JS Event Loop

JS Event Loop
BY SAAI VIGNESH P

Agenda
• Before getting on to the main topic, we’ve
to re-iterate on:
• What is JavaScript?
• JS Runtime Environment
• Synchrony and Asynchrony
• Event Loop
• What is Event Loop?
• How it works?
• Why do we need it?
• Code Example Demonstration

What is JavaScript?
• JavaScript is a lightweight, cross-platform, and interpreted scripting language. It is used
for web development, both client-side and server-side.
• It is based on the ECMAScript Specification (ECMA-262 standard).
• The language is:
• Single Threaded (Synchronous)
• Event-driven (events are emitted and handled)
• Imperative (explicit instructions)
• Functional (creating and using functions)

JS Runtime Environment
• The JavaScript runtime environment provides access to built-in libraries and objects that
are available to a program so that it can interact with the outside world and make the code
work.
• The environment consists of:
• Execution Engine (Google V8)
• Web API – Browser; Low Level API – Node.js
• Callback Queue
• Event Loop

Synchrony and Asynchrony
Synchronous Calls:
• Code executes in the order they are called.
• The previous operation/function call should be
completed or done for the control to get transferred.
• i.e., they are blocking.
• Guess the output !

Output:
I’m Function 1
I’m Function 2
I’m Function 3

Explanation
Call Stack
fun1()
console.log(“I’m Function 1”)
Output: I’m Function1

Explanation
Call Stack
fun1()
fun2()

Explanation
Call Stack
fun1()
fun2()
I’m Function 2

Explanation
Call Stack
fun1()
fun2()
fun3()
I’m Function 2

Explanation
Call Stack
fun1()
fun2()
fun3()
I’m Function 2
I’m Function 3

Explanation
Call Stack
fun1()
fun2()
I’m Function 2
I’m Function 3

Explanation
Call Stack
fun1()
I’m Function 2
I’m Function 3

Explanation
Call Stack
I’m Function 2
I’m Function 3

Blocking Call Example
• Imagine downloading a 10 MB image from Internet with JavaScript, with an internet speed
of 1 MB/sec in a browser.
• It would ideally take 10 seconds to load the image.
• For these 10 seconds, no other operations can be done including rendering.
• Once download completes, rest of the operations can be executed.
• This is called a synchronous/blocking call.
• Solution: Async calls!

Synchrony and Asynchrony
Asynchronous Calls:
• Code executes and the control is returned no matter
whether the operation is completed or not.
• i.e., they are non-blocking.
• But how to know that they are complete? Use
callbacks!
Output:
Hello World
I’m JavaScript
Timeout!

Callbacks? What are they?
• A callback is a function passed into another function as an argument, which is then
invoked inside the function to complete some kind of routine or action.
• They are usually used with asynchronous operations. eg: background operations.
• Example:
setTimeout() and setInterval() works asynchronously.

Background Ops?
SOUNDS GOOD… BUT HOW??
Cause JavaScript is:
• Single Threaded, which means it has only one Call
Stack!!

One more confusion…
If JavaScript is Single Threaded,
Then how setTimeout() / setInterval() works?

Here’s where,
We’ve to know some facts…

Timer Functions & I/O
• JavaScript – No native asynchrony
• JavaScript does not have support for Timer and I/O, they’re not part of the language
(ECMAScript specification).
• Timer functions such as:
• setTimeout()
• setInterval()
• Input/output functions such as:
• console.log()
• console.error()
• Which means they are implemented by someone else.
• Who? It’s the runtime environment (Browser (Web API) / Node.js).

JavaScript
Event Loop
• The event loop is a programming construct
or design pattern that waits for and
dispatches events or messages in a
program.
Source: Wikipedia
• It is by which the JavaScript language
behaves like a multi-threaded, being
single-threaded by nature.
• It is a form of concurrency achieved to
support asynchronous calls in JavaScript.
• So how it works? Let’s see…

Coding to demonstrate Event Loop
Try guessing the output?

Hello World
I’m JavaScript
Timeout!
Output:

Hello World
I’m JavaScript
Timeout!
Output: Hello World
I’m JavaScript
Timeout!
Output:

Let’s see how,
THAT STUFF HAPPENS…

Call Stack
Callback Queue
Event
Loop
Web API
Let’s take the example:
Start Execution!

Call Stack
Callback Queue
Event
Loop
Web API
console.log(“Hello World”)
Executing…

Call Stack
Callback Queue
Event
Loop
Web API
Timer()
Duration: 0ms
Callback: anonymous()
setTimeout(anonymous())
Executing…

Call Stack
Callback Queue
Event
Loop
Web API
console.log(“I’m JavaScript’”)
Timer()
Duration: 0ms
Callback: anonymous()
COMPLETED
anonymous()
Executing…
Event Loop’s Job:
1. Check if call stack is empty.
2. If empty, pop the callback queue,
and push the callback function to
the call stack, else wait.
3. Execute the call stack.

Call Stack
Event
Loop
Web API
anonymous()
anonymous()
Callback Queue
Executing…
Event Loop’s Job:

Call Stack
Event
Loop
Web API
anonymous()
Callback Queue
console.log(“Timeout!”)
Executing…
Event Loop’s Job:

Call Stack
Event
Loop
Web API
anonymous()
Callback Queue
Executing…
Event Loop’s Job:

Call Stack
Event
Loop
Web API
Callback Queue
Terminated
Event Loop’s Job:

Synopsis of the Demo
• So event loop helps in concurrency of multiple operations. Yes it is true.
• But still JS is single-threaded and event loop has to wait for the call stack to get empty.
• Event Loop is also responsible for firing the callbacks associated with events.
• So JavaScript without it’s runtime having the event loop, and Web API, event handling or
asynchronous execution cannot be achieved.
• Call Stack and Events Demo: https://bit.ly/event-loop-demo

Why Asynchrony is needed?
• Asynchronous execution is needed, because Browsers!.
• Browser’s webpage rendering and JS execution engine runs on the same thread.
• So, if JS is busy executing some stuff, browser rendering is stuck until JS’s call stack gets empty.
• [Code Demo]

Fun Fact about Callbacks
• Callbacks need not be always asynchronous!
• Callback functions can also be synchronous.
• So which callbacks are asynchronous? Event handlers and Timers are.
• Which is not? Array.forEach()
• Custom callbacks need not be.
• For Example:
Array.forEach(callback) isn’t asynchronous.
https://bit.ly/array-foreach-demo

Thank You!
That’s how event handling and asynchrony works in JavaScript.
Thanks to Event Loop!

JS Event Loop

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