Event Loop Model : Javascript

AnKiT KaMbOj
3 min readSep 7, 2018



EventLoop: What the Heck is Event Loop..?

The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn’t start processing the event loop until the code after an async function has executed.

In web browsers, messages are added anytime an event occurs and there is an event listener attached to it. If there is no listener, the event is lost. So a click on an element with a click event handler will add a message — likewise with any other event.

The function setTimeout is called with 2 arguments: A message to add to the queue, and a time value (optional; defaults to 0). The time value represents the (minimum) delay after which the message will actually be pushed into the queue. If there is no other message in the queue, the message is processed right after the delay; however, if there are messages, the setTimeout message will have to wait for other messages to be processed. For that reason, the second argument indicates a minimum time and not a guaranteed time.

Here is an example that demonstrates this concept (setTimeout does not run immediately after its timer expires.

EventLoop example:

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Zero delays doesn’t actually mean the call back will fire-off after zero milliseconds. Calling setTimeout with a delay of 0 (zero) milliseconds doesn’t execute the callback function after the given interval.

The execution depends on the number of waiting tasks in the queue. In the example below, the message ‘’this is just a message’’ will be written to the console before the message in the callback gets processed, because the delay is the minimum time required for the runtime to process the request, but not a guaranteed time.

Basically, the setTimeout needs to wait for all the code for queued messages to complete even though you specified a particular time limit for your setTimeout.

SetTimeout Eventloop model example:-

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Several runtimes communicating together

A web worker or a cross-origin iframe has its own stack, heap, and message queue. Two distinct runtimes can only communicate through sending messages via the postMessage method. This method adds a message to the other runtime if the latter listens to message events.

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Never blocking

A very interesting property of the event loop model is that JavaScript, unlike a lot of other languages, never blocks. Handling I/O is typically performed via events and callbacks, so when the application is waiting for an IndexedDB query to return or an XHR request to return, it can still process other things like user input.

Legacy exceptions exist like alert or synchronous XHR, but it is considered as a good practice to avoid them. Beware, exceptions to the exception do exist (but are usually implementation bugs rather than anything else).

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