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<h4 class="bd-toc-link">Web Applications II</h4>
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<a class="bd-toc-link" href="/web3/sprint/recq59MvEIZfUAohy">
1. Advanced React
</a>
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<a href="/web3/module/recnDKrWb24lgaziC/">
Class Components
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The React Lifecycle
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Composing and Sharing Non-Visual Behavior
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Testing Web Applications
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2. Advanced State Management
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<li class="active bd-sidenav-active">
<a href="/web3/module/rectpk6uIV3pCHJh8/">
The Reducer Pattern
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<a href="/web3/module/recba2NUdVmgKYyR3/"> Redux </a>
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<a href="/web3/module/recSoiWNJzGURrXYh/"> Async Redux </a>
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<a href="/web3/module/recByjgF7FwwH5Z7W/"> Context API </a>
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Testing React
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Client-Side Authentication
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HTTP / AJAX II
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Deploying Web Apps
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<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>
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<h1 class="bd-title" id="content">The Reducer Pattern</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
The difficulties of storing and efficiently performing
logic on data within components becomes more and more
apparent as the amount of data increases. Consequently, as
components grow and deal with larger sets of data, the
storing and transportation of state across the entire
application increasingly becomes more and more cumbersome
as well. Reducers offer one possible way to address this
problem within the component. At the level of the
application, an elegant combination of the Context API
with reducers provides one possible way that React
developers can manage global state.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>
explain what immutability is in programming and
demonstrate its benefits
</li>
<li>describe reducer functions</li>
<li>
employ the `useReducer` hook to manage state in a
component
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>We all build on the work of others.</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/EY8lIHdBA-w"
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 explain what immutability is in programming and
demonstrate its benefits
</h4>
<p>
Redux is built on the programming principle of immutability.
This means that the Redux store cannot be mutated, which will
save us from running into more bugs and weird side effects,
and will make it easier to reason about our code.
</p>
<h5>Overview</h5>
<p>
Mutable objects are objects whose state is allowed to change
over time. An immutable value is the exact opposite – after it
has been created, it can never change. There are some real
benefits from making your state immutable. We won’t go over
all the benefits here, but a couple we will talk about are
predictability and mutation tracking.
</p>
<h4 id="predictability">Predictability</h4>
<p>
Mutation hides change, which can create (unexpected) side
effects. This can lead to some nasty bugs in our code. When we
enforce immutability, we can keep our application architecture
and mental model simple, which makes it easier to reason about
the application. Simply put, it is very easy to predict how
the state object will change based on certain actions/events.
Without immutability, our state object can be changed or
updated in unpredictable ways, causing weird behavior and/or
bugs.
</p>
<h4 id="mutation-tracking">Mutation Tracking</h4>
<p>
Immutability makes it really easy to see if anything has
changed. For example when we change the state in Redux, our
components props will update. We can check our previous props
against our new props to know what change occurred, and know
how to handle those changes. If a user adds a task to the todo
list, the
<code class="highlighter-rouge">TodoList</code> component will
update since it is receiving new props. But what if we want to
run an animation on the new todo? We can’t just run it on
every render because it would run when the user toggles a task
to complete, or deletes a task. Since Redux state management
is immutable, we can track the changes that happen on the
state, and only run our animation when a new task is added.
</p>
<h4 id="redux-and-immutability">Redux and Immutability</h4>
<p>
Redux has a single immutable state tree (referred to as the
store) where all state changes are explicitly handled by
dispatching actions. Dispatched actions are processed by a
reducer that accepts the previous state and the action and
returns the next state of your application. It is easy to
predict how the state tree is going to change based on actions
that are dispatched. It is also easy to predict which action
will be dispatched based on some event or interaction. This
all leads to very predictable state management.
</p>
<p>
Writing immutable code can be tough - your Javascript skills
will really be tested here - and it may seem pretty tedious,
especially since we will be building very small apps with
small state trees during this sprint. Because of that, it may
be pretty hard to see the real benefits of immutable code in
class. However, when you start working with a large
application, with a huge state tree, you will quickly grow to
appreciate the benefits of writing immutable code, and the
extra effort it takes will seem much more worth it.
</p>
<h5>Follow Along</h5>
<p>
Here is a replit that will help you understand mutable code.
In the next objective below, we will learn how to write
immutable code.
</p>
<p>
Follow along with the exercises
<a href="https://repl.it/@snowkid314/understandingMutations"
>here</a
>
</p>
<h5>Challenge</h5>
<p>
Read
<a
href="https://codeburst.io/explaining-value-vs-reference-in-javascript-647a975e12a0"
>this article</a
>. Write a paragaraph or two explaining what you learned. Send
that paragraph to your PM.
</p>
<hr />
<h4 class="mark mb-3 p-2">
Learn to describe reducer functions
</h4>
<p>
The reducer pattern is a great state management pattern that
allows us to write pure functions to manage state changes in a
predictable manner.
</p>
<h5>Overview</h5>
<p>
Reducer functions take two arguments – the current state and
action – and return a new, updated state object based on both
arguments.
</p>
<p>In pseudocode, this idea looks like:</p>
<div class="language-text highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> (state, action) => newState
</code></pre>
</div>
</div>
<p>
More specifically, consider a function in JavaScript that,
when passed an integer, would return that value + 1, without
mutating the original integer’s value. Notice we could pass is
our initialState value -
<code class="highlighter-rouge">0</code> - and the return a
new value - <code class="highlighter-rouge">1</code> - without
overriding the initialState.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="mi">0</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">newState</span> <span class="o">=</span> <span class="nx">state</span> <span class="o">+</span> <span class="mi">1</span>
<span class="k">return</span> <span class="nx">newState</span><span class="p">;</span>
<span class="p">}</span>
<span class="kd">const</span> <span class="nx">newStateValue</span> <span class="o">=</span> <span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</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="nx">initialState</span><span class="p">,</span> <span class="nx">newStateValue</span><span class="p">);</span> <span class="c1">// 0, 1</span>
</code></pre>
</div>
</div>
<p>
Often, returning something such as an integer or a string is
not the best choice, especially as data grows more complex
than previous examples.
</p>
<p>
Consider the previous example, where component’s state
utilizes an object as its data structure of choice:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">}</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
Again, we are returning a <em>new</em> object and are not
directly mutating or overriding the
<code class="highlighter-rouge">initialState</code> object.
</p>
<p>
This reducer function is a pure function without any
side-effects. Reducer functions are the perfect fit for
managing changes in state while maintaining the immutability
we want in our components.
</p>
<p>
We’ve discussed the nature of the incoming state value, but
what about the action?
</p>
<p>
The action, represented by an object, contains properties
related to some action that happens in our apps. Every action
object is required to have a
<code class="highlighter-rouge">type</code> property, which
will “inform” the reducer actions happening in the app. The
<code class="highlighter-rouge">type</code> allows the reducer
to know what part of the state needs to change.
</p>
<p>
Let’s look at how we can use this to manage state in our apps.
</p>
<h5>Follow Along</h5>
<p>
Looking again at reducer above, let’s show it that we want to
increment our count state by passing in an
<code class="highlighter-rouge">action</code> with
<code class="highlighter-rouge">'increment'</code> as the
type.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span> <span class="o">===</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span> <span class="p">})</span>
</code></pre>
</div>
</div>
<p>
This strategy is especially powerful when we want our reducer
to be able to <em>reduce</em> the state. Take a look at our
reducer now:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span> <span class="o">===</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">}</span>
<span class="p">}</span> <span class="k">else</span> <span class="k">if</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span> <span class="o">===</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">-</span> <span class="mi">1</span> <span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span> <span class="p">});</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span> <span class="p">});</span>
</code></pre>
</div>
</div>
<p>
Now our state management is <em>very</em> predictable. Our
current state passes into the reducer, and an action follows
to tell the reducer <em>how</em> to update the state.
</p>
<p>
We can also add a
<code class="highlighter-rouge">payload</code> property to our
action objects (sometimes called
<code class="highlighter-rouge">data</code>). Our reducer
needs to have some data passed into it through the action to
be able to update the state correctly, and this is where that
data would live.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">name</span><span class="p">:</span> <span class="dl">'</span><span class="s1">Donald Duck</span><span class="dl">'</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span> <span class="o">===</span> <span class="dl">'</span><span class="s1">changeName</span><span class="dl">'</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// how do we know what to change the name to? The action payload!</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">name</span><span class="p">:</span> <span class="nx">action</span><span class="p">.</span><span class="nx">payload</span> <span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">changeName</span><span class="dl">'</span><span class="p">,</span> <span class="na">payload</span><span class="p">:</span> <span class="dl">'</span><span class="s1">Mickey Mouse</span><span class="dl">'</span> <span class="p">});</span>
</code></pre>
</div>
</div>
<p>
As you will see in the follow along, the action, and its
associated property
<code class="highlighter-rouge">type</code>, allow us to use
the reducer to perform conditional state transformations.
</p>
<p>
There’s one last edit we need to make to get to production
quality. As you can imagine, or
<code class="highlighter-rouge">if</code>,
<code class="highlighter-rouge">if else</code>,
<code class="highlighter-rouge">if else</code> … etc,
statements are going to get very complex and long. We’ll use
JavaScript’s
<code class="highlighter-rouge">switch</code> statement to
make that part of our reducer a lot more readable:
</p>
<p>Back to the count example, look at the change here:</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="c1">// if (action.type === 'increment') {</span>
<span class="c1">// return { count: state.count + 1 }</span>
<span class="c1">// } else if (action.type === 'decrement') {</span>
<span class="c1">// return { count: state.count - 1 }</span>
<span class="c1">// }</span>
<span class="c1">// we pass in the value we want to look at (action.type):</span>
<span class="k">switch</span><span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span><span class="p">)</span> <span class="p">{</span>
<span class="c1">// then we make a "case" for each possible value we expect:</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">};</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">-</span> <span class="mi">1</span> <span class="p">}</span>
<span class="c1">// finally, we give a "catch-all" which is just to return state untouched. Never leave this out. There should always be a default:</span>
<span class="nl">default</span><span class="p">:</span>
<span class="k">return</span> <span class="nx">state</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span> <span class="p">});</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span> <span class="p">});</span>
</code></pre>
</div>
</div>
<p>Cleaned up, the reducer now looks like this:</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="kd">const</span> <span class="nx">reducer</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="k">switch</span><span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">};</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">-</span> <span class="mi">1</span> <span class="p">}</span>
<span class="nl">default</span><span class="p">:</span>
<span class="k">return</span> <span class="nx">state</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">increment</span><span class="dl">'</span> <span class="p">});</span>
<span class="nx">reducer</span><span class="p">(</span><span class="nx">initialState</span><span class="p">,</span> <span class="p">{</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">decrement</span><span class="dl">'</span> <span class="p">});</span>
</code></pre>
</div>
</div>
<p>
(Read more about
<code class="highlighter-rouge">switch</code> statements
<a
href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/switch"
>here</a
>)
</p>
<h5>Challenge</h5>
<p>Create a reducer function that can do the following:</p>
<ol>
<li>
<p>
Take in an
<code class="highlighter-rouge">initialState</code> value
of an array of objects. Each object should represent a
to-do item, and should contain only one property,
<code class="highlighter-rouge">description</code>, which
should be a string, a short description of the to-do item.
</p>
</li>
<li>
<p>
Take in an action object with a
<code class="highlighter-rouge">type</code> property and a
<code class="highlighter-rouge">payload</code> property.
The
<code class="highlighter-rouge">payload</code> property
should have a description key and a value equal to a new
description entered by a user. (Don’t worry about making
inputs now, just write the reducer.)
</p>
</li>
<li>
<p>
If the type is equal to ‘ADD’, then return a new array
with a shallow copy of the previous state, and spread in a
new object that contains the new description key and its
corresponding value.
</p>
</li>
<li>
<p>Return the previous state as a default case.</p>
</li>
</ol>
<p>
For additional practice, and challenge, how might you
implement logic that would contain a type of ‘DELETE’ or
‘EDIT’?
</p>
<hr />
<h4 class="mark mb-3 p-2">
Learn to employ the `useReducer` hook to manage state in a
component
</h4>
<p>
Writing reducers from the ground up can end up being a lot of
boilerplate and time spent for a React developer. The goal is
the make state management <em>easier</em> in components. With
that said, the
<code class="highlighter-rouge">useReducer</code> hook
provides a nice abstraction that allows a developer to
efficiently <code class="highlighter-rouge">use</code> the
power of <code class="highlighter-rouge">reducers</code> in
React components, quite easily.
</p>
<p>
The React documentation is an excellent resource for
information about this hook and something to keep in your
collection of references moving forward:
<a
href="https://reactjs.org/docs/hooks-reference.html#usereducer"
>React docs - useReducer</a
>
</p>
<h5>Overview</h5>
<p>
The <code class="highlighter-rouge">useReducer</code> hook is
an alternative to
<code class="highlighter-rouge">useState</code> (<code
class="highlighter-rouge"
>useState</code
>
actually uses
<code class="highlighter-rouge">useReducer</code> hook under
the hood). It is preferable when you have complex logic that
you have to deal with in a component, or when you find
yourself with a lot of state properties (more than 3) in a
single component.
<code class="highlighter-rouge">useReducer</code>, takes in a
<code class="highlighter-rouge">reducer</code> function (that
we build), as well as a value for the
<code class="highlighter-rouge">initialState</code>. Then it
returns both the current state and a dispatch method in an
array, just like
<code class="highlighter-rouge">useState</code> does.
</p>
<p>
<code class="highlighter-rouge"
>const [state, dispatch] = useReducer(reducer,
initialState);</code
>
</p>
<p>
The dispatch method is a significant addition to our arsenal
here. It will “dispatch” an action to our reducer when
specific events occur in our application. The dispatch allows
us to powerfully combine the reducer function from our
previous section, with the ability to maintain our state at
the level of the component.
</p>
<p>
The <code class="highlighter-rouge">useReducer</code> hook has
all the functionality we love from the
<code class="highlighter-rouge">useState</code> hook and
combines it with the power of the reducers we are building
ourselves. In doing so, it provides access to both the state
and a function that dispatch actions to our reducer.
</p>
<h5>Follow Along</h5>
<p>
Let’s build out a component to go along with our counter
reducer. Pay attention to the comments in the code that will
walk us through this process.
</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="p">,</span> <span class="p">{</span> <span class="nx">useReducer</span> <span class="p">}</span> <span class="k">from</span> <span class="dl">'</span><span class="s1">react</span><span class="dl">'</span>
<span class="kd">const</span> <span class="nx">initialState</span> <span class="o">=</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="mi">0</span> <span class="p">}</span>
<span class="c1">// Initial count is established</span>
<span class="c1">// We will use the same reducer we created in the previous section</span>
<span class="kd">function</span> <span class="nx">reducer</span><span class="p">(</span><span class="nx">state</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="p">{</span>
<span class="k">switch</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">INCREASE</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="p">}</span>
<span class="k">case</span> <span class="dl">'</span><span class="s1">DECREASE</span><span class="dl">'</span><span class="p">:</span>
<span class="k">return</span> <span class="p">{</span> <span class="na">count</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">-</span> <span class="mi">1</span> <span class="p">}</span>
<span class="nl">default</span><span class="p">:</span>
<span class="k">return</span> <span class="nx">state</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="c1">// Create a functional component</span>
<span class="kd">function</span> <span class="nx">Counter</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Use the useReducer hook by destructuring its two properties: state, and dispatch and pass in the reducer and the initialState to the useReducer function</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">state</span><span class="p">,</span> <span class="nx">dispatch</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useReducer</span><span class="p">(</span><span class="nx">reducer</span><span class="p">,</span> <span class="nx">initialState</span><span class="p">)</span>
<span class="c1">// Return JSX that displays the count for the user</span>
<span class="c1">// Note the two button elements which allow the user to increase and decrease the count. Each of them contains an onClick event that dispatches the desired action object, with its given type. Each action, when fired, is dispatched to the reducer and the appropriate logic is applied.</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o"><></span>
<span class="p">{</span><span class="cm">/* Note, we have access to the current state and the dispatch method from the useReducer hook, so we can utilize them to display the count as well as couple the dispatching of the actions from the appropriate buttons.*/</span><span class="p">}</span>
<span class="o"><</span><span class="nx">div</span> <span class="nx">className</span><span class="o">=</span><span class="dl">"</span><span class="s2">count</span><span class="dl">"</span><span class="o">></span><span class="nx">Count</span><span class="p">:</span> <span class="p">{</span><span class="nx">state</span><span class="p">.</span><span class="nx">count</span><span class="p">}</span><span class="o"><</span><span class="sr">/div</span><span class="err">>
</span> <span class="o"><</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{()</span> <span class="o">=></span> <span class="nx">dispatch</span><span class="p">({</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">INCREASE</span><span class="dl">'</span> <span class="p">})}</span><span class="o">>+</span><span class="mi">1</span><span class="o"><</span><span class="sr">/button</span><span class="err">>
</span> <span class="o"><</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{()</span> <span class="o">=></span> <span class="nx">dispatch</span><span class="p">({</span> <span class="na">type</span><span class="p">:</span> <span class="dl">'</span><span class="s1">DECREASE</span><span class="dl">'</span> <span class="p">})}</span><span class="o">>-</span><span class="mi">1</span><span class="o"><</span><span class="sr">/button</span><span class="err">>
</span> <span class="o"><</span><span class="sr">/</span><span class="err">>
</span> <span class="p">)</span>