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JS illustrated: The event loop 🔁

John Kapantzakis on August 15, 2019

Javascript is single threaded, yet, developers can write asynchronous code in Javascript! But how is it possible? The event loop makes it possible...
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q3flat

Very nice article! Thank you for your effort!

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John Kapantzakis

Thank you very much!

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amineamami

Great article, but one question how’s this asynchronous ?

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John Kapantzakis

Thanks @amineamami !
According to MDN: Asynchronous , asynchronous code may refer to multiple related tasks happening without waiting for each other to complete.

In our case, if the code was not asynchronous, the setTimeout call whould have blocked the execution of the program until it was completed printing "Two" at the console.

But this is not the case. The program continues executing, printing "three" and, after that, the callback of the setTimeout is called, printing "Two" at the end.

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amineamami • Edited

Hmm, it seemed to me just like synchronous execution with a bit of instructions reordering.

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John Kapantzakis • Edited

Thats right! Javascript language itself does not have any asynchrony built in.
The environment (browser, node etc) is the one that arranges the events resulting in asynchronous execution (the use o callbacks, promises, ajax calls etc)

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Stephen Borsay

Hi,

On your description it shows the Web API putting "two" on the message queue but when the message is extracted from the message queue you are showing that is done by the event loop instead, Is this correct? Does the Web API always push to the message queue while the event loop extracts from the queue, or are you just missing showing the web API managing the both inserting and extracting to the message queue after being triggered by the event loop?

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Vikas Chauhan

One of the greatest articles to clear all the doubts about event loop.
Thanks man

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Sepehr Soltanieh

Awesome! Maybe make it a gif someday?!

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Gabriel Manussakis

Awesome, thanks for this article!

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Талант Асанкулов

Thank you!

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Zhuxy

cool

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mohamedNeddam

Thank you