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React Components and Component State
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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">Component Side Effects</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
Side effects in web apps are very common. It’s important
to understand what side effects are so we can handle them
properly in our components.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>explain what side effects are in React components</li>
<li>
demonstrate how to sync side effects with state and props
using the effect hook
</li>
<li>fetch data from an API using the effect hook</li>
<li>
demonstrate how to clean up side effects in the effect
hook
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
</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/M5yZxpyub48"
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/7oCm8Ih-ZCA"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
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<div class="embed-responsive embed-responsive-16by9">
<iframe
class="embed-responsive-item"
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<iframe
class="embed-responsive-item"
src="https://www.youtube.com/embed/rCtq5zvWI0Y"
frameborder="0"
allowfullscreen
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</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 side effects are in React components
</h4>
<p>
Side effects in web apps are very common. It’s important to
understand what side effects are so we can handle them
properly in our components.
</p>
<h5>Overview</h5>
<p>
What are “side effects”? A side effect is anything that
affects something outside the scope of the function being
executed. Fetching data from an API, timers, logging, and
manually manipulating the DOM are all examples of side
effects. There are two categories of side effects in React
components - those that don’t require cleanup and those that
do require cleanup. We will discuss effects which require
cleanup later in this module.
</p>
<p>
A React component without side effects is called a pure
component. A component is considered pure if it always renders
the same output for the same state and props. Similarly, a
side effect is something that can cause a component to return
a different output for the same state and props. Pure
components don’t have any side effects. React offers us tools
for managing side effects so we can avoid bugs and
inconsistencies in our app. The effect hook (<code
class="highlighter-rouge"
>useEffect()</code
>) is one of those.
</p>
<h3 id="the-effect-hook">The Effect Hook</h3>
<p>
The effect hook tells React that a component needs to run, or
execute, some side effect. This hook takes in two parameters.
The first is a callback function where we can run the side
effect. Let’s take a look at an effect hook that is handling a
<code class="highlighter-rouge">console.log</code> side
effect:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">useEffect</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="s2">Hello from the effect hook!</span><span class="dl">"</span><span class="p">);</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
Used inside the component, puts the effect function inside the
component’s function scope. This gives it access to state,
props, and local variables. So we could also do something like
this:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">useEffect</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="nx">props</span><span class="p">.</span><span class="nx">someProp</span><span class="p">,</span> <span class="nx">stateValueOne</span><span class="p">);</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
Here are some basic examples of other common side effects we
might see in React Components:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="c1">// Making API calls</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">user</span><span class="p">,</span> <span class="nx">setUser</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">error</span><span class="p">,</span> <span class="nx">setError</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">fetchUserData</span><span class="p">(</span><span class="nx">userId</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setUser</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">.</span><span class="nx">user</span><span class="p">))</span>
<span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">err</span> <span class="o">=></span> <span class="nx">setError</span><span class="p">(</span><span class="nx">err</span><span class="p">.</span><span class="nx">response</span><span class="p">.</span><span class="nx">message</span><span class="p">));</span>
<span class="p">});</span>
<span class="c1">// Manipulating the DOM</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">count</span><span class="p">,</span> <span class="nx">setCount</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">title</span> <span class="o">=</span> <span class="s2">`Count is: </span><span class="p">${</span><span class="nx">count</span><span class="p">}</span><span class="s2">`</span><span class="p">;</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">useEffect</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="s2">The component has mounted.</span><span class="dl">"</span><span class="p">);</span>
<span class="p">},</span> <span class="p">[]);</span>
</code></pre>
</div>
</div>
<hr />
<h4 class="mark mb-3 p-2">
Learn to demonstrate how to sync side effects with state and
props using the effect hook
</h4>
<p>
There are many times when we may only want a side effect to
run when certain data changes. Learning how to sync the effect
hook with changes in our state or props is essential to
handling side effects in the most efficient manner.
</p>
<h5>Overview</h5>
<p>
In the examples we saw above, the effect hooks used were not
synced with any particular pieces of data. Because of that,
each of those hooks would run every single time a piece of
state or a prop changed. With a lot of side effects, this
would be somewhat concerning from a performance standpoint.
Or, in the worst-case scenarios, where a side effect causes
the state to change, we would find ourselves in an infinite
loop and eventually crash our browser 😳.
</p>
<p>
Luckily we can sync our side effects with state and props
changes. We do this by passing in a dependency array as the
second argument to the effect hook. Note that this argument is
optional. If we don’t pass in an array after the callback
function, the effect will fire after every change. Let’s take
a look at a data fetch that sets state in our component.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="p">[</span><span class="nx">user</span><span class="p">,</span> <span class="nx">setUser</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">error</span><span class="p">,</span> <span class="nx">setError</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">fetchUserData</span><span class="p">(</span><span class="nx">props</span><span class="p">.</span><span class="nx">userId</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setUser</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">.</span><span class="nx">user</span><span class="p">))</span>
<span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">err</span> <span class="o">=></span> <span class="nx">setError</span><span class="p">(</span><span class="nx">err</span><span class="p">.</span><span class="nx">response</span><span class="p">.</span><span class="nx">message</span><span class="p">));</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
When this effect fires, it calls either
<code class="highlighter-rouge">setUser</code> or
<code class="highlighter-rouge">setError</code>. That means
that one of those pieces of state will change, which would, in
turn, cause the effect to fire again, causing the state to
change, firing the effect again, and placing us in an infinite
loop!
</p>
<p>
If you look closely at the effect hook, you’ll notice that
this effect also relies on another piece of data. That is
<code class="highlighter-rouge">props.userId</code>. As we
think about the component that this hook would be in, we can
imagine that we only want this effect to fire if that
<code class="highlighter-rouge">userId</code> prop changes,
right? We definitely don’t want it to fire if
<code class="highlighter-rouge">user</code> or
<code class="highlighter-rouge">error</code> change! So, that
means that we want this effect to sync up with
<code class="highlighter-rouge">props.userId</code> only -
nothing else. Let’s go ahead and add a dependency array
argument with that particular dependency in the array.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="p">[</span><span class="nx">user</span><span class="p">,</span> <span class="nx">setUser</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">error</span><span class="p">,</span> <span class="nx">setError</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">fetchUserData</span><span class="p">(</span><span class="nx">props</span><span class="p">.</span><span class="nx">userId</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setUser</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">.</span><span class="nx">user</span><span class="p">))</span>
<span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">err</span> <span class="o">=></span> <span class="nx">setError</span><span class="p">(</span><span class="nx">err</span><span class="p">.</span><span class="nx">response</span><span class="p">.</span><span class="nx">message</span><span class="p">));</span>
<span class="p">},</span> <span class="p">[</span><span class="nx">props</span><span class="p">.</span><span class="nx">userId</span><span class="p">]);</span> <span class="c1">// Dependency array added</span>
</code></pre>
</div>
</div>
<p>
Now this effect will fire when and only when
<code class="highlighter-rouge">props.userId</code> changes!
</p>
<p>
But wait, wouldn’t we want this effect to fire when the
component first mounts? Yes, definitely. Even with a
dependency array added to the effect hook, it will fire when
the component mounts, and then only fire when the given
dependencies change.
</p>
<p>
This brings up one last thought. How can we add an effect to
our component that we only want to fire off once as the
component mounts, and not any other times during the life of
the component? Essentially, we would want to tell the effect
hook to not synchronize that particular effect with any
state/props changes, right? We can solve this by adding the
dependency array to the effect hook, but leaving it blank.
Let’s set up an effect hook that runs a data fetch side effect
that we don’t want to sync with any state/props changes.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="p">[</span><span class="nx">data</span><span class="p">,</span> <span class="nx">setData</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">error</span><span class="p">,</span> <span class="nx">setError</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">getInitialData</span><span class="p">()</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setData</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">))</span>
<span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">err</span> <span class="o">=></span> <span class="nx">setError</span><span class="p">(</span><span class="nx">err</span><span class="p">.</span><span class="nx">response</span><span class="p">));</span>
<span class="p">},</span> <span class="p">[]);</span>
</code></pre>
</div>
</div>
<h3 id="summary">Summary</h3>
<p>
Using a dependency array as the second argument in the effect
hook, we can tell it with which state or props the effect
should be synced. This is a handy guide to use as you begin
the build the mental model for this principle:
</p>
<blockquote>
<p>
… the question is “with which state and props does this
effect synchronize with”
</p>
</blockquote>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">useEffect</span><span class="p">(</span><span class="nx">fn</span><span class="p">);</span> <span class="c1">// all state and props</span>
<span class="nx">useEffect</span><span class="p">(</span><span class="nx">fn</span><span class="p">,</span> <span class="p">[]);</span> <span class="c1">// no state or props</span>
<span class="nx">useEffect</span><span class="p">(</span><span class="nx">fn</span><span class="p">,</span> <span class="p">[</span><span class="nx">these</span><span class="p">,</span> <span class="nx">states</span><span class="p">,</span> <span class="nx">props</span><span class="p">]);</span>
</code></pre>
</div>
</div>
<hr />
<h4 class="mark mb-3 p-2">
Learn to fetch data from an API using the effect hook
</h4>
<p>
A common use case for the Effect Hook is fetching data.
Fetching data from an API depends on the API as an external
resource, and so it is a side effect. Because of this, we put
our axios call inside the effect function.
</p>
<h5>Overview</h5>
<p>
Now that we know how to sync effects with data, and we know
how to avoid infinite loops, let’s take a more in-depth look
at fetching data using an effect hook. There are a couple of
possible situations we want to look at when fetching data in a
component. First is writing an effect that is not synced with
any state or props so that it only fetches data once as the
component mounts. Next, is writing an effect that makes an API
call that could fire again during the life of the component,
depending on a piece of state or a prop.
</p>
<h5>Follow Along</h5>
<h4 id="fetching-data-when-a-component-mounts">
Fetching Data when a Component Mounts
</h4>
<p>
Let’s fetch a random dog image to render when this component
mounts. We’ll do this using the “dogs” API. Open a new sandbox
and follow along.
</p>
<p>
The first thing we’ll do is get our
<code class="highlighter-rouge">App</code> component ready
with some state and the proper JSX:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">App</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Initialize state to hold the image URL</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">dogPic</span><span class="p">,</span> <span class="nx">setDogPic</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">""</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="nx">className</span><span class="o">=</span><span class="dl">"</span><span class="s2">App</span><span class="dl">"</span><span class="o">></span>
<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">We</span> <span class="nx">Love</span> <span class="nx">Puppers</span><span class="o"><</span><span class="sr">/h1</span><span class="err">>
</span> <span class="o"><</span><span class="nx">img</span> <span class="nx">src</span><span class="o">=</span><span class="p">{</span><span class="nx">dogPic</span><span class="p">}</span> <span class="nx">alt</span><span class="o">=</span><span class="dl">"</span><span class="s2">a random dog</span><span class="dl">"</span> <span class="o">/></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>
Next, we’ll add the effect hook (minus the data fetch logic)
with an empty dependency array.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">App</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Initialize state to hold the image URL</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">dogPic</span><span class="p">,</span> <span class="nx">setDogPic</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">""</span><span class="p">);</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{},</span> <span class="p">[]);</span> <span class="c1">// Not synced with any data, so this effect only fires once</span>
<span class="k">return</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">App</span><span class="dl">"</span><span class="o">></span>
<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">We</span> <span class="nx">Love</span> <span class="nx">Puppers</span><span class="o"><</span><span class="sr">/h1</span><span class="err">>
</span> <span class="o"><</span><span class="nx">img</span> <span class="nx">src</span><span class="o">=</span><span class="p">{</span><span class="nx">dogPic</span><span class="p">}</span> <span class="nx">alt</span><span class="o">=</span><span class="dl">"</span><span class="s2">a random dog</span><span class="dl">"</span> <span class="o">/></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>
Now we can add the fetch logic. We’ll use Axios here as we’ve
previously learned.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">App</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// Initialize state to hold the image URL</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">dogPic</span><span class="p">,</span> <span class="nx">setDogPic</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">""</span><span class="p">);</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="c1">// This axios GET request will return a single image</span>
<span class="nx">axios</span>
<span class="p">.</span><span class="kd">get</span><span class="p">(</span><span class="dl">"</span><span class="s2">https://dog.ceo/api/breeds/image/random</span><span class="dl">"</span><span class="p">)</span>
<span class="c1">// Which we then set to state</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setDogPic</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">.</span><span class="nx">message</span><span class="p">))</span>
<span class="c1">// Always include error handling</span>
<span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">err</span> <span class="o">=></span> <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">err</span><span class="p">));</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="nx">className</span><span class="o">=</span><span class="dl">"</span><span class="s2">App</span><span class="dl">"</span><span class="o">></span>
<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">We</span> <span class="nx">Love</span> <span class="nx">Puppers</span><span class="o"><</span><span class="sr">/h1</span><span class="err">>
</span> <span class="o"><</span><span class="nx">img</span> <span class="nx">src</span><span class="o">=</span><span class="p">{</span><span class="nx">dogPic</span><span class="p">}</span> <span class="nx">alt</span><span class="o">=</span><span class="dl">"</span><span class="s2">a random dog</span><span class="dl">"</span> <span class="o">/></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>
See a working example of the completed example
<a
href="https://codesandbox.io/s/affectionate-chaplygin-9p27c"
>here</a
>.
</p>
<h3 id="fetching-data-multiple-times-with-synced-effect-hooks">
Fetching Data Multiple Times with Synced Effect Hooks
</h3>
<p>
Often, we will want to make the same fetch call multiple times
during the life of a component, all based on when certain data
changes. It can be tempting to write a function outside of the
effect hook that calls an API, then call that function from
the effect hook during the mounting stage, and then
subsequently call it from a handler function later. Like this:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> <span class="kd">const</span> <span class="p">[</span><span class="nx">data</span><span class="p">,</span> <span class="nx">setData</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">query</span><span class="p">,</span> <span class="nx">setQuery</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">"</span><span class="s2">react</span><span class="dl">"</span><span class="p">);</span>
<span class="c1">// This effect will only fire once when the component mounts</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">fetchData</span><span class="p">();</span> <span class="c1">// calls an external function that is dependent on state or props ⚠️</span>
<span class="p">},</span> <span class="p">[]);</span>
<span class="c1">// external function that relies on (or is dependent on) query</span>
<span class="kd">const</span> <span class="nx">fetchData</span> <span class="o">=</span> <span class="p">()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">axios</span><span class="p">.</span><span class="kd">get</span><span class="p">(</span><span class="dl">"</span><span class="s2">some/api/endpoint/</span><span class="dl">"</span> <span class="o">+</span> <span class="nx">query</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setData</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">));</span>
<span class="p">}</span>
<span class="kd">const</span> <span class="nx">handleChange</span> <span class="o">=</span> <span class="nx">e</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">setQuery</span><span class="p">(</span><span class="nx">e</span><span class="p">.</span><span class="nx">target</span><span class="p">.</span><span class="nx">value</span><span class="p">)</span>
<span class="nx">fetchData</span><span class="p">();</span> <span class="c1">// calls same external function after setting query</span>
<span class="p">}</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o"><></span>
<span class="o"><</span><span class="nx">input</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">query</span><span class="p">}</span> <span class="nx">onChange</span><span class="o">=</span><span class="p">{</span><span class="nx">handleChange</span><span class="p">}</span> <span class="sr">/</span><span class="err">>
</span> <span class="p">...</span>
<span class="o"><</span><span class="sr">/</span><span class="err">>
</span> <span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
This is not safe, as the effect hook calls
<code class="highlighter-rouge">fetchData</code> which relies
on <code class="highlighter-rouge">query</code>. According to
the
<a
href="https://reactjs.org/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies"
>React docs</a
>
</p>
<blockquote>
<p>
It’s difficult to remember which props or state are used by
functions outside of the effect. This is why usually you’ll
want to declare functions needed by an effect inside of it.
Then it’s easy to see what values from the component scope
that effect depends on.
</p>
</blockquote>
<p>
Let’s put the
<code class="highlighter-rouge">fetchData</code> function
inside the effect hook, and sync the hook with
<code class="highlighter-rouge">query</code>:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> <span class="kd">const</span> <span class="p">[</span><span class="nx">data</span><span class="p">,</span> <span class="nx">setData</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">();</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">query</span><span class="p">,</span> <span class="nx">setQuery</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">"</span><span class="s2">react</span><span class="dl">"</span><span class="p">);</span>
<span class="c1">// This effect will fire when the component mounts, AND each time "query" is updated! ✅</span>
<span class="nx">useEffect</span><span class="p">(()</span> <span class="o">=></span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">fetchData</span> <span class="o">=</span> <span class="p">()</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">axios</span><span class="p">.</span><span class="kd">get</span><span class="p">(</span><span class="dl">"</span><span class="s2">some/api/endpoint/</span><span class="dl">"</span> <span class="o">+</span> <span class="nx">query</span><span class="p">)</span>
<span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">res</span> <span class="o">=></span> <span class="nx">setData</span><span class="p">(</span><span class="nx">res</span><span class="p">.</span><span class="nx">data</span><span class="p">));</span>
<span class="p">}</span>
<span class="nx">fetchData</span><span class="p">();</span> <span class="c1">// calls an external function</span>
<span class="p">},</span> <span class="p">[</span><span class="nx">query</span><span class="p">]);</span>
<span class="kd">const</span> <span class="nx">handleChange</span> <span class="o">=</span> <span class="nx">e</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">setQuery</span><span class="p">(</span><span class="nx">e</span><span class="p">.</span><span class="nx">target</span><span class="p">.</span><span class="nx">value</span><span class="p">)</span>
<span class="p">}</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o"><></span>
<span class="o"><</span><span class="nx">input</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">query</span><span class="p">}</span> <span class="nx">onChange</span><span class="o">=</span><span class="p">{</span><span class="nx">handleChange</span><span class="p">}</span> <span class="sr">/</span><span class="err">>
</span> <span class="p">...</span>
<span class="o"><</span><span class="sr">/</span><span class="err">>
</span> <span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
So, to examine this more closely, we will fetch data from the
algolia API based on a query string typed into an input.
Again, open a new sandbox to follow along.
</p>
<p>To start out, we will have our state and JSX ready.</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">App</span><span class="p">()</span> <span class="p">{</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">data</span><span class="p">,</span> <span class="nx">setData</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">({</span> <span class="na">hits</span><span class="p">:</span> <span class="p">[]</span> <span class="p">});</span>
<span class="kd">const</span> <span class="p">[</span><span class="nx">query</span><span class="p">,</span> <span class="nx">setQuery</span><span class="p">]</span> <span class="o">=</span> <span class="nx">useState</span><span class="p">(</span><span class="dl">"</span><span class="s2">react</span><span class="dl">"</span><span class="p">);</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o"><></span>