现在有一个宏任务，又有一个微任务两者同一层级，在微任务里面又有一个宏任务和一个微任务，请问执行顺序是什么,为什么？

Let's break down the execution order in this JavaScript scenario. You're describing a situation involving the event loop, macrotasks, and microtasks.

Here's the setup:

• Macrotask 1 (Outer)
• Microtask 1
  • Macrotask 2 (Inner)
  • Microtask 2

Here's the execution order:

1. Macrotask 1 (Outer): Executes completely.

2. Microtask 1: Executes.

3. Microtask 2: Executes. Microtasks are processed in FIFO (First-In, First-Out) order, and all pending microtasks are executed before the next macrotask. This is why Microtask 2 runs before Macrotask 2.

4. Macrotask 2 (Inner): Executes. Even though this macrotask was queued within a microtask, it doesn't execute until the current event loop tick is complete (meaning all pending microtasks are finished).

Why this order?

JavaScript's event loop prioritizes microtasks over macrotasks within a single tick. The goal is to ensure that tasks related to the current operation (often UI updates or promise resolutions) are handled promptly. Here's a more detailed explanation:

• Event Loop Ticks: The JavaScript engine processes tasks in a series of "ticks." Each tick generally corresponds to handling one macrotask.

• Macrotask Queue: Macrotasks (like setTimeout, setInterval, user interactions, network events, etc.) are placed in a queue. The event loop picks one macrotask from this queue and executes it.

• Microtask Queue: Microtasks (like promises' .then() callbacks, queueMicrotask, MutationObserver callbacks) are also placed in a queue. However, after a macrotask completes, the event loop checks the microtask queue.

• Microtask Priority: The event loop processes all pending microtasks before moving on to the next macrotask. This ensures that things like UI updates triggered by promises happen as soon as possible, making the application feel more responsive.

In your example, even though Macrotask 2 was queued during the execution of Microtask 1, it has to wait its turn in the macrotask queue. The event loop won't start the next macrotask until the microtask queue is empty. This is crucial for predictable and responsive web applications.