What is a JavaScript compiler?

A JavaScript compiler (also called a JS compiler or JS engine) parses and executes JavaScript code. Our online JavaScript compiler not only runs your code but also visualizes the execution process step-by-step, showing you exactly how the Call Stack, Event Loop, and Memory work together.

How does the JavaScript Event Loop work?

The Event Loop is the mechanism that handles asynchronous operations in JavaScript. It continuously checks the Call Stack and Task Queues (Microtasks and Macrotasks), executing callbacks when the stack is empty. Our JS visualizer lets you watch this process step-by-step.

What is hoisting in JavaScript?

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope during the compilation phase. Variables declared with var are hoisted and initialized with undefined, while let and const are hoisted but remain in the Temporal Dead Zone (TDZ) until their declaration is reached.

What is a closure in JavaScript?

A closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has returned. Closures are created every time a function is created. Our visualizer shows how closures maintain their scope chain and access outer variables.

What is the difference between Microtasks and Macrotasks?

Microtasks (like Promises, queueMicrotask) have higher priority than Macrotasks (like setTimeout, setInterval). After each task completes, all pending Microtasks are executed before any Macrotask. Use our online JS compiler to visualize this difference.

Is this JavaScript playground free to use?

Yes! StackTools JavaScript compiler and execution visualizer is completely free. No login required, no data collected. All code runs entirely in your browser for maximum privacy and speed.

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JavaScript Execution Visualizer

Watch JavaScript code execute step-by-step. Visualize the Call Stack, Scope Chain, Event Loop, Hoisting, Closures & more.

Nothing leaves your browser. All processing happens locally.

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script.js

Script.jsTS / JS

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Call Stack

Stack EmptyRun code to see execution

Scopes & Memory

No Active ScopeRun code to see memory

Output will appear here...