JavaScript Tester

Paste a JavaScript snippet and run it instantly — see console output, errors, and execution time in your browser.

Last updated: May 29, 2026

Runs in your browser: Code executes directly in this tab with access to your browser APIs. Only run code you trust.

JavaScript Code

Output

Click Run to execute your code…

Client-Side Processing

Input Data Stays on Device

Instant Local Execution

Find this tool useful? Support the project to keep it free!

Buy me a coffee

What is JavaScript Tester?

JavaScript Tester executes JavaScript snippets in your browser using the browser's own JavaScript engine. It captures console.log, console.warn, console.error, and console.info output and displays it in a console-style panel. Runtime errors are caught and shown with their type and message. Code runs in the current browser tab context, so modern ES6+ syntax is fully supported. No server is involved — execution is entirely client-side.

How to Use JavaScript Tester

Paste or type JavaScript code.

Click Run.

Review the console output panel.

Fix errors and re-run as needed.

Common Use Cases

Quickly testing a function or algorithm before adding it to a project.
Verifying a regex pattern or string manipulation result.
Checking the output of a JSON transformation or data processing snippet.
Learning JavaScript by experimenting with expressions and built-in APIs.

Example Input and Output

A loop logs numbers to the console, which the tester captures.

JavaScript

for (let i = 1; i <= 5; i++) {
  console.log("Number:", i);
}

Console output

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Privacy

JavaScript code runs locally in your browser tab and is never uploaded to any server.

Frequently Asked Questions

Is this safe to use?

The code runs in your own browser tab with access to browser APIs. Only run JavaScript you trust — the tool adds a warning before execution. Avoid running code from untrusted sources.

Does it support async/await?

Top-level await is not supported since the executor wraps code in a synchronous function. Wrap async code in an immediately-invoked async function: (async () => { const result = await fetch(...); console.log(await result.json()); })();

Can it access the DOM?

Yes — the code runs in the same browser context as this page, so window, document, and other browser APIs are accessible.

Is my code uploaded anywhere?

No. Code executes entirely in your browser.