JAVASCRIPT EVENT LOOP

To give a little background, javascript is single threaded, which means that any process that it is asked to run only has one thread to perform this task. This is unlike many other languages and this poses a problem for javascript when it comes to processes that are reliant on returning information that takes time to get a response. Some examples of this are fetching information form a database or even a simple get/post request.

Javascript solves part of the limitations that come with being single threaded by using what is called the event loop. The event loop allows for javascript to return asynchronous operations with a single thread. Furthermore, requests can be still be performed before the initial request has been returned, which allows the ability to get functions to “wait” and then perform other tasks.

EVENT LOOP STEPS

3a. No blocking process — returns the process

3b. Blocking Process — gets delegated to a ‘thread pool’

4. When block process completes, the thread Pool prepares response to event loop. The event loop then calls the callback function and returns the response back to the client.

EXAMPLES

Here is a basic example utilizing setTimeout and consol.log()

The console would log First, Third, Second in this instance due to the setTimeout. This is possible because Javascript doesnt get held up with the setTimeout here.

Another almost ubiquitous example of the elegance of the event loop, is the event listening functionality, which essentially runs a asynchronous function which waits for a ‘click’ event in order to fire off its callback.

There are some great resources online for this, which I will link to here.

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