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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 II</h4>
</div>
<div class="bd-toc-item">
<a class="bd-toc-link" href="/web3/sprint/recq59MvEIZfUAohy">
1. Advanced React
</a>
<ul class="nav bd-sidenav">
<li>
<a href="/web3/module/recnDKrWb24lgaziC/">
Class Components
</a>
</li>
<li class="active bd-sidenav-active">
<a href="/web3/module/rec1DPoOZSmocXfak/">
The React Lifecycle
</a>
</li>
<li>
<a href="/web3/module/recAxmnYmusEi8JuL/">
Composing and Sharing Non-Visual Behavior
</a>
</li>
<li>
<a href="/web3/module/rechTQVfCg4p85Hoi/">
Testing Web Applications
</a>
</li>
</ul>
</div>
<div class="bd-toc-item">
<a class="bd-toc-link" href="/web3/sprint/recukritK1B1pFrcM">
2. Advanced State Management
</a>
<ul class="nav bd-sidenav">
<li>
<a href="/web3/module/rectpk6uIV3pCHJh8/">
The Reducer Pattern
</a>
</li>
<li>
<a href="/web3/module/recba2NUdVmgKYyR3/"> Redux </a>
</li>
<li>
<a href="/web3/module/recSoiWNJzGURrXYh/"> Async Redux </a>
</li>
<li>
<a href="/web3/module/recByjgF7FwwH5Z7W/"> Context API </a>
</li>
</ul>
</div>
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3. Advanced Web Applications
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<li>
<a href="/web3/module/recS9H6qCj0dQe32B/">
Testing React
</a>
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Client-Side Authentication
</a>
</li>
<li>
<a href="/web3/module/recupVjaAKPqbuk7Y/">
HTTP / AJAX II
</a>
</li>
<li>
<a href="/web3/module/receOMvCB7dSyX5FL/">
Deploying Web Apps
</a>
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<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>
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</div>
<div class="wrapper limit-width">
<div class="flex-wrapper">
<div class="limit-width">
<div class="jumbotron">
<h1 class="bd-title" id="content">The React Lifecycle</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
Every component in React has what we call a lifecycle. The
Lifecycle is a a part of how our components operate under
the hood. Being able to demonstrate understanding of the
React Component Lifecycle by knowing how to use a few of
the “Methods” that are provided by it, will help you
understand how and why react behaves the way it does at
times.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>
describe the three phases of the React Component Lifecycle
</li>
<li>
properly explain what the `constructor` and `render`
methods do and what their place is in the React Component
Lifecycle
</li>
<li>
use the `componentDidMount` function to set a component's
state post render
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>
Write all code knowing someone else will be working on it
after you.
</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/KRZXwWOa18c"
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/nFEorPNRNd8"
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/t985N_qPhEU"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
<li class="list-group-item">
<a
target="_blank"
href="https://engineering.musefind.com/react-lifecycle-methods-how-and-when-to-use-them-2111a1b692b1"
>React Lifecycle methods</a
>
<p>
A great introduction to what we have at our disposal
when working with React.
</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 describe the three phases of the React Component
Lifecycle
</h4>
<p>
The React component lifecycle is an important concept to
leverage when it comes to writing React apps. For example,
they are necessary in order for React apps to make API
requests and then render based on the data that was received.
</p>
<h5>Overview</h5>
<p>
React is, in essence, a combination of multiple components. A
component can be as simple as a single piece of user interface
that represents a small portion of our application.
Conceptually, a component lifecycle happens in three phases.
This idea is displayed nicely in the following diagram from
one of the maintainers of React “Dan Abramov”.
</p>
<p>
<img
src="https://image.ibb.co/j8CzEd/lifecycle.jpg"
alt="drawing"
width="1000px"
/>
</p>
<p>
As you can see, the three React lifestyle phases are 1)
Birth/Mounting, 2) Growth/Updating, and 3) Death/Unmounting.
</p>
<h3 id="the-birthmounting-phase">The Birth/Mounting Phase</h3>
<p>
This is the phase when the component is being built out from
the ground up. A few things are happening here: Whatever
initial data you want access to will be defined on the
constructor of this phase
</p>
<ul>
<li>Your render method is invoked.</li>
<li>
<code class="highlighter-rouge">componentDidMount</code>
gets called as well.
</li>
</ul>
<h3 id="growthupdating-phase">Growth/Updating Phase</h3>
<p>
In the Growth/Updating phase you’re updating compnent data.
</p>
<ul>
<li>
<code class="highlighter-rouge">setState</code> can be used
to change the component’s state data, forcing a call to
<code class="highlighter-rouge">render</code>.
</li>
<li>
<code class="highlighter-rouge">shouldComponentUpdate</code>
is a method one could use here to stop a component from
calling render if necessary.
</li>
</ul>
<h3 id="deathun-mounting-phase">Death/Un-mounting Phase</h3>
<p>
Again, self-explanatory, but the unmounting phase includes
removing the component from the screen.
</p>
<ul>
<li>Component is removed from the screen.</li>
<li>
<code class="highlighter-rouge">componentWillUnmount</code>
is called and can be used for any clean up you may need to
do.
</li>
</ul>
<h5>Follow Along</h5>
<p>
Dive into the documentation at
<a
href="https://reactjs.org/docs/react-component.html#the-component-lifecycle"
>ReactJS</a
>and look into some of the key pieces of the LifeCycle API.
</p>
<p>The methods that we’re going to look at are:</p>
<ul>
<li><code class="highlighter-rouge">constructor</code></li>
<li><code class="highlighter-rouge">render</code></li>
<li>
<code class="highlighter-rouge">componentDidMount</code>
</li>
<li>
<code class="highlighter-rouge">componentDidUpdate</code>
</li>
<li>
<code class="highlighter-rouge">componentWillUnmount</code>
</li>
</ul>
<p>
Let’s also compare where each of these methods belong within
the react lifecycle by taking a look at
<a
href="http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/"
>this diagram</a
>.
</p>
<p>
We will explore these in depth later on but for now, focus on
warming up to the documentation and the idea that components
have a lifecycle.
</p>
<h5>Challenge</h5>
<p>
Read
<a
href="https://medium.com/@baphemot/understanding-reactjs-component-life-cycle-823a640b3e8d"
target="_blank"
>this medium article</a
>
about the component lifecycle. Take notes and share what you
learned in a paragraph to your Project Manager via Slack.
</p>
<h5>Dig Deeper</h5>
<ul>
<li>
<a
href="https://blog.bitsrc.io/react-16-lifecycle-methods-how-and-when-to-use-them-f4ad31fb2282"
target="blank_"
>React Lifecycle Methods: How and When to Use Them</a
><br />
An article thoroughly walks through when to use each React
component lifecycle method, and why
</li>
<li>
<a
href="https://tylermcginnis.com/react-interview-questions/"
target="blank_"
>React Interview Questions</a
><br />
An amazing breakdown of commonly seen react interview
questions by _Tyler McGinnis_
</li>
<li>
<a href="https://youtu.be/GY8zKD4AHl0" target="blank_"
>CS8 - React II Lifecycle Methods</a
><br />
</li>
</ul>
<hr />
<h4 class="mark mb-3 p-2">
Learn to properly explain what the `constructor` and `render`
methods do and what their place is in the React Component
Lifecycle
</h4>
<p>
<code class="highlighter-rouge">render()</code> is the life
blood of every react class component. We have to use it when
building our UI. You can think of our
<code class="highlighter-rouge">render</code> function as the
‘thing that tells react what will be mounted to the screen’.
Learning about how this function works, and when it works,
will help us all understand more about the React ecosystem and
components in general.
</p>
<h5>Overview</h5>
<h3 id="the-constructor-function">The Constructor Function</h3>
<p>
You already know all about the
<code class="highlighter-rouge">constructor</code> function as
it pertains to
<code class="highlighter-rouge">classes</code> in JavaScript.
It’s not much different in React. The constructor’s purpose in
React is to create components with inciting state data for the
initial render. Any other props that the component receives on
state can be done through the constructor function. We also
used to bind all of our event handlers to the component via
the constructor, and now we don’t have to because of some
special <code class="highlighter-rouge">ESNext</code> syntax
that allows us to use arrow functions on our
<code class="highlighter-rouge">class methods</code>. The
following snippet is an example of this.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">eventHandler</span> <span class="o">=</span> <span class="p">()</span> <span class="o">=></span> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span> <span class="na">foo</span><span class="p">:</span> <span class="dl">'</span><span class="s1">bar</span><span class="dl">'</span> <span class="p">});</span>
</code></pre>
</div>
</div>
<p>
We will have ample time to practice application set up with
state data via the constructor, so for now, soak in these
examples.
</p>
<p>
Let’s say we have some data from an external file living
within our application. Let’s say, too, that we want our
component to render a list of that data out to the DOM. We
would need to import the external data as an array, ( we don’t
care about the shape, or what type of data just that it lives
on an array ) and use the constructor to set it up on state.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="k">import</span> <span class="nx">React</span> <span class="k">from</span> <span class="dl">'</span><span class="s1">react</span><span class="dl">'</span><span class="p">;</span>
<span class="k">import</span> <span class="p">{</span> <span class="nx">data</span> <span class="p">}</span> <span class="k">from</span> <span class="dl">'</span><span class="s1">./extraneousSource.js</span><span class="dl">'</span><span class="p">;</span>
<span class="kd">class</span> <span class="nx">MyComponent</span> <span class="kd">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
<span class="kd">constructor</span><span class="p">()</span> <span class="p">{</span> <span class="c1">// if I wanted to receive some props here I could pass them in through the constructor! constructor(props)...</span>
<span class="k">super</span><span class="p">();</span> <span class="c1">// if i receive props through the constructor I will need to pass them back through super(props);</span>
<span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span>
<span class="na">arbitraryStateData</span><span class="p">:</span> <span class="nx">data</span><span class="p">,</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
This how we use the constructor in the mounting phase of our
component’s lifecycle. Now that our data is set up on state,
we can access it during our render portion of the mounting
phase.
</p>
<p>
In conclusion, the constructor function on a React Class’
Component’s purpose is to serve up initial state data for when
the time comes to mount up your DOM elements.
</p>
<h3 id="the-render-method">The Render Method</h3>
<p>
The <code class="highlighter-rouge">render()</code> method is
one of the React lifecycle methods that is used to tell React,
to return some piece of DOM. The React virtual DOM will then
handle the steps to mount those DOM pieces.
</p>
<p>
The <code class="highlighter-rouge">render()</code> method is
required for a class component, and without it, the component
won’t work.
</p>
<p>
<code class="highlighter-rouge">render</code> should be a pure
function, meaning it should return the same thing each time
you call it. Its concern is to look at
<code class="highlighter-rouge">this.props</code> and
<code class="highlighter-rouge">this.state</code> and return
some DOM element, a boolean, an array of DOM elements, and a
couple of other things that you may want to reference or look
into later.
</p>
<p>
The function is there to return what your component should
render to the screen. Though many developers ignore it,
<code class="highlighter-rouge">Component</code> is an
important lifecycle method and should be regarded as such.
</p>
<p>
Another essential item to remember about
<code class="highlighter-rouge">render</code> is that it is
called not only in the
<code class="highlighter-rouge">Mounting Phase</code> of the
component lifecycle but also during the
<code class="highlighter-rouge">Updating Phase</code>. This
duality makes the
<code class="highlighter-rouge">render()</code> method unique
because it exists across multiple phases.
</p>
<p>
Now that we have our state data set up for us, we can use that
state data to give a list of elements to React and let it do
it’s <code class="highlighter-rouge">thing</code>. Inside the
render method of our component, let’s return a list of items
that the
<code class="highlighter-rouge">arbitraryStateData</code>
property (found on our
<code class="highlighter-rouge">state</code> object) will
generate.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">render</span> <span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o"><</span><span class="nx">div</span><span class="o">></span>
<span class="c1">// map returns an array remember? So lets give react an array of JSX elements and let it perform it's magic. For this example we'll assume this list is a string.</span>
<span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">arbitraryStateData</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">data</span> <span class="o">=></span> <span class="o"><</span><span class="nx">div</span><span class="o">></span><span class="p">{</span><span class="nx">data</span><span class="p">}</span><span class="o"><</span><span class="sr">/div></span><span class="se">)</span><span class="err">}
</span> <span class="o"><</span><span class="sr">/div</span><span class="err">>
</span> <span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
One last thing to note is that any changes from
<code class="highlighter-rouge">setState</code> invoke a call
to our <code class="highlighter-rouge">render</code> method as
well. We’ll discuss more on this later, but it’s important to
remember that
<code class="highlighter-rouge">render()</code> is called
during mounting, and will be reinvoked if anything changes in
our state object. You can think of the state object and render
methods as existing together. The state object is like the
bigger brother to the
<code class="highlighter-rouge">render</code> function, it
tells the
<code class="highlighter-rouge">render</code> function what to
do, and the render function obeys.
</p>
<p>
In conclusion, the
<code class="highlighter-rouge">render()</code> method is how
we tell React what data we want to turn into DOM elements.
<code class="highlighter-rouge">render()</code> is necessary
for every class component we create because we need it to
return
<code class="highlighter-rouge"><JSX/></code> elements.
It is involved in the
<code class="highlighter-rouge">Mounting Phase</code> and the
<code class="highlighter-rouge">Updating Phase</code> of our
component’s lifecycle.
</p>
<p>
Continue to think about these methods as the mounting methods
in our Component LifeCycle as you create more and more
professional-looking code.
</p>
<h5>Follow Along</h5>
<p>
Let’s build out some friends together using
<a href="https://codesandbox.io/s/5v3pql3y8x" target="_blank"
>this code sandbox link</a
>
</p>
<p>
Once there, you should see a page that renders out a
<code class="highlighter-rouge"
><h1> and <p></code
>
tags. You should also notice a
<code class="highlighter-rouge">people.js</code> file that
contains an array of objects with this shape.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre class="highlight"><code><span class="p">{</span>
<span class="dl">"</span><span class="s2">id</span><span class="dl">"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">first_name</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2">Suzi</span><span class="dl">"</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">last_name</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2">Claiden</span><span class="dl">"</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">email</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2"><a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="f685959a979f929398c6b697849f8c999897d8939283">[email protected]</a></span><span class="dl">"</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">occupation</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2">Biostatistician II</span><span class="dl">"</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">friends</span><span class="dl">"</span><span class="p">:</span> <span class="p">[</span>
<span class="p">{</span>
<span class="dl">"</span><span class="s2">firstName</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2">Gabe</span><span class="dl">"</span><span class="p">,</span>
<span class="dl">"</span><span class="s2">lastName</span><span class="dl">"</span><span class="p">:</span> <span class="dl">"</span><span class="s2">Lemery</span><span class="dl">"</span>
<span class="p">}</span>
<span class="p">]</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
This is the data that our application will consume. Using the
knowledge of the React LifeCycle methods, we will discuss how
to get this data from one place to another while.
</p>
<p>
Let’s start at the beginning with our constructor method.
First, let’s define a name for our state data. Our data object
is all about people, so let’s call our state object “persons”
(intentionally not “people,” so that we can tell the two
apart.)
</p>
<p>
Let’s import the people and set them up on our constructor’s
state object.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="k">import</span> <span class="p">{</span> <span class="nx">people</span> <span class="p">}</span> <span class="k">from</span> <span class="dl">"</span><span class="s2">./people</span><span class="dl">"</span><span class="p">;</span>
<span class="p">...</span>
<span class="kd">constructor</span><span class="p">()</span> <span class="p">{</span>
<span class="k">super</span><span class="p">();</span>
<span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span>
<span class="na">persons</span><span class="p">:</span> <span class="nx">people</span>
<span class="p">};</span>
<span class="p">}</span>
<span class="p">...</span>
</code></pre>
</div>
</div>
<p>
Now that we imported data, we can loop over that friend’s
array and generate a DOM element for each item in the array.
</p>
<h5>Challenge</h5>
<p>
Now you’re going to extend the Friends App that we built
together <a href="" target="_blank">link</a> by completing the
following tasks.
</p>
<ul>
<li>
Notice that each person object has an array of friends
assigned to it.
</li>
<li>
The goal for the stretch problem will be to have a
<code class="highlighter-rouge">FriendsList</code> drop-down
built out that hides the friends for each person until it is
clicked on.
</li>
<li>
Once a user clicks on your FriendsList card, display that
user’s friends.
</li>
</ul>
<hr />
<h4 class="mark mb-3 p-2">
Learn to use the `componentDidMount` function to set a
component's state post render
</h4>
<p>
Understanding the timing of
<code class="highlighter-rouge">componentDidMount</code> and
utilizing that timing to ensure that your component has the
state on it that you need when you need it, is pivotal in
designing a proper functioning React application. Today we’re
going to learn all about what
<code class="highlighter-rouge">componentDidMount</code> is,
when it gets called, and what we commonly do within it.
</p>
<h5>Overview</h5>
<p>
The
<code class="highlighter-rouge">componentDidMount</code>
method is a part of the
<code class="highlighter-rouge">mounting</code> phase in the
React Lifecycle. This method gets called as soon as the render
method is called the first time, and it begs the question…now
what?
</p>
<p>
When our component has mounted, we have the bare bones minimum
we need to put something on the screen, so now what?
</p>
<p>
Inside of
<code class="highlighter-rouge">componentDidMount</code> we
can call <code class="highlighter-rouge">setState</code> which
forces a re-render of our component. This way, any
asynchronous actions should be performed inside of our
<code class="highlighter-rouge">componentDidMount</code>
function, especially when it comes to fetching data via HTTP.
Data fetching is the de-facto purpose for using