BFE.dev solution for JavaScript Quiz
1. Promise order

Background Knowledge of Event Loop

Event loop in JavaScript determines how the code is executed.

For details, please refer to The explanation on JavaScript.info, it is very helpful and easy to understand.

Simply put

1. Event loop keeps executing the tasks.
2. Macro Tasks are created by running scripts, event handlers, setTimeout(), .etc.
3. Micro Tasks are created by function calls like Promise.then(), queueMicrotask() .etc.
4. After each Macro task is executed, if there are Micro tasks created, Event loop executes them first before executing next Macro task.

OK, now let's break down the code in the question.

Step-by-step Question Analysis

console.log(1)
const promise = new Promise((resolve) => {
  console.log(2)
  resolve()
  console.log(3)
})

console.log(4)

promise.then(() => {
  console.log(5)
}).then(() => {
  console.log(6)
})

console.log(7)

setTimeout(() => {
  console.log(8)
}, 10)

setTimeout(() => {
  console.log(9)
}, 0)

When above script is executed, the execution of the whole script is one Macro task.

console logs	Macro	Micro
	script run

We meet the first console.log()

console logs	Macro	Micro
1	script run

Now comes the creation of Promise.

const promise = new Promise((resolve) => {
  console.log(2)
  resolve()
  console.log(3)
})

The arrow function passed to Promise() is the executor, from the ECMA262 Spec, we can see that the executor function is executed during the construction of Promise object, so it is synchronous.

resolve() changes the internal state of Promise object to fulfilled, but doesn't block the next line of code, thus we have following tasks and logs.

console logs	Macro	Micro
1 2 3	script run

Again we meet a console.log() call.

console logs	Macro	Micro
1 2 3 4	script run

promise.then(() => {
  console.log(5)
}).then(() => {
  console.log(6)
})

Promise.then() sets up the success and failure callbacks for the Promise object. From the ECMA262 Spec, we can clearly see that when the Promise object is already fullfilled or rejected, then() would HostEnqueuePromiseJob, which basically means it is add to the Micro Tasks.

console logs	Macro	Micro
1 2 3 4	script run	() => console.log(5)

Promise.then() also returns a new Promise, since there is no return value in the callback () => console.log(5), the new Promise is resolved with value undefined, and next then() does repeats above process.

console logs	Macro	Micro
1 2 3 4	script run	() => console.log(5), () => console.log(6)

Another console.log() call.

console logs	Macro	Micro
1 2 3 4 7	script run	() => console.log(5), () => console.log(6)

Here comes setTimeout().

setTimeout(() => {
  console.log(8)
}, 10)

setTimeout(() => {
  console.log(9)
}, 0)

setTimeout() is not part of JavaScript language spec, but it is a host API, which is under HTML spec, basically the host(most of the time, it means the browser or node.js) tries to add the Macro task after certain delay.

Notice that the delay you set is the minimum delay since the timer is to add Macro tasks, the execution is still handled by Event loop.

console logs	Macro	Micro	host timer
1 2 3 4 7	script run	() => console.log(5), () => console.log(6)	0ms, 10ms

Now all code in the first Macro task are executed, Event loop tries to execute all the Micro tasks

console logs	Macro	Micro	host timer
1 2 3 4 7 5		() => console.log(6)	0ms, 10ms

console logs	Macro	Micro	host timer
1 2 3 4 7 5 6			0ms, 10ms

Now all tasks are empty, and time is up for the 1st timer

console logs	Macro	Micro	host timer
1 2 3 4 7 5 6	() => console.log(9)		10ms

Event loop picks up the Macro task.

console logs	Macro	Micro	host timer
1 2 3 4 7 5 6 9			10ms

The last timer gets executed.

console logs	Macro	Micro	host timer
1 2 3 4 7 5 6 9	() => console.log(8)

Here is our final result.

console logs	Macro	Micro	host timer
1 2 3 4 7 5 6 9 8