I don't understand why following execution output is 1 3 4 2 6 5;
Promise.resolve().then(() => console.log(2));
(async() => console.log(await 6))();
console.log(1);
(async() => await console.log(3))();
Promise.resolve().then(() => console.log(5));
console.log(4);Here's how your code executes:
Promise.resolve() creates a resolved promise which puts () => console.log(2) in the micro-task queue
queue: [
() => console.log(2)
]
The IIFE executes in which you have await 6. Awaiting 6 is similar to wrapping it in Promise.resolve(6) and then awaiting it: await Promise.resolve(6).
When the operand of await is not a promise, await creates a native promise and uses the operand's value to resolve that promise
queue: [
resolve(6),
() => console.log(2)
]
1 is logged on the console
queue: [
resolve(6),
() => console.log(2)
]
console output: 1
Second IIFE executes where you have await console.log(3). This is like await undefined because console.log(3) will execute, 3 will be logged on the console and the return value of console.log, i.e. undefined will be awaited.
In short, you are awaiting undefined.
queue: [
resolve(undefined),
resolve(6),
() => console.log(2)
]
console output: 1 3
Promise.resolve() creates a resolved promise which puts () => console.log(5) in the micro-task queue
queue: [
() => console.log(5),
resolve(undefined),
resolve(6),
() => console.log(2)
]
console output: 1 3
4 is logged on the console
queue: [
() => console.log(5),
resolve(undefined),
resolve(6),
() => console.log(2)
]
console output: 1 3 4
Script execution end
Event loop will now begin processing the micro-task queue
2 will be logged on the console because it was queued first
queue: [
() => console.log(5),
resolve(undefined),
resolve(6)
]
console output: 1 3 4 2
await 6 resolves with the value 6 which is then passed to console.log. As a result 6 is logged on the console
queue: [
() => console.log(5),
resolve(undefined)
]
console output: 1 3 4 2 6
Promise created as a result of await console.log(3) resolves with the value of undefined
queue: [
() => console.log(5)
]
console output: 1 3 4 2 6
5 is logged on the console
queue: [ ]
console output: 1 3 4 2 6 5
Done.
Note: Your code shouldn't rely on the timing and order in which different unrelated promises are resolved.
There are two sequences here: 1 3 4 and 2 6 5. The first is produced on the first tick (before any promises have resolved, and the second on the second tick (after promise resolution).
(async() => await console.log(3))(); writes 3 to the log before the await, so the 3 appears before any promises resolve, and between 1 and 4 because of the order of the statements. (Note that the await in this line resolves to undefined, because console.log doesn't return anything).
For the sequence 2 6 5, all of the console logs come after a promise resolves (or, equivalently, an await statement), and they're in natural order otherwise.
1 is printed first because of order of execution.
3 is printed as code is executed because console.log() is not an asynchronous operation. Does not matter if you put await in front of it.
4 is again printed because of order of execution.
Now, rest of the order of execution is due to asynchronous operations. The 3 statements are printed in an asynchronous manner after the above 3 statements are executed.
