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TraceLab
Start coding
Free online compiler & code visualizer

Don’t just run code. Watch it think.

Run programs with input, then trace important steps across Python, JavaScript, TypeScript, C, C++, Java, Go, Swift, Ruby, PHP, Perl, Bash, SQL, HTML and CSS.

Choose your workspace

What do you want to build?

Pick a language to open its example, write code, and run it in the editor.

main.py
Python runs locally in your browser● Checking backend…
Live previewRendered in a sandboxed frame
Program flowStep 1 of 1
Start⚡
Analyze◆
Process?
Update↔
Output✓

Start — waiting for your program.

Readyline —
Start
What’s happening now

Ready. Run your program to build a source-driven trace.

Live variables
Main logic program
Key source lines appear here when you enter a program.
Main code explanation
Run the walkthrough to see the important steps in plain language.
15 live · Kotlin walkthroughHTML / CSSPythonJavaScriptTypeScriptC / C++Java / GoKotlin · trace onlySwiftRuby / PHPPerl / BashSQL

Use ← → to move through the generated trace, or press Play flow.

How it works

TraceLab reads the structure of your source and builds a flow from the instructions it actually finds.

1

Write or paste code

Use the built-in example or paste your own program.

2

Choose a language

Choose one of 15 live runtimes, or use Kotlin for a source walkthrough.

3

Analyze & visualize

The flow is generated from your source instead of a fixed five-step story.

What actually runs

Browser languages execute locally; hosted languages use the TraceLab backend.

Live

Python

Runs in the browser with Pyodide.

Live

JavaScript

Runs in a sandboxed browser frame.

Live

HTML & CSS

Rendered in the live preview.

Hosted

11 runtimes

TypeScript, C, C++, Java, Go, Swift, Ruby, PHP, Perl, Bash and SQL run through TraceLab. Kotlin walkthroughs work, but Wandbox currently has no Kotlin compiler.

Frequently asked

Does the flow use a fixed five-step algorithm?

No. The number and meaning of timeline steps change with the source code.

Will every possible algorithm be understood perfectly?

Not yet. The frontend provides a broad source-driven trace and recognizes common DSA patterns; exact runtime tracing for every language requires backend instrumentation.

Where does hosted code run?

Hosted languages are sent to the TraceLab execution backend when you press Run.

Can I keep updating the public site?

Yes. Replace and redeploy the HTML whenever you fix bugs or add features.