javascript Promise excution order - 


promise.resolve() .then(() => console.log(2)) .then(() => console.log(3));  promise.resolve() .then(() => console.log(4)) .then(() => console.log(5));  console.log(1);

why above code snippet result 1 2 4 3 5 instead of 1 2 3 4 5 .

what execution order?

it runs in order because that's order callbacks entered callback queue.

lets follow through code does:

promise.resolve() .then(() => console.log(2)) .then(() => console.log(3));  promise.resolve() .then(() => console.log(4)) .then(() => console.log(5));  console.log(1);

  • first, main code ran, 2 promises created , resolved, , 2 callbacks added callback queue; callbacks 2 , 4. queue [2,4]. console.log(1) happens , function ends.

  • now function done, next callback in callback queue runs, logging 2, pushes new callback callback queue, callback 3. queue [4,3].

  • next, callback 4 runs , logs 4, , pushes callback 5 queue. queue [3,5]

  • next, callbacks 3 , 5 each run in order, bringing queue [].

if of functions asynchronous, order have changed drastically based on how long each took complete.


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