You're observing different execution orders because of how JavaScript's event loop handles **macrotasks** and **microtasks**.

Event Loop Basics

- `setTimeout(..., 0)` is a **macrotask** (added to the macrotask queue).

- `.then()` from a `fetch` is a **microtask** (added to the microtask queue).

- Microtasks are **always executed before** the next macrotask, after the current execution stack is empty.

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In Browser

When you use `setTimeout(..., 0)` and `fetch(...)`, here’s what happens:

1. Synchronous code runs first.

2. Microtasks (like `.then()` from `fetch`) are processed.

3. Then macrotasks (like `setTimeout`) are processed.

That’s why in most browsers:

console.log(“A”);        // sync

fetch(…).then(…)     // microtask

setTimeout(…, 0)       // macrotask

console.log(“B”);        // sync

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Why `1ms` setTimeout changes order:

Changing to `setTimeout(..., 1)` gives the event loop more time, so the fetch may resolve before the timeout happens — but it's **not guaranteed**. It's a race condition depending on network timing and browser internals.

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In Node.js

Node uses a slightly different event loop model. The key difference is:

- `setTimeout` goes into the **Timers phase**

- `fetch` is not native to Node and uses the **microtask queue** after a Promise resolves

So in Node, `setTimeout(..., 0)` often logs before `.then()` due to **phase timing differences**.

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Summary

- Microtasks (`Promise.then`, `fetch`) run **before** macrotasks (`setTimeout`).

- Timing differences in browsers vs. Node.js are due to **event loop phase priorities**.

- `setTimeout(..., 0)` does **not mean immediate execution**, just “as soon as possible after current tasks.”