MicroTask Queue 和 MacroTask Queue 的混淆

Question

I am a beginner in JS and While going through Event Loop and Promises , I came across the below example at mdn .

const wait = ms => new Promise(resolve => setTimeout(resolve, ms));

wait().then(() => console.log(4));
Promise.resolve().then(() => console.log(2)).then(() => console.log(3));
console.log(1); // 1, 2, 3, 4

Shouldn't the O/P be 1,4,2,3 ?

These are the steps, which according to my understanding should have happened

1. setTimeout CB in MacroTask Q
2. log(4) CB in MicroTask Q
3. log(2) CB in MicroTask Q
4. log(3) CB in MicroTask Q

log(1) and then empty the MicroTask Q in the FIFO order. What am I getting wrong here, please explain

Answer 1

The wait function's promise resolves only after the setTimeout finishes - after a macrotask (a few milliseconds) In contrast, the Promise.resolve gets put onto the microtask queue, which will run first (often a millisecond or less). After the console.log(1) line runs, you'll have the following:

Microtask queue: .then(() => console.log(2)).then(() => console.log(3));

Macrotask queue: resolve (due to the setTimeout )

Microtask queue runs first, so 2 gets logged, then the next .then gets put onto the microtask queue, and 3 gets logged. Finally, the setTimeout macrotask runs, resolving the wait Promise, putting another .then onto the microtask queue, and shortly causing 4 to be logged.