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JavaScript Event Loop Visualizer

Visualize the JavaScript event loop with call stack, task queue, and microtask queue. Understand async execution interactively.

console.log("Start");

setTimeout(() => {
  console.log("setTimeout");
}, 0);

Promise.resolve().then(() => {
  console.log("Promise");
});

console.log("End");
SlowFast
Idle
Call Stack
(empty)
Web APIs / Timers
(empty)
Microtask Queue
(empty)
Task Queue (Macrotasks)
(empty)
(empty)
Call Stack
Microtask Queue
Task Queue (Macrotasks)
Web APIs / Timers
Console Output

Understanding the JavaScript Event Loop

The JavaScript event loop is the mechanism that handles asynchronous operations. It continuously checks the call stack and queues to determine what code to execute next. setTimeout callbacks go to the task queue (macrotask), while Promise callbacks go to the microtask queue, which has higher priority.

Key Concepts

  • The call stack executes synchronous code first (LIFO order)
  • Microtasks (Promises) always run before macrotasks (setTimeout)
  • Each macrotask is followed by all pending microtasks
  • The event loop checks queues only when the call stack is empty
  • Web APIs like setTimeout run outside the main thread

Frequently Asked Questions

What is the JavaScript event loop?
The JavaScript event loop is a concurrency model that allows JavaScript to perform non-blocking I/O operations despite being single-threaded. It works by offloading operations to the browser (Web APIs), then queueing their callbacks to be executed when the call stack is empty.
What is the difference between microtasks and macrotasks?
Microtasks (Promise.then, queueMicrotask, MutationObserver) have higher priority and are processed after each macrotask. Macrotasks (setTimeout, setInterval, I/O, UI rendering) are processed one at a time, with all pending microtasks drained between each macrotask.
Why does Promise.then execute before setTimeout?
Promise callbacks are microtasks, which are processed immediately after the current synchronous code finishes and before any macrotask (like setTimeout). The event loop always drains the entire microtask queue before picking the next macrotask.
What happens when the call stack is empty?
When the call stack is empty, the event loop first checks the microtask queue and processes all pending microtasks. Once the microtask queue is empty, it picks the next macrotask from the task queue and pushes it onto the call stack for execution.
Is this tool free to use?
Yes, this JavaScript event loop visualizer is completely free. It simulates the event loop behavior in your browser to help you understand asynchronous JavaScript execution patterns.

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How to Use

  1. Enter or paste your data in the input field
  2. Configure any options if available
  3. Click the action button to process
  4. Copy the result to your clipboard

Use Cases

  • Development and debugging workflows
  • Data format conversion
  • Code generation and formatting
  • Quick calculations and validation

FAQ

Is this Javascript Event Loop Visualizer tool free to use?
Yes, the Javascript Event Loop Visualizer tool is completely free. No registration or payment required.
Is my data secure?
Absolutely. All processing happens client-side in your browser. Your data never leaves your device or is sent to any server.
What formats does the Javascript Event Loop Visualizer tool support?
The tool supports all standard formats for IP address calculation. Check the tool interface for specific format options.