JavaScript Tester

What is JavaScript Tester?

Paste JavaScript, click Run, and see its console.log output, executed in a sandboxed iframe with no access to this page, your cookies, or the network, so it's safe to try code you don't fully trust. Useful for quickly testing a code snippet, checking how a method behaves, or running an untrusted script from a tutorial or Stack Overflow answer without pasting it into your own project or browser devtools console first.

How to Use the JavaScript Tester

1. 1

   Paste the JavaScript: Enter the code snippet you want to run.

2. 2

   Click Run: Execute the code inside the sandboxed iframe.

3. 3

   Read the Console Output: View console.log/warn/error output, and any uncaught errors, in the output panel.

How It Works

Clicking Run injects the pasted code into a fresh iframe sandboxed with `allow-scripts` only (no `allow-same-origin`), which strips it of access to this page's DOM, cookies, and local storage, and blocks meaningful network requests. The iframe's `console.log`/`warn`/`error`/`info` calls are intercepted and streamed back to the output panel, along with any uncaught runtime error, instead of going to the browser's own devtools console.

Example

Input:
const nums = [1, 2, 3, 4];
const doubled = nums.map(n => n * 2);
console.log(doubled);
console.log('sum:', doubled.reduce((a, b) => a + b, 0));

Output:
[2, 4, 6, 8]
sum: 20

What It Catches

Runtime errors, not just syntax errors. Since the code actually executes, a thrown exception (like calling `.map()` on `undefined`, or referencing an undeclared variable) shows up in the output as an uncaught error, the same way it would in a real browser console.