken++; const token = runToken; // Recreate the iframe so prior state resets and runaway code is discarded. destroyFrame(); clearConsole(); ghost.style.display = 'none'; statusEl.dataset.state = 'idle'; statEl.textContent = '…'; frame = document.createElement('iframe'); frame.setAttribute('sandbox', 'allow-scripts'); frame.setAttribute('aria-hidden', 'true'); frame.style.cssText = 'position:absolute;width:0;height:0;border:0;left:-9999px;top:-9999px'; frame.srcdoc = buildSandboxDoc(code, token); document.body.appendChild(frame); } window.addEventListener('message', function (ev) { const d = ev.data; if (!d || d.__jst !== true) return; if (d.token !== runToken) return; // ignore stale frames if (d.kind === 'done') { let info = { ok: true, ms: 0 }; try { info = JSON.parse(d.text); } catch (e) {} statusEl.dataset.state = info.ok ? 'ok' : 'error'; if (info.ok) { statEl.innerHTML = tr(L.finished, { ms: nf.format(info.ms) }); } else { statEl.textContent = L.error; } if (lineCount === 0 && info.ok) ghost.style.display = 'flex'; return; } appendLine(d.kind, d.text); }); /* ---------------- controls ---------------- */ root.querySelector('[data-jst-run]').addEventListener('click', run); root.querySelector('[data-jst-clearlog]').addEventListener('click', clearConsole); root.querySelector('[data-jst-clear]').addEventListener('click', function () { input.value = ''; renderGutter(); input.focus(); }); root.querySelector('[data-jst-sample]').addEventListener('click', function () { input.value = [ "// Try editing and press Run (Ctrl/Cmd+Enter)", "const nums = [5, 2, 9, 1, 7];", "console.log('sorted:', [...nums].sort((a, b) => a - b));", "", "const user = { name: 'Ada', skills: ['math', 'code'], active: true };", "console.info(user);", "", "console.warn('heads up: this is a warning');", "", "async function ping() {", " await new Promise((r) => setTimeout(r, 50));", " return 'pong';", "}", "ping().then((v) => console.log('async →', v));", ].join('\n'); renderGutter(); run(); input.focus(); }); /* ---------------- editor behaviour ---------------- */ let autoTimer = null; input.addEventListener('input', function () { renderGutter(); if (autorunCb.checked) { clearTimeout(autoTimer); autoTimer = setTimeout(run, 600); } }); // Tab inserts a tab instead of moving focus. input.addEventListener('keydown', function (e) { if (e.key === 'Tab') { e.preventDefault(); const s = input.selectionStart, en = input.selectionEnd; input.value = input.value.slice(0, s) + '\t' + input.value.slice(en); input.selectionStart = input.selectionEnd = s + 1; renderGutter(); return; } if ((e.ctrlKey || e.metaKey) && e.key === 'Enter') { e.preventDefault(); run(); } }); autorunCb.addEventListener('change', function () { if (autorunCb.checked) run(); }); renderGutter(); })();

JavaScript Tester

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