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d="M747.1,121.1V79.3c0-15.8-9.4-22.4-26.6-22.4c-15,0-26.8,6.6-26.8,19.3c0,2.4,0.2,3.3,0.8,5h16.6 c-0.5-2-0.7-5-0.7-7.5c0-9.6,3.6-12.4,9.4-12.4c6.4,0,9.7,2,9.7,14.4v11.2l-20.8,7.7c-10.6,4.1-19.3,8.8-19.3,20.7 c0,10.9,7.5,17.8,18.6,17.8c9.9,0,17.4-6.2,21.9-12.1v-0.1h0.1l0,11.1l26,0.1v-4.5C749.3,127.1,747.1,126.2,747.1,121.1z M729.6,117.8c-4.4,3.3-7.6,5.3-12.1,5.4c-7.7,0-11.2-5.2-11.2-12.6c0-7.7,3.6-11.2,9.8-13.5l13.4-5.4L729.6,117.8L729.6,117.8z"
></path>
</g>
<path
d="M0,0v90.4c0,65.7,50.9,88.5,88.1,105.2h0.1l3,1.4c0.7-0.4,1.4-0.7,2.2-1c0.4-0.2,0.9-0.4,1.3-0.6 c37.3-16.6,88.4-39.5,88.4-105V0H0z M116,132l-2-5.3l-26.6-70c-1.6,4.1-5.3,14-9.3,24.6l-15.3,41.3c-0.7,2-0.3,3,0.2,3.7 c1.1,1.3,3.5,1.3,7.3,1.3H71l0,4.4H42.4l0-4.4h2.2c3.9,0,7.1-1.7,9.5-6.5l2.9-6.6l26.5-67.8l-4.7-12.5h24.7l34.1,89.7l3.1,8.1H116z"
></path></svg
></a>
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<nav class="bd-links collapse" id="bd-docs-nav">
<div class="bd-toc-item">
<h4 class="bd-toc-link">Web Applications I</h4>
</div>
<div class="bd-toc-item">
<a class="bd-toc-link" href="/web2/sprint/recPSZMPrmESUYo2C">
1. Applied JavaScript
</a>
<ul class="nav bd-sidenav">
<li>
<a href="/web2/module/rectn5PUU5ubcQkPu/"> DOM I </a>
</li>
<li class="active bd-sidenav-active">
<a href="/web2/module/recJWv3RIfa4NFXbn/"> DOM II </a>
</li>
<li>
<a href="/web2/module/rec847sNXZX9CVDNl/"> Components I </a>
</li>
<li>
<a href="/web2/module/recd6kDKS6eMapSRq/">
Components II
</a>
</li>
</ul>
</div>
<div class="bd-toc-item">
<a class="bd-toc-link" href="/web2/sprint/recYL2HDPPpkDmGEm">
2. Intro to React
</a>
<ul class="nav bd-sidenav">
<li>
<a href="/web2/module/recZau7hH8vzww14N/">
React Components and Component State
</a>
</li>
<li>
<a href="/web2/module/recgzSGQtp2HYwgSR/">
Composing React Components and Passing Data Via Props
</a>
</li>
<li>
<a href="/web2/module/recKe8PW6ZMwjL1Qg/">
Component Side Effects
</a>
</li>
<li>
<a href="/web2/module/recNDoSqyUw3eq1y3/">
Advanced Styling Techniques
</a>
</li>
</ul>
</div>
<div class="bd-toc-item">
<a class="bd-toc-link" href="/web2/sprint/recH2pYM3qVavZTRN">
3. Single Page Applications
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<ul class="nav bd-sidenav">
<li>
<a href="/web2/module/recd7jGy7tfVkcFlX/"> React Router </a>
</li>
<li>
<a href="/web2/module/rect081xiYT2cfxGF/">
Form Management
</a>
</li>
<li>
<a href="/web2/module/recKK5C7wV0WiECfr/">
Advanced Form Management
</a>
</li>
<li>
<a href="/web2/module/recduYpjgZBQaQM8X/"> cypress.io </a>
</li>
</ul>
</div>
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<a class="bd-toc-link" href="/web2/sprint/recwKyMueNtoTTmUD">
4. WEB Unit 2 Build
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<main class="content-container flex-wrapper" role="main">
<div class="sticky-nav">
<div class="sprint-nav">
<ul class="section-nav">
<li class="toc-entry toc-h2"><a href="#prepare">Prepare</a></li>
<li class="toc-entry toc-h2"><a href="#learn">Learn</a></li>
<li class="toc-entry toc-h2"><a href="#project">Project</a></li>
<li class="toc-entry toc-h2"><a href="#review">Review</a></li>
</ul>
</div>
</div>
<div class="wrapper limit-width">
<div class="flex-wrapper">
<div class="limit-width">
<div class="jumbotron">
<h1 class="bd-title" id="content">DOM II</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
Events are the way we users interact with the page. Any
time there is some interaction by way of a mouse,
keyboard, etc., the DOM creates and propagates an event
object. This event object carries information about the
event so that it may be handled at any point up the tree
from the point of origin.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>install Node on Windows (if needed)</li>
<li>install Node on Mac (if needed)</li>
<li>
explain what events are in the browser window, and how
event listeners can listen for those events.
</li>
<li>add event listeners and event handlers to elements</li>
<li>
recognize event propagation and the method used to prevent
it
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>
Treat everyone with as much respect and integrity as you
wish to be treated with.
</p>
</div>
<h2 class="display-4">
<a
id="prepare"
class="anchor"
href="#prepare"
aria-hidden="true"
><span class="octicon octicon-link"></span></a
>Prepare
</h2>
<p class="lead">Review each preclass resource before class.</p>
<div class="preclass">
<ul class="list-group list-group-flush">
<li class="list-group-item">
<div class="embed-responsive embed-responsive-16by9">
<iframe
class="embed-responsive-item"
src="https://www.youtube.com/embed/uTTgynLLz-M"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
<li class="list-group-item">
<div class="embed-responsive embed-responsive-16by9">
<iframe
class="embed-responsive-item"
src="https://www.youtube.com/embed/ttMay5P_NEM"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
</ul>
</div>
<h2 class="display-4">
<a id="learn" class="anchor" href="#learn" aria-hidden="true"
><span class="octicon octicon-link"></span></a
>Learn
</h2>
<h4 class="mark mb-3 p-2">
Learn to install Node on Windows (if needed)
</h4>
<hr />
<h4 class="mark mb-3 p-2">
Learn to install Node on Mac (if needed)
</h4>
<p>
Package managers allow developers to install several code
dependencies with just a few commands. We will be using npm as
our go-to package manager here at Lambda.
</p>
<h5>Overview</h5>
<p>
In preprocessing, there are three main concepts we need to
understand:
</p>
<ol>
<li>
The language or syntax that we use to write our code (LESS)
</li>
<li>The compiler that converts the language (Node)</li>
<li>The CSS output from the compiler (Normal CSS)</li>
</ol>
<p>
This module focuses on the compiler. For us to use our
language (LESS), we need to have a compiler that understands
that specific language input.
</p>
<p>
A compiler is a program that converts language into code. In
our case, LESS is the language, and CSS is the outputted code.
We utilize compilers to provide more features than standard
CSS can offer.
</p>
<h5>Follow Along</h5>
<p>
Installing our working environment can be daunting at first.
We will be covering a lot of ground. Get through each step
before going back and trying to learn everything you did.
</p>
<p>
If you are on a mac, follow the mac instructions below. If you
are on a PC, scroll down after the mac instructions to the PC
instructions. If you are on Linux, chances are Node is already
installed in your system. Run
<code class="highlighter-rouge">node -v</code> in a shell to
make sure, and then you can look at step 3 in the mac
instructions.
</p>
<h3 id="step-by-step-install-instructions-for-mac">
Step By Step Install Instructions for Mac
</h3>
<p>
<strong
><em
>Step 1:
<a href="https://brew.sh/"
>Install Homebrew and Node</a
></em
></strong
>
</p>
<ul>
<li>
Follow the instructions on how to install Homebrew using
your terminal
</li>
<li>
Pay attention to your terminal as the installer will ask for
a password
</li>
<li>
Once Homebrew is installed, run
<code class="highlighter-rouge">brew install node</code> in
your terminal
</li>
<li>
Pay attention to your terminal, as there might be small
issues to troubleshoot
</li>
<li>Restart your terminal after installation</li>
<li>
Run <code class="highlighter-rouge">node -v</code> in your
terminal to make sure node was installed correctly
</li>
</ul>
<p>
<strong><em>Step 2: Install Less</em></strong>
<em>NOTE:</em> You will see our commands have
<code class="highlighter-rouge">-g</code> in them. Please
write the commands exactly as you see them. Globally
installing packages is ideal for our situation.
</p>
<ul>
<li>
Open terminal and run
<code class="highlighter-rouge">npm install -g less</code>
</li>
<li>
If it’s not working, restart your terminal and try again
</li>
</ul>
<p>
<strong><em>Step 3: Install less-watch-compiler</em></strong>
</p>
<ul>
<li>
Open the terminal and run
<code class="highlighter-rouge"
>npm install -g less-watch-compiler</code
>
</li>
</ul>
<p>
<strong
><em
>Step 4: Use less-watch-compiler On Your Project</em
></strong
>
</p>
<ul>
<li>
Fork and clone
<a
href="https://github.com/LambdaSchool/Preprocessing-Part-I"
>Preprocessing-Part-I</a
>
</li>
<li>
<code class="highlighter-rouge">cd</code> into your project
on your local machine
</li>
<li>
Once you are in the root of your project run
<code class="highlighter-rouge"
>less-watch-compiler less css index.less</code
>
</li>
<li>
Your project should be up and running, change some styles
around in your .less file, and save the file to make sure
you see a change in your CSS.
</li>
<li>Great work, you’re all set up!</li>
</ul>
<h3 id="step-by-step-install-instructions-for-pc">
Step By Step Install Instructions for PC
</h3>
<p>
<strong
><em
>Step 1:
<a href="https://nodejs.org/en/">Install Node.js</a></em
></strong
>
</p>
<p>
<strong
><em
>Step 2:
<a href="https://git-scm.com/download/win"
>Install Git Bash (Git SCM)</a
></em
></strong
>
</p>
<ul>
<li>
After the installer finishes, open git bash and run
<code class="highlighter-rouge">node -v</code> to make sure
node is installed correctly
</li>
<li>
If you don’t see some node version numbers display in your
git bash window, restart git bash and try again
</li>
</ul>
<p>
<strong><em>Step 3: Install Less</em></strong>
</p>
<p>
<em>NOTE:</em> You will see our commands have
<code class="highlighter-rouge">-g</code> in them. Please
write the commands exactly as you see them. Globally
installing packages is ideal for our situation.
</p>
<ul>
<li>
Open git bash and run
<code class="highlighter-rouge">npm install -g less</code>
</li>
<li>
If it’s not working, restart your git bash and try again
</li>
</ul>
<p>
<strong><em>Step 4: Install less-watch-compiler</em></strong>
</p>
<ul>
<li>
Open git bash and run
<code class="highlighter-rouge"
>npm install -g less-watch-compiler</code
>
</li>
</ul>
<p>
<strong
><em
>Step 5: Use less-watch-compiler On Your Project</em
></strong
>
</p>
<ul>
<li>
Fork and clone
<a
href="https://github.com/LambdaSchool/Preprocessing-Part-I"
>Preprocessing-Part-I</a
>
</li>
<li>
<code class="highlighter-rouge">cd</code> into your project
on your local machine
</li>
<li>
Once you are in the root of your project run
<code class="highlighter-rouge"
>less-watch-compiler less css index.less</code
>
</li>
<li>
Your project should be up and running, change some styles
around in your .less file, and save the file to make sure
you see a change in your CSS.
</li>
<li>Great work, you’re all set up!</li>
</ul>
<h5>Challenge</h5>
<ol>
<li>
Follow the instructions provided to you in the tutorial
above.
</li>
<li>
Once in your project, change the background color of your
body tag to <code class="highlighter-rouge">red</code>.
</li>
<li>
Once the background turns red, your compiler is working
correctly.
</li>
</ol>
<hr />
<h4 class="mark mb-3 p-2">
Learn to explain what events are in the browser window, and
how event listeners can listen for those events.
</h4>
<p>
We will learn what events and event listeners are, and you
will be able to explain those concepts. This is the foundation
for adding dynamic usability to our sites.
</p>
<h5>Overview</h5>
<p>
We use Javascript to make our pages dynamic and usable. Now
that we can access and manipulate elements on our page, you
might be wondering to yourself, “Why don’t we just change the
elements in the HTML file directly? Why do we have to go
through all of this just to change some styling on an
element?” Those are valid questions, and the way we have been
using Javascript so far, it would make sense to change our
HTML instead.
</p>
<p>
But, Javascript’s real power lies in its ability to make
webpages interactive. In addition to manipulating elements,
Javascript also allows us to add features and make
modifications to our site by directly reacting to user
interactions. Think about a button click, drag and drop, zoom,
or any number of user interactions. We create functionality
for those interactions using JavaScript. Now that we know how
to manipulate and modify DOM nodes, we can learn about events.
</p>
<p>
Every user interaction with a site is an event: a click,
moving the mouse, scrolling the page, pressing a key on the
keyboard, these are all events on the page, and the browser
can detect all of them. There are tons of different events the
browser tracks. When an event happens on a page, it is known
as a trigger.
</p>
<p>
Now that we know what an event is and we know the browser is
always tracking them, we need to be able to listen for
specific events on specific elements. Did a user click that
button? Did a users mouse hover over an element? Was there
text entered into the input field? There are dozens of events
an element can listen for. When one of those events happens on
that element, we can do something about it. The tracking
process and subsequent action taken is called an event
listener. We put an event listener on an element and give it a
callback. When that event is triggered on the element, the
callback is run.
</p>
<hr />
<h4 class="mark mb-3 p-2">
Learn to add event listeners and event handlers to elements
</h4>
<p>
We can chose specific elements to listen to specific events
and fire a callback when that event occurs. This is called an
event listener.
</p>
<h5>Overview</h5>
<h3 id="addeventlistener">.addEventListener</h3>
<p>
Once we have an element selected, we can use the
‘.addEventListener’ method on that element.
<code class="highlighter-rouge">.addEventListener</code> takes
two arguments, first the event to listen for and, second, the
callback to fire when that event is triggered. For a complete
list of events see the resources below. In use, this looks
something like
<code class="highlighter-rouge"
>element.addEventListener('click', callback);</code
>.
</p>
<h3 id="events">Events</h3>
<p>
We learned before that there are different types of events we
can listen for. The most common are mouse events highlighted
by the ‘click’ event, but there are dozens of more events we
can listen for. We can add event listeners for as many events
as there are for each element, meaning we can listen for a
<code class="highlighter-rouge">mouseclick</code>,
<code class="highlighter-rouge">keypress</code>,
<code class="highlighter-rouge">hover</code>, and more all on
the same element. Although to do so we need to call
<code class="highlighter-rouge">.addEventListener</code> and
pass in a callback for each one.
</p>
<p>
Take 5 minutes and study this MDN page. Choose an event that
is NOT a mouse event, and read about it and how it works.
</p>
<p>
<a href="https://developer.mozilla.org/en-US/docs/Web/Events"
>MDN Event Listeners</a
>
</p>
<h3 id="the-callback-and-the-event-object">
The callback and the
<code class="highlighter-rouge">Event Object</code>
</h3>
<p>
The callback (also known as an event handler) will take a
single argument; this is known as the
<code class="highlighter-rouge">Event Object</code>. This is a
Javascript Object and contains all we need to know about the
event and the element it happened on.
</p>
<ul>
<li>
eg:
<code class="highlighter-rouge"
>element.addEventListener('click', (event) => {//Handle
event});</code
>
</li>
<li>
One of the most important properties of the event object is
<code class="highlighter-rouge">.target</code>, this
property will give us all of the info about the DOM node
where the event happened. It has many of the same properties
as a regular DOM node,
<code class="highlighter-rouge">.children</code>,
<code class="highlighter-rouge">.parent</code>,
<code class="highlighter-rouge">.style</code>,
<code class="highlighter-rouge">innerText</code>, etc. We
can use these properties to manipulate the element itself,
or it’s relatives.
</li>
<li>
We can use this to manipulate the target in any way we want,
for example to change the background color we would write
the following:
<code class="highlighter-rouge"
>element.addEventListener('click', (event) => {
event.target.style.backgroundColor = 'blue'; });</code
>
</li>
<li>
Depending on the type of event listened for, we can have
access to other information about the event as well, such as
the key pressed (in the form of a code) and other things.
Spend 5 minutes reading about the event object here:
<a
href="https://developer.mozilla.org/en-US/docs/Web/API/Event#Properties"
>MDN Event Object: Properties</a
>
</li>
</ul>
<h5>Challenge</h5>
<p>
Follow the instructions in the Javascript section of this
codepen on your own. Use the
<code class="highlighter-rouge">sampleButton</code> element at
the top as a reference, and add the appropriate event
listeners to the elements. Please Fork and save your code, and
share it with your TL.
</p>
<p>
<a
href="https://codepen.io/lambdaschool/pen/gqmrwx?editors=0010"
>Codepen: DOM-II Events</a
>
</p>
<h5>Dig Deeper</h5>
<ul>
<li>
<a
href="https://developer.mozilla.org/en-US/docs/Web/Events"
target="blank_"
>DOM Events - MDN</a
><br />
An extensive list of web events
</li>
</ul>
<hr />
<h4 class="mark mb-3 p-2">
Learn to recognize event propagation and the method used to
prevent it
</h4>
<p>
Event propagation can cause many headaches when dealing with
events. It is best to learn how to recognize this feature, and
how to properly prevent it from happening, when need be.
</p>
<h5>Overview</h5>
<p>
Think of a pyramid. Pyramids are made up of many layers, the
bottom layer is the largest and as it goes higher, ever
decreasing sized layers are added to the one before it. If you
are standing on the uppermost layer of a pyramid, you are also
standing on every layer below it. When we look at a DOM tree
we can see those child elements are nested inside parent
elements, like a pyramid. And, like a pyramid, if you
stand(trigger an event) on a child element, you are also
triggering that same event on every parent element all the way
up to the body. This process is called event propagation. If
you have an event of the same type on a parent element and a
child element, and you trigger that event on the child
element, it will also trigger on the parent. We have to be
careful not to create unwanted interactions because of this.
In the future, you may consider this is a feature, for now,
this is something we would like to avoid.
</p>
<p>
In our event handler, we are passed the event object. The
event object has lots of methods and properties on it
including one called
<code class="highlighter-rouge">.stopPropagation()</code> if
we call this method in our event handler, it will effectively
stop our event from bubbling any further up the chain.
</p>
<ul>
<li>
eg:
<code class="highlighter-rouge"
>const eventHandler = (event) => {
event.stopPropagation() };</code
>
</li>
</ul>
<h4 id="preventdefault">.preventDefault</h4>
<p>
Some elements have a native default reaction to certain
events. For example, form elements will refresh the page on
submit.
<code class="highlighter-rouge">.preventDefault</code> is a
method on the event object, and it will stop an HTML element
from reacting in its default way.
<code class="highlighter-rouge">.preventDefault</code> will be
used less than
<code class="highlighter-rouge">.stopPropagation</code>, but
it is important to know about as well.