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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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<h4 class="bd-toc-link">Web Fundamentals</h4>
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1. User Interface and Git
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User Interface
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Git for Web Development
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Responsive Design
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Intro to JavaScript
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<a href="/web1/module/recXRqGjyTS6uDzSA/"> Arrays </a>
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<a href="/web1/module/recQV3tKcEDXa840D/"> Objects </a>
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3. JavaScript Fundamentals
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Scope and Closure
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Advanced Array Methods
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Prototypes and Inheritance
</a>
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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="jumbotron">
<h1 class="bd-title" id="content">Advanced Array Methods</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
This module will give you practice with advanced array
methods such as
<code class="highlighter-rouge">.map</code>,
<code class="highlighter-rouge">.filter</code>, and
<code class="highlighter-rouge">.reduce</code> and
introduce you to the idea of callbacks. Both advanced
array methods and callbacks will make it easier to
interact with large data sets, like we started in the last
sprint. As you progress in your journey as a web
developer, these will likely become some of your favorite
tools for both displaying data, and dealing with user
data.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>
demonstrate the proper use a of a callback function by
passing a function to another function as a parameter
</li>
<li>
demonstrate ability to use `.map` `.reduce` `.filter` and
describe their use cases
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>Omit needless words.</p>
<footer class="blockquote-footer">
<p>Strunk and White</p>
</footer>
</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/zIRN-Gn8phU"
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/YNubYbAhtys"
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 demonstrate the proper use a of a callback function
by passing a function to another function as a parameter
</h4>
<p>
Callback functions come from a programming paradigm known as
functional programming (FP). Is not a new paradigm nor is it
unique to JavaScript. In fact, many have wondered (due to its
dynamic nature) whether JavaScript is an Object Oriented
language or a Functional Programming language. The answer to
that is simply, both. We get the ability to choose the
patterns to use in order to accomplish tasks.
</p>
<p>
That really is all we’re trying to do here: provide the
computer a set of instructions to achieve a task. And the make
up of many many tasks simply becomes… software.
</p>
<p>
At the end of the day, the language isn’t written for the
computer, that would be binary. The language we use as
developers was written for us, so we as developers get to
choose the paradigm we are to use to achieve the task.
</p>
<p>
So, callback functions are just functions that are passed into
other functions as arguments. And the concept is so commonly
used throughout the JavaScript language that one day you’ll
look back and barely remember a time when you didn’t know how
to use them.
</p>
<h5>Overview</h5>
<p>
You’ll hear these words everywhere: “Functions in JavaScript
are first-class citizens.” And if you’re like me, you’ll get
puzzled, trying to dissect meaning from those words.
</p>
<p>
Simply put, this means that functions in JavaScript are a Type
like any other Type (Boolean, String, Number), and they can be
passed around as arguments to other functions.
</p>
<p>Take the following example into consideration:</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">functionFeeder</span> <span class="o">=</span> <span class="kd">function</span><span class="p">(</span><span class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">callback</span><span class="p">(</span><span class="dl">'</span><span class="s1">Hello from the inside of Function Feeder</span><span class="dl">'</span><span class="p">);</span>
<span class="p">};</span>
</code></pre>
</div>
</div>
<p>
What we have done here is define a function called
<code class="highlighter-rouge">functionFeeder</code> and
passed it a
<code class="highlighter-rouge">parameter</code> called
<code class="highlighter-rouge">callback</code>. So when
<code class="highlighter-rouge">functionFeeder</code> is
invoked, we can feed it a function as an argument. That
function, in turn, will run the string
<code class="highlighter-rouge"
>Hello from the inside of Function Feeder</code
>
back to the callback function it receives. And thus, when we
call <code class="highlighter-rouge">functionFeeder</code> we
can handle that string that comes back to us like so:
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">functionFeeder</span><span class="p">((</span><span class="nx">string</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span> <span class="c1">// invoking the function</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">string</span><span class="p">);</span> <span class="c1">// alert a function that pops up a box in the browser.</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
<code class="highlighter-rouge">functionFeeder</code>
respectively becomes what is known as a
<code class="highlighter-rouge">higher-order function</code>
or a <code class="highlighter-rouge">callBack function</code>,
allowing it to take in a function as a parameter and executes
a function when called with a function as the argument.
</p>
<p>Let’s take a look at a few more examples.</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> <span class="kd">function</span> <span class="nx">sayHello</span><span class="p">(</span><span class="nx">name</span><span class="p">)</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="s2">`Hello, </span><span class="p">${</span><span class="nx">name</span><span class="p">}</span><span class="s2">`</span><span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
<code class="highlighter-rouge">sayHello</code> is a function
with one purpose, to say hello to the name that is passed into
it. Because we’ve built a function with such a straightforward
goal, we can use it over and over again. Once we set up a
function that will define the name we want passed to
<code class="highlighter-rouge">sayHello</code>, all we’ll
have to do is pass it to our function and let it perform is
routine / action.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> <span class="kd">function</span> <span class="nx">callSayHelloWithLars</span><span class="p">(</span><span class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">innerName</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Lars</span><span class="dl">'</span><span class="p">;</span>
<span class="nx">callback</span><span class="p">(</span><span class="nx">innerName</span><span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
The <code class="highlighter-rouge">callback</code> in the
above function will become
<code class="highlighter-rouge">sayHello</code> when we invoke
it like this:
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code> <span class="nx">callSayHelloWithLars</span><span class="p">(</span><span class="nx">sayHello</span><span class="p">);</span>
</code></pre>
</div>
</div>
<p>
And
<code class="highlighter-rouge">callSayHelloWithLars</code>
executes <code class="highlighter-rouge">sayHello</code> as
it’s callback.
</p>
<p>
I could re-use
<code class="highlighter-rouge">sayHello</code> in another
context now. Check it out.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">callSayHelloWithRyan</span><span class="p">(</span><span class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">newName</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Ryan</span><span class="dl">'</span><span class="p">;</span>
<span class="nx">callback</span><span class="p">(</span><span class="nx">newName</span><span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
Now let’s combine a few of the things we’ve learned today. I
mentioned that callbacks are used everywhere. They’re even
used in our Array methods… ALL OF THE TIME! EVERYWHERE! In the
case of <code class="highlighter-rouge">.forEach()</code>, an
iterator function, we use a callback to access the items in
the array that come back with each pass.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">items</span> <span class="o">=</span> <span class="p">[</span><span class="dl">'</span><span class="s1">feather</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">coupon</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">cup</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">drill</span><span class="dl">'</span><span class="p">];</span>
</code></pre>
</div>
</div>
<p>
We could use a
<code class="highlighter-rouge">native for loop</code> to loop
over this list and log out every item.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="k">for</span><span class="p">(</span><span class="kd">let</span> <span class="nx">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="o"><</span> <span class="nx">items</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span> <span class="nx">i</span> <span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">items</span><span class="p">[</span><span class="nx">i</span><span class="p">]);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
This works for sure! But we have
<code class="highlighter-rouge">.forEach</code> built into the
language to allow us not to have to write that every time. So,
we could do something like this to clean it up a bit.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">items</span><span class="p">.</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">item</span> <span class="o">=></span> <span class="nx">alert</span><span class="p">(</span><span class="nx">item</span><span class="p">)</span> <span class="p">);</span>
</code></pre>
</div>
</div>
<p>
<strong>NOTICE</strong>
this part is the callback
<code class="highlighter-rouge">item => alert(item)</code>
</p>
<p>
This is some fancy syntax that we will continue to go over as
the week progresses.
</p>
<h5>Follow Along</h5>
<p>
So now lets practice some things together. First, let’s create
a list of elements we want to play around with as our data —
literally, elements.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">elements</span> <span class="o">=</span> <span class="p">[</span><span class="dl">'</span><span class="s1">earth</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">wind</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">fire</span><span class="dl">'</span><span class="p">,</span> <span class="dl">'</span><span class="s1">water</span><span class="dl">'</span><span class="p">];</span>
</code></pre>
</div>
</div>
<p>
Remember that with functions we can pass as many parameters as
we want to them. So let’s create a couple of functions that we
can use to consume this list of elements.
</p>
<p>
Create a function called show first that passes back the first
item in the given array.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">showFirst</span><span class="p">(</span><span class="nx">array</span><span class="p">,</span> <span class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">callback</span><span class="p">(</span><span class="nx">array</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span>
<span class="p">}</span>
<span class="nx">showFirst</span><span class="p">(</span><span class="nx">elements</span><span class="p">,</span> <span class="p">(</span><span class="nx">firstItem</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">firstItem</span><span class="p">);</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
Create a function like ‘shoFirst’, but this time, show the
length of the array passed.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">showLength</span><span class="p">(</span><span class="nx">array</span><span class="p">,</span> <span class="nx">callback</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">callback</span><span class="p">(</span><span class="nx">array</span><span class="p">.</span><span class="nx">length</span><span class="p">);</span>
<span class="p">}</span>
<span class="nx">showLength</span><span class="p">(</span><span class="nx">elements</span><span class="p">,</span> <span class="p">(</span><span class="nx">length</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
<span class="nx">alert</span><span class="p">(</span><span class="nx">length</span><span class="p">);</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<p>
Now, let’s use
<code class="highlighter-rouge">forEach</code> again to loop
over our array and alert each item in the array.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">elements</span><span class="p">.</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">element</span> <span class="o">=></span> <span class="nx">alert</span><span class="p">(</span><span class="nx">element</span><span class="p">));</span>
</code></pre>
</div>
</div>
<p>
Remember map? The most significant difference between
<code class="highlighter-rouge">.forEach</code> and
<code class="highlighter-rouge">.map</code> is that
<code class="highlighter-rouge">map</code> returns a new array
of elements while in turn passing each element back to the
callback.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">newArray</span> <span class="o">=</span> <span class="nx">elements</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">item</span> <span class="o">=></span> <span class="dl">'</span><span class="s1">Element: </span><span class="dl">'</span> <span class="o">+</span> <span class="nx">item</span><span class="p">);</span>
</code></pre>
</div>
</div>
<h5>Challenge</h5>
<p>
Complete the following exercises included here in this JSBin
<a href="http://jsbin.com/sadikid/edit?js,console"
>Callbacks</a
>.
</p>
<h5>Dig Deeper</h5>
<ul>
<li>
<a
href="https://codeburst.io/javascript-what-the-heck-is-a-callback-aba4da2deced"
target="blank_"
>What the heck is a callback?</a
><br />
Further instruction on callbacks
</li>
</ul>
<hr />
<h4 class="mark mb-3 p-2">
Learn to demonstrate ability to use `.map` `.reduce` `.filter`
and describe their use cases
</h4>
<p>
Earlier on we covered what callbacks are and gave you ample
practice to get familiar with their use. Today we’re going to
dive deeper and cover some more crucial examples of callback
functions as well as get you the reps you need.
</p>
<p>
The callback is such a fundamental piece of the JavaScript
language that you need to have a good grasp of if you want to
be able to push your skills further.
</p>
<h5>Overview</h5>
<p>
In this lesson, we’re going to get a lot of practice playing
around with data and transforming it with more loop-like
functions. Data transformation, after all, is one of the main
goals in our jobs as web engineers since we ultimately display
data in some way that makes sense to the end user.
</p>
<p>
Functional programming is a very fun programming paradigm
because it lends itself to this idea of
<code class="highlighter-rouge">immutability</code>. Which is
basically creating objects (variables, functions, etc) that
can’t be changed by other code later on in a codebase. When we
get to Redux, you’ll learn all about immutability, and you’ll
have a better grasp of what that concept means by going
through some of these exercises today.
</p>
<p>
First, let’s talk about what functions we have at our disposal
to make variables and functions immutable. The
<code class="highlighter-rouge">.map</code>
<code class="highlighter-rouge">.filter</code> and
<code class="highlighter-rouge">.reduce</code> functions are
commonly used to achieve immutability and take a set of data
and transform it in some way.
</p>
<p>
Let’s consider the following data set, an array of object
literals. You will often see this data type stored in imported
<code class="highlighter-rouge">.JSON</code> files, including
in guided practice today. Here though, our data is written
inline, which is also fine.
</p>
<div class="language-json highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="err">const</span><span class="w"> </span><span class="err">data</span><span class="w"> </span><span class="err">=</span><span class="w"> </span><span class="p">[</span><span class="w">
</span><span class="p">{</span><span class="nl">"city"</span><span class="p">:</span><span class="s2">"seattle"</span><span class="p">,</span><span class="w"> </span><span class="nl">"state"</span><span class="p">:</span><span class="s2">"WA"</span><span class="p">,</span><span class="w"> </span><span class="nl">"population"</span><span class="p">:</span><span class="mi">652405</span><span class="p">,</span><span class="w"> </span><span class="nl">"land_area"</span><span class="p">:</span><span class="mf">83.9</span><span class="p">},</span><span class="w">
</span><span class="p">{</span><span class="nl">"city"</span><span class="p">:</span><span class="s2">"new york"</span><span class="p">,</span><span class="w"> </span><span class="nl">"state"</span><span class="p">:</span><span class="s2">"NY"</span><span class="p">,</span><span class="w"> </span><span class="nl">"population"</span><span class="p">:</span><span class="mi">8405837</span><span class="p">,</span><span class="w"> </span><span class="nl">"land_area"</span><span class="p">:</span><span class="mf">302.6</span><span class="p">},</span><span class="w">
</span><span class="p">{</span><span class="nl">"city"</span><span class="p">:</span><span class="s2">"boston"</span><span class="p">,</span><span class="w"> </span><span class="nl">"state"</span><span class="p">:</span><span class="s2">"MA"</span><span class="p">,</span><span class="w"> </span><span class="nl">"population"</span><span class="p">:</span><span class="mi">645966</span><span class="p">,</span><span class="w"> </span><span class="nl">"land_area"</span><span class="p">:</span><span class="mf">48.3</span><span class="p">},</span><span class="w">
</span><span class="p">{</span><span class="nl">"city"</span><span class="p">:</span><span class="s2">"kansas city"</span><span class="p">,</span><span class="w"> </span><span class="nl">"state"</span><span class="p">:</span><span class="s2">"MO"</span><span class="p">,</span><span class="w"> </span><span class="nl">"population"</span><span class="p">:</span><span class="mi">467007</span><span class="p">,</span><span class="w"> </span><span class="nl">"land_area"</span><span class="p">:</span><span class="mi">315</span><span class="p">}</span><span class="w">
</span><span class="p">]</span><span class="err">;</span><span class="w">
</span></code></pre>
</div>
</div>
<p>
Let’s say we want to represent this data in some way that is
useful to our users.
</p>
<h3 id="map"><code class="highlighter-rouge">.map</code></h3>
<p>
<code class="highlighter-rouge">.map</code> asks us to create
an array with just city and state names. We could do this by
copying just city and state data into a new array in a
<code class="highlighter-rouge">for</code> loop like so.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">cityStates</span> <span class="o">=</span> <span class="p">[];</span>
<span class="k">for</span><span class="p">(</span><span class="kd">let</span> <span class="nx">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="o"><</span> <span class="nx">data</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">let</span> <span class="nx">mappedObj</span> <span class="o">=</span> <span class="p">{};</span>
<span class="nx">mappedObj</span><span class="p">.</span><span class="nx">city</span> <span class="o">=</span> <span class="nx">data</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">city</span><span class="p">;</span>
<span class="nx">mappedObj</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="nx">data</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">state</span><span class="p">;</span>
<span class="nx">cityStates</span><span class="p">.</span><span class="nx">push</span><span class="p">(</span><span class="nx">mappedObj</span><span class="p">);</span>
<span class="nx">mappedObj</span> <span class="o">=</span> <span class="p">{};</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
That works! It’s a simple solution, and simplicity is the gold
standard for programming. That said, it’s a bit hard to read,
so is there a better way? absolutely!
</p>
<p>
<code class="highlighter-rouge">.map</code> is a higher-level
function that takes 2 arguments, a callback and a context. The
callback is a function to be called “for” each item in the
array, the context is the object value(s) to be used. You’ll
often people say they are going to “map” some data. That means
they are going to apply a set of rules to a data set. For
example “mapping” the array
<code class="highlighter-rouge">[1,2,3]</code> times two would
result in <code class="highlighter-rouge">[2,4,6]</code>.
</p>
<p>
<img
src="https://i.imgur.com/YxcSyFZ.png"
alt=".map annotated code"
/>
</p>
<p>
The map function can also take in a callback that passes back
a couple of things to us. The three things you’d get back from
a callback passed to Map would be:
</p>
<ul>
<li>
The current item of the array
<code class="highlighter-rouge">state</code>
</li>
<li>
The current index of the current item
<code class="highlighter-rouge">index</code>
</li>
<li>
The entire array <code class="highlighter-rouge">data</code>
</li>
</ul>
<p>
In the example below, the value given in the callback is
<code class="highlighter-rouge">state</code> since task 1 asks
us to look at state data.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">mappedCityStates</span> <span class="o">=</span> <span class="nx">data</span><span class="p">.</span><span class="nx">map</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="dl">'</span><span class="s1">city</span><span class="dl">'</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">city</span><span class="p">,</span> <span class="dl">'</span><span class="s1">state</span><span class="dl">'</span><span class="p">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">state</span><span class="p">};</span>
<span class="p">});</span>
</code></pre>
</div>
</div>
<h3 id="filter">
<code class="highlighter-rouge">.filter</code>
</h3>
<p>
What if we only want to see states who’s
<code class="highlighter-rouge"
>population is greater than</code
>
650,000? With your prior JavaScript knowledge it might make
sense to create something like this:
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">largeStates</span> <span class="o">=</span> <span class="p">[];</span>
<span class="k">for</span><span class="p">(</span><span class="kd">let</span> <span class="nx">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="o"><</span> <span class="nx">data</span><span class="p">.</span><span class="nx">length</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span><span class="p">(</span><span class="nx">data</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">population</span> <span class="o">>=</span> <span class="mi">650000</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">largeStates</span><span class="p">.</span><span class="nx">push</span><span class="p">(</span><span class="nx">data</span><span class="p">[</span><span class="nx">i</span><span class="p">]);</span>
<span class="p">}</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
Now, we can use
<code class="highlighter-rouge">.filter</code> to iterate over
all items in the array and return only what we want. This is
useful anytime we need a subset of a larger group of data.
</p>
<p>
<em
>Filter takes exactly the same arguments as .map, namely:
value, index, and array.</em