practise2:

console.log("1");

const promise = new Promise((resolve,reject)=>{
    console.log("2");
    resolve("3")
    console.log("4")
})

promise.then((res)=>{
    console.log("5")
    console.log(res);
    console.log("6")
})

console.log("7")

🔄 Step-by-Step Execution Flow

1. console.log("1")

• Synchronous → logs: 1

2. Creating the Promise

const promise = new Promise((resolve, reject) => {
    console.log("2");
    resolve("3");
    console.log("4");
});

• Enters the executor function of the Promise immediately

• Logs: 2

• Calls resolve("3") — this doesn't execute .then() immediately, it schedules it as a microtask

• Logs: 4

🔸 At this point, the Promise is resolved, but the .then() callback is waiting in the microtask queue.

3. Register .then()

promise.then((res) => {
    console.log("5");
    console.log(res);
    console.log("6");
});

• .then() is added to the microtask queue

4. console.log("7")

• Synchronous → logs: 7

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🕳️ Microtask Queue Executes

Now that all synchronous code is done, JavaScript checks the microtask queue (which has the .then() callback).

• Logs: 5

• Logs: 3 (this is the resolved value)

• Logs: 6

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🧾 Final Output: 2 4 7 5 3 6

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🧠 Why This Order?

🎯 Key Concept: .then() Does Not Exist Yet When You Call resolve("3")

That’s right — here's how this unfolds internally:

🔹 Step-by-Step Breakdown:

1. The Promise constructor is invoked:

   • The executor function runs immediately.

   • resolve("3") is called before the .then() is attached.

2. What happens internally when resolve("3") is called?

   • The Promise transitions to a "fulfilled" state.

   • At this point, no .then() callback is registered yet.

   • So the Promise simply remembers the value ("3").

3. Later, .then() is attached:

   • The .then() method checks: "Is the Promise already fulfilled?"

   • Yes → it queues the callback immediately in the microtask queue (not immediately executed, just queued).

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🧠 So, Where Is the .then() at the Moment of resolve()?

✅ Answer: Nowhere yet.

• It hasn’t been registered.

• The resolve() function just fulfills the Promise and stores the value.

• Once the synchronous code completes and the .then() is actually called, the JS engine checks the fulfilled Promise, grabs its value, and queues the .then() callback in the microtask queue.

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📦 Think of It Like This:

• 🔐 resolve("3") locks in the result.

• 🗃 .then(callback) later checks: “Oh, you already have the result? Cool, I’ll queue myself with that.”

• 🚚 Then the engine queues the callback with the value.

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🔄 Visual Timeline

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🧪 Summary

• .then() is only added to the microtask queue after it’s called.

• resolve() doesn’t know or care about .then() — it just sets the internal state.

• When .then() is later attached, and it sees the promise is fulfilled, it schedules the callback right away (as a microtask).

Would you like a visual diagram showing this timing with stack and queues?