DEV Community

Adrian Matei for Codever

Posted on • Edited on

2 1

Javascript runtime interview question

I would definitely put the following question in an interview, if am interested in knowing if the candidate knows a bit about the Javascript runtime.

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');
Enter fullscreen mode Exit fullscreen mode

If you are not sure about the answer read this In depth: Microtasks and the JavaScript runtime environment article before


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
Enter fullscreen mode Exit fullscreen mode

Usually it takes a couple of milliseconds in this case before scheduling and execution of the statements in setTimeout. So even if you set the value ov 1 in the first setTimeout, it will still be shown before. Play around with it to see where is the limit for your system:

console.log('1 - start');
const timeScheduled = Date.now();
setTimeout(() => console.log(`2 - setTimeout1 ${Date.now()-timeScheduled}ms have passed since I have been scheduled`), 1);
Promise.resolve('Success')
    .then(()=> console.log('3 - promise1'))
    .then(()=> console.log('4 - promise2'));
setTimeout(() => console.log(`5 - setTimeout2 ${Date.now()-timeScheduled}ms have passed since I have been scheduled`), 0);
console.log('6 - end');
Enter fullscreen mode Exit fullscreen mode

Shared with ❤️ from Codever.   👉   use the copy to mine functionality to add it to your personal snippets collection.

Sentry blog image

How to reduce TTFB

In the past few years in the web dev world, we’ve seen a significant push towards rendering our websites on the server. Doing so is better for SEO and performs better on low-powered devices, but one thing we had to sacrifice is TTFB.

In this article, we’ll see how we can identify what makes our TTFB high so we can fix it.

Read more

Top comments (0)

Image of Timescale

Timescale – the developer's data platform for modern apps, built on PostgreSQL

Timescale Cloud is PostgreSQL optimized for speed, scale, and performance. Over 3 million IoT, AI, crypto, and dev tool apps are powered by Timescale. Try it free today! No credit card required.

Try free

👋 Kindness is contagious

Please leave a ❤️ or a friendly comment on this post if you found it helpful!

Okay