Tasks, microtasks execution order in javascript - snippet example

What is the order in which the following texts are logged via console.log ?

console.log('1 - start');
setTimeout(() => console.log('2 - setTimeout1'), 0);
Promise.resolve('Success')
    .then(()=> console.log('3 - promise1'))
    .then(()=> console.log('4 - promise2'));
setTimeout(() => console.log('5 - setTimeout2'), 0);
console.log('6 - end');

Output

1 - start// statement is executed directly in the script (first "Run script" task)
5 - end // part of the first "Run script" task gets executed too
3 - promise1 // placed in microTasks queue and executed between tasks, after first "Run script" task is ready
4 - promise2 // microtask added  during previous microtask execution  and executed immediately
2 - setTimeout1 // callback execution scheduled in another task in the "macroTasks" queue (also task queue), executed in the next interation of the event-loop
5 - setTimeout2 // callback execution scheduled in another task in the task queue, executed in the next iteration of event-loop

Reference - https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API/Microtask_guide/In_depth


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Adrian Matei

Adrian Matei
Life force expressing itself as a coding capable human being