Tasks, microtasks, queues and schedules - JakeArchibald.com

The way that Javascript schedules timeouts and promise callbacks is more complicated than you think. This can be the reason why callbacks are not executed in the order that they are queued.

Please read this article!

This is an extract of the example in the article which demonstrates the execution order of tasks and microtasks.

Copyconsole.log('script start');

setTimeout(function() {
  console.log('setTimeout');
}, 0);

Promise.resolve().then(function() {
  console.log('promise1');
}).then(function() {
  console.log('promise2');
});

console.log('script end');

Copy// Task
script start
script end

// Microtask
promise1
promise2

// Task
setTimeout

The summary of the article says:

• Tasks execute in order, and the browser may render between them

• Microtasks execute in order, and are executed:

  • after every callback, as long as no other JavaScript is mid-execution
  • at the end of each task

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Further resources:

• JavaScript Visualized: Promise Execution Show archive.org snapshot - a good article with excellent visualizations how promises effect the execution order of tasks and microtasks