关于async、Promise与事件循环的疑问：无Web API时Promise是否有用？

Hey there! Let's walk through all your questions clearly using your code as a reference:

1. 你的示例里有没有使用async？ #

Nope, you didn't use async here. Your add function is a regular function that explicitly returns a Promise object—no async keyword anywhere, which matches your note about removing it per the hint.

2. add函数的执行逻辑：事件循环队列还是其他线程？ #

Let's break down exactly what happens when your code runs:

• First, add(3, 6) runs synchronously in the main JS thread. When you create the Promise, the executor function (the one with resolve/reject) runs immediately—since you call resolve(a + b) right away with no async operations, this resolves the Promise instantly.
• The .then() callback doesn't go to the main event loop's macro task queue (like setTimeout would). Instead, it gets added to the microtask queue—a special queue that's processed right after the current synchronous code finishes, before any macro tasks run.
• No other threads are involved here! Since there's no Web API call (like HTTP requests, timers, or DOM events), everything happens in the single main JS thread. The Promise resolution is handled entirely by the JS engine itself.

执行顺序的具体步骤： #

1. Run add(3,6): create Promise, resolve it with 9, queue the .then() callback to microtasks.
2. Run console.log('test 1'): outputs test 1 immediately.
3. Current synchronous code finishes. Now process all microtasks: run the .then() callback, output 9.

3. 无Web API场景下使用Promise是不是毫无意义？ #

Absolutely not! Promise has plenty of value even when you're not using Web APIs:

• Unified API for sync/async logic: If you have a function that might return a value synchronously sometimes and asynchronously others, wrapping it in a Promise lets callers use .then() or await consistently, no need to branch logic.
• Clean chained logic: Even for synchronous steps, using .then() lets you chain operations in a readable way without nesting (though this is more impactful if you might switch to async later).
• Centralized error handling: You can attach a single .catch() to a chain of operations to handle any errors along the way, which is cleaner than scattered try/catch blocks for complex sequences.
• Utility methods: Tools like Promise.all() or Promise.race() let you coordinate multiple operations (even sync ones) and handle their results as a group—if later you swap those sync operations for async ones, your coordination code stays the same.

内容的提问来源于stack exchange，提问作者Sérgio Campos da Fonseca