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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>
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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"
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<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>
<a href="/web2/module/recJWv3RIfa4NFXbn/"> DOM II </a>
</li>
<li>
<a href="/web2/module/rec847sNXZX9CVDNl/"> Components I </a>
</li>
<li class="active bd-sidenav-active">
<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>
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<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>
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<div class="wrapper limit-width">
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<div class="limit-width">
<div class="jumbotron">
<h1 class="bd-title" id="content">Components II</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
Now that we have some understanding of what a component
is, we can learn to utilize asynchronous JavaScript and
Promises to make HTTP requests and get data from a server.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>
understand and utilize asynchronous Javascript and
Promises
</li>
<li>
understand HTTP requests and get data from a server using
axios
</li>
<li>
iterate over a list of data received from a server,
creating a set of components and adding them to the DOM
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>Life is a marathon, not a sprint.</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/7I3TeN2Sj1k"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
<li class="list-group-item">
<a
target="_blank"
href="https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes"
>Data Attributes in HTML</a
>
<p>
Learn how to read and update data attributes in HTML
</p>
</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 understand and utilize asynchronous Javascript and
Promises
</h4>
<p>
Data attributes give developers another way to hook into the
DOM without having to use classes and IDs as a unique way to
make selections.
</p>
<h5>Overview</h5>
<h3 id="asynchronous-code">Asynchronous code</h3>
<p>
In JavaScript we have the concept of ‘asynchronous’ code. This
simply means code that does not run instantly in line. Perhaps
the code needs to wait a moment, wait for something to happen,
or in the case we will explore today, wait until data comes
back from a server. Using asynchronous code can be challenging
at first because we need to be able to recognize which code
will be asynchronous and which will run instantly. Take the
following example, which will run instantly and which will
wait to run?
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">setTimeout</span><span class="p">(</span> <span class="p">()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello!</span><span class="dl">'</span><span class="p">);</span>
<span class="p">},</span> <span class="mi">1000</span><span class="p">);</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="dl">'</span><span class="s1">Over here!</span><span class="dl">'</span><span class="p">);</span>
</code></pre>
</div>
</div>
<p>
Even if you have never seen
<code class="highlighter-rouge">setTimeout</code> before, you
probably realized that it will wait a moment to run, where
console.log will run instantly. If you run this code in your
console you should see
<code class="highlighter-rouge">Over here!</code> print first
because setTimeout will wait 1 second (1000 ms) to run.
</p>
<p>
Asynchronous code is everywhere in Javascript. It is an
important concept to begin to understand.
</p>
<h3 id="promises">Promises</h3>
<p>
We have a few different ways to approach asynchronous code, in
the last example we saw the use of callbacks (which you
already understand). Other concepts are
<code class="highlighter-rouge">async/await</code> and
<code class="highlighter-rouge">Promises</code>. Today we will
be using Promises.
</p>
<p>
Promises are a design pattern for use when handling
asynchronous code in JavaScript. We use them as an alternative
to nesting multiple callbacks. You may have heard of a problem
called <code class="highlighter-rouge">callback</code> hell.
Promises are a way to avoid this problem.
</p>
<p>
Implementing promises is not something you’ll need to do very
often, especially in front end development. However,
understanding how they work under the hood will help you deal
with them, for example when requesting data from web servers.
</p>
<p>
Learning how to handle promises is pretty straight forward. In
fact, on a
<code class="highlighter-rouge">Promise</code> object there
are really only two methods that we need to use to handle
promises <code class="highlighter-rouge">then</code> and
<code class="highlighter-rouge">catch</code>. For what we’re
trying to accomplish in terms of transferring data between our
apps and some server somewhere, we will actually be using what
is called a
<code class="highlighter-rouge">Promise Library</code>. I want
to ensure that you understand that the built in
<code class="highlighter-rouge">Promise</code> object you’re
about to see, is <strong>NOT</strong> a promise library,
rather it is a feature built into the JavaScript language as
of ES6. Now, let’s dig in.
</p>
<p>Consider the following code and what it is doing here.</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>let time = 0;
const timeMachine = () => {
return setTimeout(() => {
return time += 1000;
}, 1000);
};
timeMachine();
console.log(time); --> OUTPUTS: 0;
</code></pre>
</div>
</div>
<p>
What we have seen here is that at some point in time, the
setTimeout function (1 second later) will actually end up
manipulating the
<code class="highlighter-rouge">time</code> variable. However,
when we
<code class="highlighter-rouge">console.log</code> time we get
<code class="highlighter-rouge">0</code>. The reason for this,
is because of the way the
<code class="highlighter-rouge">time</code> variable is
defined. It is happening inside of a
<code class="highlighter-rouge">setTimeout</code> block, which
waits 1000ms to actually do its thing. This is an important
concept to grasp here.
<em
>Even though we’ve manipulated the data, we don’t have
access to the data that was manipulated quite yet</em
>. In a nutshell this is asynchronous JavaScript.
</p>
<p>
Ok, so the problem, is that we’re expecting
<code class="highlighter-rouge">time</code> to be equal to
<code class="highlighter-rouge">1000</code> by the time we
want to work with it. One way we can solve this is by using
Promises. Before we do that let’s figure out what a
<code class="highlighter-rouge">promise</code> is first.
</p>
<p>Straight from MDN:</p>
<blockquote>
<p>
<em
>A Promise is a proxy for a value not necessarily known
when the promise is created. It allows you to associate
handlers with an asynchronous action’s eventual success
value or failure reason. This lets asynchronous methods
return values like synchronous methods: instead of
immediately returning the final value, the asynchronous
method returns a promise to supply the value at some point
in the future.</em
>
</p>
</blockquote>
<p>
Simply put, a Promise is just that, a promise from the object
that it will let us know when it has completed (or errored)
what we have asked it to do. A promise can exist in one of
three states:
</p>
<ul>
<li>
<code class="highlighter-rouge">Pending</code>: a state
where the promise is neither rejected nor fulfilled. (this
is the state it is in when we first call it)
</li>
<li>
<code class="highlighter-rouge">Fulfilled</code>: a state
where <em>all’s well</em> and a resolved value can be used
by our code.
</li>
<li>
<code class="highlighter-rouge">Rejected</code>: a state
where <em>something went wrong</em> and there is an error
that needs to be dealt with.
</li>
</ul>
<p>
If the promise succeeds, it will return the value as a
parameter into a callback passed into
<code class="highlighter-rouge">.then()</code>. If the promise
fails, the callback passed into the
<code class="highlighter-rouge">.catch()</code> runs, taking
an error as its argument.
</p>
<h5>Follow Along</h5>
<p>Ok that’s a lot of words, now lets see some code.</p>
<ul>
<li>
Reminder, we <em>are</em> building our own promise here,
although you will <em>not</em> need to do this on your own
today.
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>let time = 0;
const timeMachine = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve((time += 1000));
}, 1000);
});
};
</code></pre>
</div>
</div>
</li>
</ul>
<p>
Notice here that we’ve now wrapped our
<code class="highlighter-rouge">setTimeout</code> function in
a <code class="highlighter-rouge">new Promise</code> and we’re
resolving the addition of the
<code class="highlighter-rouge">time += 1000</code> and
passing that resolved result. This allows us to do what we
call
<code class="highlighter-rouge">promise chaining</code> when
we invoke our
<code class="highlighter-rouge">timeMachine</code> function.
Remember that every promise ships with
<code class="highlighter-rouge">then and catch</code> methods
and we can use those methods to either receive a
<code class="highlighter-rouge">resolved promise</code> or a
<code class="highlighter-rouge">rejected promise</code>. Lets
find out how!
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>timeMachine()
.then(newTime => {
console.log(newTime); --> OUTPUTS: 1000
});
</code></pre>
</div>
</div>
<p>
Ok, in the above code, the
<code class="highlighter-rouge">thenified</code> timeMachine
invocation is now receiving a callback itself, (this is how
all promise chains should look) with some item being received
as a parameter. We can chain multiple
<code class="highlighter-rouge">thens</code> together. Inside
of each <code class="highlighter-rouge">then</code> block
when/if we return something new, we can be directed into a new
<code class="highlighter-rouge">then</code> block which is
really neat. Or maybe we’re
<code class="highlighter-rouge">resolving</code> more than one
process, and we want to see the next result in the next
<code class="highlighter-rouge">then</code> block. Lets
manipulate the data in the
<code class="highlighter-rouge">.then</code> block we have
now, and see what happens when we use another
<code class="highlighter-rouge">then</code>.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>timeMachine()
.then(newTime => {
const myTime = newTime / 1000;
return `${myTime} seconds have passed`;
})
.then(newString => {
console.log(newString); --> OUTPUTS "1 seconds have passed"
});
</code></pre>
</div>
</div>
<p>
In our first <code class="highlighter-rouge">then</code> block
we are manipulating the time that is originally being resolved
by the <code class="highlighter-rouge">Promise</code> and then
sequentially returning it with some text concatenated onto the
time. Because we are returning a value from our first
<code class="highlighter-rouge">then</code> statement, we can
now chain on another
<code class="highlighter-rouge">then</code>. And thus we have
achieved some promise chaining. This isn’t a terribly useful
program at this point. But we’re about to see some power.
</p>
<p>
Now lets make another function that we can use to return yet
another promise. This is where some of the
<code class="highlighter-rouge">then chaining</code> starts to
really come in handy. We’re going to have to refactor our
code, ONLY where we’re calling
<code class="highlighter-rouge">timeMachine</code>. The
function we’re going to write is called
<code class="highlighter-rouge">parseTime</code> and it will
receive a
<code class="highlighter-rouge">ms</code> milliseconds
parameter that we can use to reject a promise in the future.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>const parseTime = ms => {
return new Promise((resolve, reject) => {
const timeString = time / 1000;
resolve(`${timeString} seconds have passed`);
});
};
</code></pre>
</div>
</div>
<p>
Ok! Now we’ve offloaded some work from our
<code class="highlighter-rouge">then</code> block into a
function that resolves a promise. We can now use it like
above, but this time, when we call our
<code class="highlighter-rouge">timeMachine</code> function
we’ll pass this
<code class="highlighter-rouge">parseTime</code> function as
an argument to our first
<code class="highlighter-rouge">then</code> block.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>timeMachine()
.then(parseTime)
.then(timePassed => {
console.log(timePassed); --> OUTPUTS: "1 seconds have passed"
});
</code></pre>
</div>
</div>
<p>
We’ve effectively achieved the same thing, but now we’re
chaining promises and their resolves together. This is Promise
chaining. Now the last thing we need to cover is a
<code class="highlighter-rouge">rejected promise</code>
because up until now all the things have just… worked. That is
not the case in the real world, so because of that, we need to
learn how to handle rejections. Lets tweak our
<code class="highlighter-rouge">parseTime</code> function so
that we can reject a promise instead of resolving a promise
that is, if a certain condition matches.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>const parseTime = ms => {
return new Promise((resolve, reject) => {
const timeString = time / 1000;
if (ms > 999) {
resolve(`${timeString} seconds have passed`);
} else {
reject(new Error(`ms is less than 1 second promise rejected!`));
}
});
};
</code></pre>
</div>
</div>
<p>
Here’s what’s going to be sweet, when this function actually
runs, ms will be 1000. So our condition will resolve the
promise, but lets remember where we’re actually using this
function.. it’s within a
<code class="highlighter-rouge">then</code> block on the
resolve of another promise. If we tweak our
<code class="highlighter-rouge">timeMachine</code> function to
resolve under <code class="highlighter-rouge">1000 ms</code>,
we’ll get a rejection. So lets change the
<code class="highlighter-rouge">timeMachine</code> function in
such a way that breaks our promise chain so that we can deal
with it.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>resolve((time += 999));
</code></pre>
</div>
</div>
<p>
Now we should hit our condition, and our promise will then be
rejected. So we have a
<code class="highlighter-rouge">catch</code> block for this
purpose.
</p>
<div class="highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code>timeMachine()
.then(parseTime)
.then(timePassed => {
console.log(timePassed); --> OUTPUT: DOES NOT RUN
})
.catch(err => {
console.log(err); --> OUTPUT: [Error: ms is less than 1 second promise rejected!]
});
</code></pre>
</div>
</div>
<p>
And that is that! We have successfully built out a promise
chain that allows us to see the benefit of writing them in the
first place. So now, when you’re dealing with Promises in the
future, the <code class="highlighter-rouge">.then</code> and
<code class="highlighter-rouge">.catch</code> won’t be all
that scary! That last part is the part to remember. HOW we
handle promises is what is important for us in the future. Not
HOW we implement them. We will be using Axios for that and
we’re going to teach you what that is soon.
</p>
<h5>Challenge</h5>
<p>
Using the time machine example you just built, build one more
function from scratch that returns a
<code class="highlighter-rouge">Promise</code> that has a
setTimeout inside it. In the
<code class="highlighter-rouge">.then()</code> console.log
what is resolved from inside the setTimeout.
</p>
<hr />
<h4 class="mark mb-3 p-2">
Learn to understand HTTP requests and get data from a server
using axios
</h4>
<p>
Often when building a website or application we need dynamic
data. This data is usually stored on a server. We can request
data from a server and display it on our site. In this
objective we will learn how to request data using HTTP and a
library called <code class="highlighter-rouge">axios</code>.
</p>
<h5>Overview</h5>
<h3 id="http">HTTP</h3>
<p>
HTTP is a <em>network protocol</em>, a set of rules that
govern the way web clients, like a browser, communicate with
web servers over the internet.
</p>
<p>
We, as developers, need to know what HTTP Methods are and how
they are used to perform CRUD (Create, Read, Update, Delete)
operations on server resources and what HTTP status codes are
and what they are used for.
</p>
<p>
<code class="highlighter-rouge">HTTP Methods</code> provide a
common <em>language</em> or nomenclature that the client can
use to let the server know what operation it wants to perform.
</p>
<p>
When a client needs to ask a server for information it should
do a GET request specifying a URL that points to the desired
resource.
</p>
<p>
A POST request is used to ask the server to add or
<em>create</em> new resources.
</p>
<p>
The method used by the client to ask the server to make
changes to specific resources is PUT.
</p>
<p>
To remove or delete data from the server the client needs to
send a DELETE request.
</p>
<p>
We will see examples of how to use those methods from our
React Applications.
</p>
<p>
<code class="highlighter-rouge">HTTP Status Codes</code> are
used to indicate if a <em>request</em> has been successful or
not and <strong>why</strong>. The server will set the status
code for all responses sent to the client.
</p>
<h3 id="axios">axios</h3>
<p>
<code class="highlighter-rouge">axios</code> is a Javascript
library. It is used to send HTTP requests to servers. It is
<em>not</em> necessary to do this, but it makes things much
easier. Because all server requests are asynchronous,
<code class="highlighter-rouge">axios</code> uses Promises.
Once you get the basic pattern down,
<code class="highlighter-rouge">axios</code> is incredibly
easy to use.
</p>
<p>
To read more you can check out the documentation here:
<a href="https://github.com/axios/axios"
>https://github.com/axios/axiosn</a
>
</p>
<p>
In this lesson we are only going to concern ourselves with the
HTTP GET request, this will allow us to ask a server for data.
</p>
<p>
We need to start by including some code in our HTML file, this
will essentially download axios and allow us to use it in our
own javascript file.
</p>
<p>
We will put the following line in the
<code class="highlighter-rouge">head</code> section of our
HTML:
</p>
<div class="language-html highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nt"><script </span><span class="na">src=</span><span class="s">"https://unpkg.com/axios/dist/axios.min.js"</span><span class="nt">></script></span>
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