State of Macrotask and Microtask queue during this calculation

Question

This question is very similar in intent to Difference between microtask and macrotask within an event loop context , but more specific, in that it asks for the explication of a definite example: I think for this reason it shouldn't be considered a duplicate.

What is the state Macrotask queue and Microtask queue during the execution of this code in node.js

console.log("A1");

(async ()=> {

    console.log("1")

    f = async ()=>{console.log('2')}

    await f()

    console.log("3")

})()

console.log("A2");

Output:

A1
1
2
A2
3

Output I expected: A1, A2, '1','2','3'

Based on this reasoning: log A1 --> enqueue anonymous function on microtask queue --> log A2 --> execute anaonymous function log 1, enqueue f on the microtask queue --> execute f from microtask queue --> log 2 --> log 3

Where am I going wrong? (additionally how is a top level async func enqueued?)

NOTE: actual command used to run this was npx babel-node myscript.js

Answer 1

You're seeing this behavior because an async function runs synchronously up until the first await , explicit return , or implicit return (code execution falling off the end of the function).

I'm going to change the code slightly to give the outer async function a name so it's easier to talk about:

console.log("A1");
const outer = async () => {
    console.log("1")
    f = async ()=>{console.log('2')}
    await f()
    console.log("3")
};
outer();
console.log("A2");

Here's what happens:

1. console.log("A1") runs (of course).
2. outer() is called.
3. The synchronous part of outer runs, so it:
   • Runs the console.log("1")
   • Creates f
   • Calls f()
4. The synchronous part of f runs, so it:
   • Does console.log('2')
5. At this point, f implicitly returns, and so it returns its promise to outer . That promise is already fulfilled with the value undefined . (See here in the spec.)
6. outer awaits f 's promise, so it returns its promise to the caller (which throws it away, but that doesn't matter).
7. outer awaiting f 's promise queued a microtask to continue outer because f 's promise is already settled.
8. console.log("A2") runs.
9. The microtask runs, allowing outer to continue and do console.log("3") .