Async/await:

Absolutely! Let's walk through your code line by line with precise explanations, focusing especially on what happens after each awaitand how microtasks are scheduled and executed.

---

✅ Code Recap:

jsCopyEditconsole.log("script start");

async function async1() {
  console.log("async1 start");
  await async2();
  console.log("async1 end");
}

async function async2() {
  console.log("async2");
  await null;
  console.log("async2 after await");
}

async1();

Promise.resolve().then(() => {
  console.log("promise 1");
});

queueMicrotask(() => {
  console.log("microtask");
});

console.log("script end");

---

🔍 Step-by-step Execution in Detail

---

1. console.log("script start")

• Directly executed in the call stack

• Prints: script start

---

2. Function Declarations

• async1 and async2 are defined.

• No code is executed yet—just registered in memory.

---

Let me explain why async1 end comes after promise 1 and microtask, which might seem counterintuitive at first.

Detailed Explanation of the Correct Order:

1. Initial Synchronous Execution (same as before):

   • script start

   • async1 start

   • async2

   • script end

2. Microtask Queue After Synchronous Execution:
   The microtask queue contains (in this order):

   • The continuation of async2 (console.log("async2 after await"))

   • The Promise.resolve().then callback (console.log("promise 1"))

   • The queueMicrotask callback (console.log("microtask"))

   • (The async1 end is not here yet!)

Wait, why isn't async1 end in the queue yet? Because of how await works:

• When async2 hits await null, it returns a promise and schedules async2 after await as a microtask.

• The await async2() in async1 then takes that promise and schedules async1 end to run only after that promise resolves.

3. Processing Microtasks:

   • First microtask: async2 after await runs

     • This resolves the promise that async2 returned

     • Now the await async2() in async1 can continue, so it schedules async1 end as a new microtask at the end of the queue

     • Current queue: [promise 1, microtask, async1 end]

   • Next microtask: promise 1 runs

   • Next microtask: microtask runs

   • Finally: async1 end runs

Why This Order Matters:

The key insight is that async1 end gets scheduled as a microtask while the microtask queue is being processed, after async2 after await runs. This makes it the last microtask to execute.

Visual Timeline:

Output:

script start
async1 start
async2
script end
async2 after await
promise 1
microtask
async1 end