A Complete Overview of JavaScript Events - All You Need To Know

Summary

In this informative video, Dom introduces the fundamentals of JavaScript events, ideal for beginners and those seeking a refresher. He covers HTML inline events, the crucial 'addEventListener' function, the JavaScript event object for capturing event details, and the distinction between standard functions and arrow functions in event handling. With practical examples, Dom demonstrates how to respond to user interactions like clicks and provides insights on when to use each function type for clear and effective coding practices.

Takeaways

• 😀 JavaScript events allow you to react to user interactions on a webpage, such as clicks or form submissions.
• 📌 The script introduces four main topics for understanding JavaScript events: HTML inline events, the 'addEventListener' function, the JavaScript event object, and the difference between standard and arrow functions in event handling.
• 🔗 HTML inline events are the simplest form of event handling but can be limiting due to the lack of complexity they can manage within the HTML attributes.
• 🔧 The 'addEventListener' method is a preferred approach for attaching events to elements because it keeps JavaScript separate from HTML, making the code cleaner and more manageable.
• 📚 The JavaScript event object provides detailed information about the event that occurred, such as mouse coordinates and whether modifier keys were pressed during the event.
• 👉 Inline event handlers can quickly become messy and are not recommended for complex projects, where separating JavaScript from HTML is advisable.
• 🛠️ Functions like 'onClick' and 'onMouseOver' can be attached to elements to define actions that should occur when specific events are triggered.
• 🔄 When using 'addEventListener', you can specify the event type and a function to be executed when the event occurs, promoting better organization of code.
• 🎯 The 'this' keyword behaves differently in standard functions and arrow functions within event handlers, with the former referring to the element that triggered the event and the latter to the global object.
• 👨‍🏫 The video script is aimed at beginners and those with some experience who want to review JavaScript events, emphasizing the importance of understanding the basics for effective web development.
• 🔍 The script suggests that developers should research and understand the various types of events available in JavaScript to effectively use them in their projects.

Q & A

What are JavaScript events, and why are they important?

JavaScript events allow you to react to specific interactions or occurrences on a web page, such as clicks, form submissions, or mouse movements. They are important because they enable dynamic and interactive user experiences by triggering actions when certain events happen.

What is an inline HTML event in JavaScript, and when might you use it?

An inline HTML event in JavaScript is an event handler directly placed within the HTML element, such as `onclick="alert('Hello!')"`. It's typically used in beginner tutorials for its simplicity but is not recommended for complex projects due to its limitations in maintainability and functionality.

What is the `addEventListener` function in JavaScript, and why is it preferred over inline events?

The `addEventListener` function is used to attach an event handler to an element separately from the HTML markup. It is preferred over inline events because it keeps the HTML clean, allows multiple events to be attached to a single element, and provides more flexibility in handling events within the JavaScript code.

How does the `getElementById` function relate to event handling in JavaScript?

The `getElementById` function is used to select an HTML element by its ID, allowing you to reference that element in your JavaScript code. Once selected, you can attach event handlers, such as using `addEventListener`, to respond to user interactions with that element.

What is the JavaScript event object, and what information does it provide?

The JavaScript event object, often referred to as `e` or `event`, contains information about the event that just occurred, such as the type of event (e.g., click, mouseover), the position of the mouse, and whether modifier keys like Alt or Ctrl were pressed. This information can be used to make the event handling more dynamic and responsive to user actions.

Why might you use an external function for handling events in JavaScript?

Using an external function for handling events in JavaScript, rather than defining the function inline, helps keep the code organized and reusable. It allows you to separate the logic from the event attachment, making the code easier to maintain, debug, and extend.

What is the difference between a standard JavaScript function and an arrow function in the context of event handling?

The main difference between standard JavaScript functions and arrow functions in event handling is how they treat the `this` keyword. In standard functions, `this` refers to the element that triggered the event. In arrow functions, `this` is inherited from the surrounding context, often leading to it referring to the global window object instead of the element.

Why might the `this` keyword behave differently in arrow functions compared to standard functions?

In arrow functions, the `this` keyword does not bind to the element that triggered the event but instead inherits `this` from the surrounding lexical context. This can lead to unexpected behavior, especially in event handlers, where you might expect `this` to refer to the element the event is bound to.

How can you log the event object to the console in JavaScript, and why would you do this?

You can log the event object to the console using `console.log(e)` within the event handler function. This is useful for debugging and understanding the properties of the event, such as the mouse coordinates or whether modifier keys were pressed, allowing you to tailor your event handling logic accordingly.

What are some common use cases for the information provided by the event object in JavaScript?

Common use cases for the event object include determining the position of the mouse during a click event, checking if modifier keys (like Ctrl or Alt) were held down, and accessing the target element that triggered the event. This information can be used to create more responsive and interactive user interfaces.

Outlines

00:00

📚 Introduction to JavaScript Events

This paragraph introduces the concept of JavaScript events, explaining their purpose and how they allow developers to respond to user interactions on a webpage. It sets the stage for the video by outlining the topics to be covered, including HTML inline events, the addEventListener function, the JavaScript event object, and the difference between standard functions and arrow functions in the context of event handling. The paragraph emphasizes the importance of events for beginners and experienced developers alike, aiming to provide a comprehensive understanding of how events work in JavaScript.

05:02

🔗 HTML Inline Events and Their Limitations

The second paragraph delves into the use of HTML inline events, demonstrating how they can be used to trigger JavaScript actions directly within HTML elements. It provides a practical example using a 'div' element with an 'onclick' event that displays an alert message. The paragraph also touches on the limitations of inline events, such as the inability to handle complex logic within the event attributes, and suggests that separating JavaScript code from HTML using external scripts or files is a better practice for maintainability and scalability.

10:03

🎯 The Advantages of addEventListener

This paragraph highlights the preferred method of handling events in JavaScript: the addEventListener function. It contrasts this approach with inline events, showing how addEventListener allows for more complex and modular code. The paragraph includes a step-by-step guide on how to use addEventListener, from selecting the HTML element to defining the event type and the function that will be executed upon the event's occurrence. It also discusses the benefits of externalizing event handling functions and the use of the event object to access details about the event.

15:04

📌 Understanding the JavaScript Event Object

The fourth paragraph focuses on the JavaScript event object, which contains information about the event that has occurred. It clarifies the role of the event object in providing details such as mouse position and keyboard state during an event. The paragraph demonstrates how to access the event object by passing it as a parameter to the event handler function and then using it to log information to the console. It also explains the various properties available within the event object that can be useful for different scenarios, depending on the project requirements.

⚔️ Standard Functions vs. Arrow Functions in Event Handling

The final paragraph discusses the difference between standard JavaScript functions and arrow functions when used as event handlers. It provides examples of both function types within the addEventListener method and explains the significance of the 'this' keyword in each context. The paragraph points out that 'this' refers to the element being interacted with in standard functions, whereas in arrow functions, 'this' refers to the global object, which can lead to errors if not handled correctly. The video concludes by emphasizing the importance of choosing the right function type based on the specific needs of the event handling scenario.

Keywords

💡JavaScript Events

JavaScript Events are actions or occurrences that happen in a web browser or web application, which can be responded to by JavaScript code. They are central to the video's theme as they allow for interactive web pages. For example, the script discusses how events like 'click' or 'mouseover' can trigger actions such as displaying a dropdown menu or validating a form.

💡Inline HTML Events

Inline HTML Events are JavaScript event handlers written directly within the HTML code, using attributes like 'onclick'. The video describes them as the most basic form of JavaScript event handling, but also points out their limitations, such as lack of scalability and maintainability, as seen in the example where 'onclick' is used to display an alert message.

💡addEventListener

The addEventListener method is a JavaScript function used to attach event handlers to elements. It's highlighted in the video as the preferred way to handle events because it separates JavaScript code from HTML, making the code cleaner and more manageable. The script demonstrates how to use addEventListener to react to a 'click' event on a div element.

💡Event Object

The Event Object in JavaScript contains information about an event, such as the type of event, mouse coordinates, and whether modifier keys were pressed. The video explains that this object is crucial for understanding the context of an event and provides an example of logging the event object to the console to inspect its properties.

💡Arrow Functions

Arrow Functions are a more concise syntax for writing functions in JavaScript, introduced in ES6. The video contrasts arrow functions with standard functions, particularly in the context of event handling. It notes that 'this' inside an arrow function refers to the global object (like 'window'), unlike in standard functions where 'this' refers to the element that triggered the event.

💡Standard Functions

Standard Functions, also known as function declarations or function expressions, are a traditional way of defining functions in JavaScript. The video points out that within standard functions, the 'this' keyword refers to the element that the event handler is attached to, which is important for understanding the scope and context of 'this' in event-driven programming.

💡Event Handling

Event Handling is the process of responding to user actions or browser events with JavaScript. The video's main theme revolves around this concept, showing how to set up event listeners and handle events like 'click' to trigger actions. Event handling is essential for creating interactive web pages.

💡DOM Manipulation

DOM Manipulation refers to the process of changing the Document Object Model (DOM) of a webpage using JavaScript. In the context of the video, DOM manipulation is implied when discussing how to change the behavior of HTML elements in response to events, such as showing an alert or changing styles.

💡Event Properties

Event Properties are the details contained within the Event Object that provide information about the event. The video script explains how to access these properties, such as 'clientX' and 'clientY' for mouse coordinates, and 'ctrlKey' to check if the control key was pressed during the event.

💡Function Externalization

Function Externalization is the practice of defining functions separately from the event listener invocation. The video demonstrates this by showing how to define a function named 'handleBoxClick' and then reference it in the addEventListener call, which promotes better organization and reusability of code.

💡This Keyword

The 'this' keyword in JavaScript refers to the context within which a function is executed. The video script uses the 'this' keyword to illustrate the difference in context between standard functions and arrow functions in event handlers, where 'this' can refer to the element or the global object, respectively.

Highlights

Introduction to JavaScript events for beginners and a recap for experienced developers.

Definition of JavaScript events as a way to react to actions on a webpage.

Explanation of the basic concept of event and action in JavaScript.

Overview of four main topics to be covered in the video: HTML inline events, addEventListener function, JavaScript event object, and the difference between standard and arrow functions in events.

Usage of HTML inline events such as 'onClick' for beginner tutorials.

Limitations of inline event handlers in terms of complexity and maintainability.

Introduction to the 'addEventListener' function as the preferred method for handling events.

Demonstration of adding event listeners to a div element with an 'id' of 'box'.

Explanation of how to use different JavaScript events like 'click' and 'mouseover'.

Advantages of separating JavaScript code from HTML for better organization and scalability.

Example of defining and using an external function with 'addEventListener'.

Introduction to the JavaScript event object 'e' and its role in providing information about the event.

Description of properties within the event object, such as mouse coordinates and key presses.

Difference between standard functions and arrow functions in the context of 'this' keyword usage in event handlers.

Implications of using 'this' in arrow functions versus standard functions and potential errors.

Recommendation to use standard functions when 'this' keyword is needed to refer to the element.

Conclusion summarizing the basics of JavaScript events and the importance of understanding them for web development.

Transcripts

hey guys how you going

my name is dom and today we're going to

be taking a look at the basics of

javascript events okay so today's video

is going to be perfect for people who

are just starting out to learn

javascript or

you've got a bit of experience under

your belt and you just want to get a

recap on how the events work

okay so of course the first question

here is going to be

what exactly are javascript events well

essentially they're going to allow you

to react to when

certain things happen on the page so for

example

when the user clicks on a button i want

for example

a drop down menu to show or another

example might be

when the user clicks on a submit form i

want

some sort of validation to happen so

basically you have the event

and then you have the action so in

today's video

we're going to cover four main topics

around

the events to of course gain an

understanding of how they work

so the first topic covered is going to

be the usage of the very basic

html inline events okay so these

are things like your on click which

you'll see on many beginner

tutorials the second topic covered is

going to be the usage of the add event

listener function and this one here is

arguably the most

important the third topic is going to be

the usage

of the javascript event object and these

ones right here

are going to tell you information about

the event which just occurred

and the last topic is going to be the

difference between a standard javascript

function

and an arrow function when it comes to

the events so

hope you guys enjoy and learn something

from today's video

let's get right into the first topic

here the

inline html events okay so

starting off with the most basic form of

a javascript event

handler okay so right here i've got a

fairly straightforward

html file and we can see right down here

i've got this div with an id of box and

i've applied some

fairly straightforward css styles to

this

box so now we can see in the browser

it's going to look

something like this so if i go back

inside the text editor we can see

inside the actual html for the box

itself we can see

it says on click alert hey

decode so this right here is the most

basic form

of a javascript event and you'll find

this style

on many different beginner tutorials out

there on the web

now of course it's quite

self-explanatory what this does

when i click on the box i want this to

say alert

hey decode so now if i was to go inside

the browser

we can see upon clicking on the box we

of course get hey

decode right there and that is the most

basic form like i said

of a javascript event handler now

of course if you're using a text editor

like visual

studio code if you go inside the actual

div here

and you specify for example o n or just

on we can see there are many different

events to choose from

and of course when doing your own

project you're going to be googling

these things and finding out exactly

what type of event you need to use for

your own

scenarios an example might be the on

mouse over event and essentially it's

going to allow you to react to when the

user

hovers their mouse over the actual div

or the box in this case right here

so for example i can just say alerts

then i can say

something like hover okay

just like that so now if i was to save

this we can

we can see of course we actually have

two event handlers right here

the onmouseover and the on click and of

course that is perfectly fine and valid

to have

so now saving this right here going

inside the browser

we can see if i was to hover over the

actual box we get hover

right there if i was to then click

on the box we also get hey d code so

right there we can see an example of

adding the two events

right there now what is the problem with

this

style of event handler well you can't

actually do

much inside these two

inside these quotes okay so of course

you can do quite a few things inside

here but of course

once your project gets more complex and

you want to do

many different things inside the event

handler

you're kind of restricted as to what you

can do inside here

and it can get very messy quite quickly

so

the better option is going to be to

essentially put all of your javascript

code

inside its own separate you know section

of the page or its own file

and then do your event handling inside

there instead

it makes it much easier to work with and

of course that brings us to

the add event listener function okay so

ad event listener is going to be the

preferred way for you to add events to

your javascript code

and of course your web page so right

here we can see we get a very similar

example

i've got the exact same div with an id

of box but now

i've opened up this script tag right

here and i've specified some javascript

code okay so essentially it's going to

do the exact same thing

and i can show you this right now so

going inside the browser we can see

upon clicking on the box we get right

here hey

d code so now looking at the actual code

itself

we can see in the javascript the first

line of code here is simply grabbing

the actual div from the html so right

here i'm just saying

a new constant or new variable

called box div is equal to

document.getelementbyid and i'm passing

in box right here so

for those of you who don't know

essentially if i pass in box right here

it's going to grab this div and now

essentially this div

refers to this box div right inside the

javascript code

so now i'm just simply saying box div

dot add event listener and i'm passing

through here

the click event so similar to the

previous example

where we had the mouse over you can also

pass in

mouse over inside here and that is also

going to work

like i mentioned earlier there are many

different javascript events to use so

you're going to have to do your own

searching and find out what you need for

each different scenario

but let's go back to the basic click

event right here

after this we're simply specifying a

javascript

function right inside here and i'm

accepting

the event object right there so we're

going to get to this

very shortly but for now we can simply

ignore that

and then going inside the actual

function body itself

i'm then saying alert hey d code so

essentially

we're moving the alert call from

inside the html in the previous example

to

this function right inside here and this

makes it

a lot easier to work with and

essentially

you're going to have a much better time

writing your javascript code

inside the script tag or an external

file

compared to inside the html so of course

we saw

how that one works now what if i was to

instead of specifying the function right

here

we can actually specify it externally so

this right here is another example

of what you might see so if i was to

specify a function

right inside here i can say function

and call this function handle box

click so essentially this function is

going to do the job of handling

the box click so now i can also accept

the event object

as we did earlier and then i can simply

move

the alert code inside here and then

instead of having this function right

here i can instead just say

handle box click just like that and that

is going to give us the exact same

result it is simply just a different way

of writing it

and you might see this example or style

also

so if i was to save this we can see we

get the

exact same result we can click and we

get hey

decode so now we're going to move on to

taking a look

at the event object right here so let's

look at that

right now okay so moving on to the event

object i'm referring to

this e right here now i know this e

confused me quite a bit

when i first started learning javascript

but it's actually very straightforward

in what it actually does

so essentially this e or the event

object

contains information about the event

which has just

occurred in the case of the click event

you can find many different pieces of

information

about that click for example you can see

if the user was holding down the alt

or the control key when the click

occurred or even

the position of the mouse now to gain

access to

the event object you simply pass through

a name

for the event object right here inside

the function now of course

you can call this whatever you want for

example

it doesn't need to be e we can say ev

or we can even say event right here

realistically

we could say absolutely anything like d

code for example

and that is also going to work but it is

a nice convention

to simply use e or ev as the name for

the event

object okay so that being said

we can then reference or use the object

inside the function

by saying e just like this now

right here i'm simply saying console.log

and i'm passing through e

so essentially this console.log is gonna

tell me

information about the actual object

right there

inside the javascript developer console

so now if i was to save this right here

and go

inside the browser i can then using f12

you can toggle the developer tools right

inside here

and you may need to switch to the

console output right up here

to gain access to of course the console

so now

if i was to click on the object we can

see

so on the div we can see right here we

get the mouse

event inside the console right here and

notice how it says

mouse events there are many different

event

objects in javascript because of course

this one right here

was a click event a click event relates

to the mouse so of course we get the

mouse event

there is also the keyboard event for

example

so now going inside the actual object

itself we can see

there are many different properties

related to

the event which has just occurred for

example

we gain access to the client x and the

client y

and these are just the the mouse

coordinates

when the click occurred we can see we

get 108

and 80 right inside here if i was to

click

again this time in the top left corner

we can see the client x and the client y

are much lower at 10 and 10.

we also gain access to things like the

control key

and the alt key which of course tell you

if the alt or control key were pressed

or held down when the click occurred for

example we can see now

the control key is set to false but if i

was to hold the control key

and click again we get right inside here

control key

sets a true so of course depending on

your own

scenario or use case you're gonna find

these properties useful okay so of

course

it's gonna depend on your own project

what you're doing

you may not need to use any of these

properties and that's perfectly fine but

in many cases you may need to gain

access to these different

information regarding the event to of

course do

different things okay so let's let's

move on now to

the usage of the of the arrow functions

in comparison to the standard javascript

functions

okay so moving on to the final part of

today's video and that is going to be

the difference between standard versus

arrow functions

when it comes to your event handlers

okay so right here we can see

i have two examples of the usage of the

add event

listener method okay so right here we

can see

the first example is very similar if not

the same in fact it is the same

as our previous examples we're simply

saying function and then of course

taking through the event object and then

we're simply saying right down here

console.log and i'm logging out the

value

of this keyword if you don't know what

this keyword refers to

that is perfectly fine this is still

going to make sense

in the second example this one right

here is probably

one of the more modern ways to write

your javascript event

handlers and this one uses an arrow

function

as we can see right here so these serve

a very similar purpose

and they do basically the exact same

thing it's simply just

a different way of writing out a

javascript function

this one is standard and this one is an

arrow function

now we can see i'm also logging out the

value

of the this keyword so now if i was to

save this right here

go inside the browser then click on the

div we can see

we get our two outputs right there

as we can see in the case of the first

example with the standard function

this refers to or this keyword refers to

the actual box itself so basically if i

go back inside here

in this case this refers to the exact

same thing

as box div right there and that is for

standard functions

when it comes to your arrow function

this refers to

the whole window global object itself

so the reason why i pointed this out is

because

of course if you run into errors where

you know certain things are defined or

you know certain things aren't working

just watch out for cases where you may

be using an

arrow function but then using this

keyword always make sure if using this

keyword right here

you want to make sure you use the

standard javascript function

instead of the arrow functions right

down here

if you want to learn more about the

arrow functions in javascript

i've got a whole video dedicated to that

so you can watch that too

if you like and that is the basics of

javascript

events thanks for watching guys and i'll

see you in the next

video