⚡ Event Loop in Browser JS vs Node.js

🔹 Common Concepts (Both)

• Call Stack -> Executes functions (LIFO).
• Web/Node API’s -> Handle async works (timers, HTTP, events).
• Task Queues -> Where async callbacks go before execution.
• Event loop -> Decided when to move tasks from queues into the stacks.

🔹 Browser JavaScript (Chrome, Firefox, etc.)

• Macrotasks (Task Queues) :
  • setTimeout, setInterval, DOM events, setImmediate(not in browser)
• Microtasks (Job Queues) :
  • Promise.then, queueMicrotask, MutationObserver.
• Priority Rule:
  • After each stack execution → run all microtasks → then move on to one macrotask.

    console.log("Start");
    setTimeout(() => console.log("Timeout"), 0);
    Promise.resolve().then(() => console.log("Promise"));
    console.log("End");

    // ✅ Output (Browser):
    Start
    End
    Promise
    Timeout

• 👉 Promise (microtask) runs before setTimeout (macrotask).

🔹 Node.js Event Loop

• Node.js uses libuv under the hood, and has extra phases in its event loop.

• Event Loop Phases in Node:

  • Timer Phase - Executes setTimeout & setInterval
  • Pending Callbacks - Executes I/O callbacks.
  • Idle, Prepare - Internal use.
  • Poll Phase - Retrives new I/O events.
  • Check Phase - Executes setImmediate
  • Close Callbacks - Closes connection.

• 💡 Node also has:

  • Microtasks -> Promise.then, queueMicrotask
  • process.nextTick -> Runs before microtask after every phase.

• Priority in Node.js :

  • process.nextTick
  • Microtasks (Promises, callbacks)
  • Macrotasks (timers, setImmediate, etc)

• Example in Nodejs ->

    setTimeout(() => console.log("Timeout"), 0);
    setImmediate(() => console.log("Immediate"));
    Promise.resolve().then(() => console.log("Promise"));
    process.nextTick(() => console.log("NextTick"));

    // ✅ Output (Node.js):

    NextTick
    Promise
    Timeout
    Immediate

• 👉 Explanation:
  • process.nextTick → highest priority.
  • Then Promise.then (microtask).
  • Then setTimeout (timer phase).
  • Then setImmediate (check phase).

⚡ Key Differences: Browser vs Node.js

Feature	Browser JS	Node.js
Microtasks	Promise.then, queueMicrotask	Same
Macrotasks	setTimeout, setInterval	setTimeout, setInterval, setImmediate
Special Queue	❌ None	✅ process.nextTick (higher priority than microtasks)
Order of async tasks	Microtasks → Macrotasks	process.nextTick → Microtasks → Macrotasks
Event loop phases	Simpler	Multiple phases (timers, poll, check, close, etc.)

🧠 Interview One-liners

• “In Browser” : Microtasks(Promises, callbacks) always run before macrotask(timers, setImmediate)
• “In Nodejs” : process.nextTick runs first, then Promises (microtasks), then timers and setImmediate depending on the phase.”