Execution Visualizer — See Code Run Step by Step
Watch JavaScript run one line at a time, entirely in your browser. The Execution Visualizer steps through your code and shows the current line, every variable, the call stack, recursion and console output at each step — forwards and backwards. It is the fastest way to actually understand how code executes instead of just whether it works. Nothing is uploaded.
How to Use
- Type or paste JavaScript into the editor — or load one of the examples (factorial, Fibonacci, bubble sort, FizzBuzz, array map/filter).
- Press <strong>Visualize ▶</strong>. Your code is parsed and run locally; if there is a syntax or runtime error, the line and reason are shown.
- Use <strong>Next / Prev</strong> (or the slider, or ▶ Play) to move through execution. The current line is highlighted as you go.
- Watch the <strong>Call Stack</strong> grow and shrink — each function call is its own frame with its own variables, so recursion is easy to see. The <strong>Output</strong> panel shows console.log results as they happen.
- Step <strong>backwards</strong> any time to re-examine what changed. Everything runs in your browser; nothing you write is uploaded.
Watch JavaScript or Python run one line at a time — see the current line, every variable, the call stack and the output at each step, forwards and backwards. The fastest way to understand how code actually executes. Everything runs in your browser.
What you can watch
Supported JavaScript
The visualizer runs a teaching subset of JavaScript: let/const/var, numbers, strings, booleans, arrays and objects; arithmetic, comparison and logical operators; if/else, while and C-style for loops; function declarations, arrow functions and recursion; array methods (map, filter, forEach, push, pop, join, slice); console.log and a small Math. Not yet supported: classes, for…of/in, switch, async/await, modules, destructuring, spread and template-string interpolation.
About the Execution Visualizer — See Code Run Step by Step
Working on everyday tasks? The Execution Visualizer — See Code Run Step by Step is a free browser tool that gives you the answer in seconds. Watch JavaScript run one line at a time, entirely in your browser. The Execution Visualizer steps through your code and shows the current line, every variable, the call stack, recursion and console output at each step — forwards and backwards. It is the fastest way to actually understand how code executes instead of just whether it works. Nothing is uploaded.
How it works
Type in what you have, and the answer shows up right away. Change anything and it updates by itself. Everything runs in your browser, so it is fast and nothing you type is sent away.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What can it run?
A teaching subset of JavaScript: variables (let/const/var), numbers, strings, booleans, arrays and objects, arithmetic / comparison / logical operators, if/else, while and C-style for loops, function declarations, arrow functions, recursion, and array methods like map, filter, forEach, push and join, plus console.log and a small Math. It does not yet run classes, for-of/in, switch, async, modules, destructuring or template-string interpolation.
How is this different from just running the code?
Running code only shows the final output. The visualizer pauses on <em>every</em> step and shows what's happening inside — the value of each variable, the call stack, and how arrays and objects change. You can also step backwards, which a normal run can't do. It's built for understanding, not just results.
Does my code get uploaded?
No. The interpreter runs entirely in your browser. Nothing you type is sent to any server.
Why did it stop with a “too many steps” message?
To protect your browser from infinite loops, execution stops after 5,000 steps. If you hit that, your loop probably never ends — check its condition. You can still step through everything that ran up to that point.
Is this like Python Tutor?
Same idea — step-by-step visualization of how code executes — but it runs fully in your browser with no server round-trip, and currently focuses on JavaScript. More languages are on the Code Lab roadmap via a shared stepping engine.
How do I use the Execution Visualizer — See Code Run Step by Step?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Understand recursion
Watch the call stack build up and unwind on factorial or Fibonacci.
Debug a loop
See exactly how variables change on every iteration.
Teach beginners
Show what “the computer does” line by line, visually.
Trace a sort
Step through bubble sort and watch the array reorder.
Check your mental model
Predict the next value, then step to confirm.
Last updated: