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Javascript Event Loop - Why so Serious!

Let me put a dramatic start to this one. This is not just a blog, story or any other technical article. This is rather a realization that I had very recently. I have seen two categories of Javascript developers in my surroundings. One that know on how to write program(or application) with it. The another category of people who know(or have the thirst to know) how Javascript was written fundamentally as a programming language. There is no good or bad between these two categories. However it is mostly about the transition and balance between one to another.

Now coming back to my own realization that I was talking about, it all started with the question I had, "What is Javascript"? I spent time on searching the answer over internet, books and the best that I got so far(and by far) is: js_def.png

The definition above opened the door for asking lots of other questions. Single-Threaded, How does it support asynchronous things? What makes it non-blocking and concurrent? How does it work internally? The shortest answer was Javascript's Event Loop Model. As I understood broadly, there are mainly three components to it for an execution context.

  • Call Stack where functions, its parameters are pushed to form a Stack Frame. This stack frame is a memory location in the stack. The memory is cleared when the function returns as it is pop out of the stack.
  • Room for Web APIs to evaluate. Example of Web APIs are, setTimeOut, onClick, onMouseOver etc.
  • Queue where CallBacks are en-queued, managed and de-queued to get executed in an order. This queue is called Task Queue. There is another variance called, Job Queue which we will see later.

"A picture is worth a thousand words". Here is the arrangement of all these components in the Event Loop Model(At this point, you don't have to understand it fully. Keep Reading) : event_loop_1.png (This Screenshot is taken from a fantastic online tool, latentflip.com/loupe)

So how does it work altogether?

  • The Call Stack executes the items in order. One at a time, and then again, One at a time! Everything else waits in Task Queue or Job Queue until the Call Stack is free.
  • If there is a Web API call(like, some one click on a button, or a setTimeOut has been called), it gets executed immediately outside of the stack. However, if there are Callbacks to be executed, those get placed in the Queue in the order they are supposed to be invoked.
  • There is something called Event Loop which constantly look into the fact, if the stack is empty then, push the items from the Task Queue to the Call Stack for execution. eventLoopSmall.png

Let's explain this with a simple Code Execution.


const tom = () => console.log('tom');
const jerry= () => console.log('jerry');

const cartoon = () => {
  console.log('cartoon '); 
  setTimeout(tom, 5000); 
  jerry(); 
}

cartoon ();

The Event Loop is in Action (Observe the sequence couple of times): (Courtesy: latentflip.com/loupe) eventLoopInAction.gif

So what is happening there? Let us see step by step.

  • Let us assume that, the Call Stack was empty at the beginning.
  • The function cartoon gets into the stack and it will be executed line by line.
  • The console.log('cartoon ') pushed into the stack and executed.
  • The next like setTimeout(tom, 5000); will be handled out side of the Call Stack as it is a Web API. It has a Callback function called tom which will be en-queued in Task Queue.
  • Next in Call Stack is the function jerry() which get executed.
  • And then finally the function cartoon exited.
  • At this point, Call stack is free. Hence the Event Loop will de-queue the callback it has in Task Queue and push it to Call Stack for execution.
  • The function tom gets executed in Stack Frame.
  • The Call Stack is free again.

That's all about it! Event Loop Model is really nothing that of so difficulty and seriousness. It is about understanding how things work in the orchestrated manner. As I mentioned before, there are different types of queues, Task Queue and Job Queue. I have generalized here as Queue. I shall be writing another post to explain the difference specifically.

Hope you liked the post, Stay Tuned!

Tapas Adhikary's photo

Tapas Adhikary

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Great post! Tapas Adhikary

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Nice post ! Looking forward for the next one about queues.

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