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Event loop

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Think of the event loop like a restaurant kitchen with one chef 🍳👨‍🍳

The Simple Story

🧑‍🍳 The Chef (Call Stack)
  • Only one chef exists (JavaScript is single-threaded)
  • The chef can only cook one dish at a time
  • Dishes stack up on the counter (call stack)
📞 The Waiter (Web APIs)
  • Takes orders that take time (timers, fetch requests, file reads)
  • Gives them to the kitchen staff in the back (browser/Node.js)
  • Doesn't make the chef wait – the chef keeps cooking other dishes
📋 The Priority List (Microtask Queue)
  • VIP orders (Promises) – must be done immediately after current dish
  • Chef clears ALL VIP orders before touching regular orders
📝 The Regular List (Task Queue)
  • Normal orders (setTimeout, clicks, I/O)
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  • Chef picks one from this list only when VIP list is empty
    • 🔄 The Event Loop (The Manager)
    • Constantly checks: "Is the chef free?"
    • If yes: Gives next task (VIPs first, then regular)
    • If no: Waits
    • Repeats forever while the restaurant is open

    Real Code Example

    JavaScript
    console.log('🍕 Order pizza'); // Chef cooks immediately

setTimeout(() => { // Waiter takes this to back kitchen
  console.log('🍟 Fries ready');
}, 0);

Promise.resolve().then(() => { // VIP order!
  console.log('🥤 Drink served');
});

console.log('🍔 Order burger'); // Chef cooks immediately

    Mermaid Diagram

    Mermaid
    flowchart TD
    A[Start script] --> B[Run synchronous code]
    B --> C[Async APIs complete]
    C --> D[Promise callbacks -> Microtask Queue]
    C --> E[setTimeout/click/I O callbacks -> Task Queue]
    D --> F{Call Stack empty?}
    E --> F
    F -->|No| G[Wait]
    G --> F
    F -->|Yes| H[Event Loop checks queues]
    H --> I[Run all Microtasks first]
    I --> J[Run one Task]
    J --> F