setTimeout strange result [duplicate]

Question

I have been playing with Promises, but I am having trouble understanding what is happening with the following code:

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 10)
})

setTimeout(() => {
  console.log('Promise log inside first setTimeout')
}, 0)

promise.then(res => {
  console.log('Promise log after fulfilled')
})

console.log('Promise made - Sync code terminated')

setTimeout(() => {
  console.log('Promise log inside second setTimeout')
}, 0)

The output is:

Promise started - Async code started 
Promise made - Sync code terminated 
Promise log inside first setTimeout 
Promise log inside second setTimeout 
Promise log after fulfilled

It is as expected.

But let check the output of the below code:

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 1)
})

setTimeout(() => {
  console.log('Promise log inside first setTimeout')
}, 0)

promise.then(res => {
  console.log('Promise log after fulfilled')
})

console.log('Promise made - Sync code terminated')
setTimeout(() => {
  console.log('Promise log inside second setTimeout')
}, 0)

Changed the to be resolved promise setTimeout timer value from 10ms to 1ms

The output is:

Promise started - Async code started 
Promise made - Sync code terminated 
Promise log after fulfilled 
Promise log inside first setTimeout 
Promise log inside second setTimeout 

Any explanation for this?

Answer 1

From Concurrency model and the event loop

• setTimeout does not run immediately after its timer expires
• Zero delay doesn't actually mean the call back will fire-off after zero milliseconds. Calling setTimeout with a delay of 0 (zero) milliseconds doesn't execute the callback function after the given interval. Basically, the setTimeout needs to wait for all the code for queued messages to complete even though you specified a particular time limit for your setTimeout.

What happen if we set 2 and 1 milliseconds:

const promise = new Promise((resolve, reject) => {
  console.log('Promise started - Async code started')
  setTimeout(() => {
    resolve('Success')
  }, 2)
})

console.log('Promise log inside first setTimeout 1')
setTimeout(() => {
  console.log('Promise log inside first setTimeout 2')
}, 1)

promise.then(res => {
  console.log('Promise log after fulfilled ❌')

})

console.log('Promise log inside second setTimeout 1')
setTimeout(() => {
  console.log('Promise log inside second setTimeout 2')
}, 1)
});

The output always will be:

Promise started - Async code started
Promise log inside first setTimeout 1
Promise log inside second setTimeout 1
Promise log inside first setTimeout 2
Promise log inside second setTimeout 2
Promise log after fulfilled ❌

Conclusion

If you want a proper behavior, worth to get rid of Zero delays.