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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"
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<li class="toc-entry toc-h2"><a href="#prepare">Prepare</a></li>
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<h1 class="bd-title" id="content">Scope and Closure</h1>
<span class="lead">
<p class="bd-lead"></p>
<p>
This module will give you practice with function scope and
closures. Closures are an important programming concept
that often come up in job interviews. Closure can help
solve complex problems with ease, and builds nicely off of
what you’ve been learning the past few days.
</p>
</span>
<strong
>At the end of this module, you should be able to:</strong
>
<ul>
<li>explain function scope</li>
<li>
describe what closure is, how closure is created in a
program and why it is important to understand closures in
JavaScript
</li>
</ul>
</div>
<div class="alert alert-warning" role="alert">
<h4>Pro Tip</h4>
<p>
Always think of yourself as a learner…It’s like being in a
community of wizards, and there’s still so much wizardry to
learn, you never can learn it all. People who have been
doing Unix command line, 30 or 40 years later, they’re still
finding cool stuff they never knew about. There’s that
moment of clarity when you discover a new trick, that you
had no idea about - it’s wonderful. I think it’s that sense
of wonder that brings people into technology in the first
place. As software developers, we get that more than any
other profession.
</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/ppb_O7UQZbY"
frameborder="0"
allowfullscreen
></iframe>
</div>
</li>
<li class="list-group-item">
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<iframe
class="embed-responsive-item"
src="https://www.youtube.com/embed/AgBoMqDO-08"
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 function scope</h4>
<h5>Overview</h5>
<h3 id="scope">Scope</h3>
<p>
Remember our cookie analogy from our original discussion on
Arrays? If not, recall that you have a robot baking cookies
for you with the function
<code class="highlighter-rouge">bakeCookies()</code>! Things
are going great. Fresh cookies every day - until suddenly,
your robot starts burning the lemon crisps! Come on, robot. To
fix this, you decide to add a parameter
<code class="highlighter-rouge">temperature</code>, such that
you can specify how your robot should be baking cookies. So
now,
<code class="highlighter-rouge"
>bakeCookies(chocolateChip, 400)</code
>
and
<code class="highlighter-rouge"
>bakeCookies(lemonCrisps, 350)</code
>
are valid. Later on, you go to print a recipe (for a
cookie-baking robot friend) with
<code class="highlighter-rouge">console.log(temperature)</code
>, but it comes back as
<code class="highlighter-rouge">undefined</code>! what? isn’t
it clearly defined in the function? You’ve just run into your
first scoping issue.
</p>
<p>
Scope is defined as which variables we currently have access
to and where. So far in this course, we have mostly been
working in
<code class="highlighter-rouge">global</code> scope, in that
we can access any variable we have created, anywhere in our
code. There are a couple different levels of scope:
<code class="highlighter-rouge">block</code> level scope is
used in <code class="highlighter-rouge">if</code> statements
and <code class="highlighter-rouge">for</code> loops. In
<code class="highlighter-rouge">block</code> level scope, a
variable declared using either
<code class="highlighter-rouge">let</code> or
<code class="highlighter-rouge">const</code> is only available
within the statement or loop (like
<code class="highlighter-rouge">i</code> in a
<code class="highlighter-rouge">for</code> loop). Similarly,
there is function scope, where
<code class="highlighter-rouge">temperature</code> exists
inside the function, but not elsewhere in your code file.
</p>
<h4 id="function-level-scope">Function Level Scope</h4>
<p>
Function scope allows us to create variables inside of
functions, that are essentially private to that function. We
can not reach into a function from the outside and get access
to these variables. But we are free to use these variables
anywhere within our function. Conversely, we <em>do</em> have
access to variables outside of the function. It is a one-way
street. Functions can reach out and grab variables outside of
their scope, but we can not reach into a function to get a
variable.
</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">var1</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Lambda</span><span class="dl">'</span>
<span class="kd">function</span> <span class="nx">myFunc</span><span class="p">(){</span>
<span class="kd">const</span> <span class="nx">var2</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">School</span><span class="dl">'</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">var1</span><span class="p">);</span> <span class="c1">// Lambda</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">var2</span><span class="p">);</span> <span class="c1">// School</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">var1</span><span class="p">);</span> <span class="c1">// Lambda</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">var2</span><span class="p">);</span> <span class="c1">// undefined</span>
</code></pre>
</div>
</div>
<p>
Because <code class="highlighter-rouge">var2</code> is defined
inside the function, it can’t be accessed later on.
<code class="highlighter-rouge">var1</code>, however, is
global, and can be accessed anywhere.
</p>
<iframe
height="400px"
width="100%"
src="https://repl.it/@sunjieming/Function-Scope?lite=true"
scrolling="no"
frameborder="no"
allowtransparency="true"
allowfullscreen="true"
sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"
></iframe>
<p>
<strong>Check For Understanding:</strong> Edit the code above
such that <code class="highlighter-rouge">var2</code> has
global scope (and can be accessed outside of the function).
</p>
<h3 id="hoisting">Hoisting</h3>
<p>
Typically when we use variables we declare and initialize them
in the same line. That looks something like this:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="c1">// declare & initialize variable</span>
<span class="kd">var</span> <span class="nx">myVariable</span> <span class="o">=</span> <span class="dl">"</span><span class="s2">this is a variable</span><span class="dl">"</span><span class="p">;</span>
</code></pre>
</div>
</div>
<p>
According to the rules of JavaScript though, we don’t actually
have to do it this way. For example, the below is also
perfectly acceptable and more closely mirrors what JavaScript
is doing under the hood. When you run JavaScript, the compiler
will declare all variables <em>before</em> initializing them
and <em>before</em> running the rest of the script. First the
compiler will declare all functions, then the variables. This
is one of the quirky things about JavaScript as a language.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="c1">// declare variable</span>
<span class="kd">var</span> <span class="nx">myVariable</span><span class="p">;</span>
<span class="c1">// initialize variable</span>
<span class="nx">myVariable</span> <span class="o">=</span> <span class="dl">"</span><span class="s2">this is a variable</span><span class="dl">"</span><span class="p">;</span>
</code></pre>
</div>
</div>
<p>
Given what you now know, consider the example below. What
would you expect the output to be?
<code class="highlighter-rouge">Null</code>?
<code class="highlighter-rouge">undefined</code>?
<code class="highlighter-rouge">this is a variable</code>?
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="c1">// use variable</span>
<span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">myVariable</span><span class="p">);</span>
<span class="c1">// declare and initialize variable</span>
<span class="kd">var</span> <span class="nx">myVariable</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">this is a variable</span><span class="dl">'</span><span class="p">;</span>
</code></pre>
</div>
</div>
<p>
The answer is undefined. This is because of how the compiler
runs. It first declares the variable with a value of
<code class="highlighter-rouge">undefined</code>, then runs
the script, including the initialization of the variables with
the values we give them. The compiler would take the code
above and interpret is as this:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">var</span> <span class="nx">myVariable</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">myVariable</span><span class="p">);</span>
<span class="nx">myVariable</span> <span class="o">=</span> <span class="dl">"</span><span class="s2">this is a variable</span><span class="dl">"</span><span class="p">;</span>
</code></pre>
</div>
</div>
<p>
Let’s consider another example. What would you expect the
output here to be?
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">sayHello</span><span class="p">();</span>
<span class="kd">function</span> <span class="nx">sayHello</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</span><span class="dl">"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
If you guessed <code class="highlighter-rouge">"hello"</code>,
you’d be correct. After declaring all of the variables and
functions, your compiler will also
<strong>initialize</strong> functions. This means that
functions can be used before they are declared
<em>or</em> initialized. Quick note that this only applies to
declared functions, not function expressions.
</p>
<p>
All of that encompasses the idea of
<strong><em>hoisting</em></strong
>, Hoisting is the ability to call functions and variables
before they are declared, like we see above. It is usually
preferable to declare your function before you call it, but
because of hoisting, you won’t get an error either way. The
only time you’d want to break that rule is if you wanted to
invoke all the functions at the top of a file so that another
developer looking at your file could see at the top what that
file does
</p>
<p>
We’ve mentioned some minor differences between variable
declaration types in the past. Those are explained by
hoisting. Declarations with
<code class="highlighter-rouge">var</code> and the
<code class="highlighter-rouge">function</code> keyword are
hoisted and can therefore be used anywhere in a script. These
will return undefined if not yet initialized. Variables
declared with <code class="highlighter-rouge">let</code> and
<code class="highlighter-rouge">const</code> are not hoisted
and thus can not be used anywhere without throwing a
<code class="highlighter-rouge">ReferenceError</code>.
Basically, you can’t use
<code class="highlighter-rouge">let</code> variables or
<code class="highlighter-rouge">const</code> variables before
you’ve given them a value. That difference is illustrated
here:
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="dl">'</span><span class="s1">1a</span><span class="dl">'</span><span class="p">,</span> <span class="nx">myName1</span><span class="p">);</span> <span class="c1">// undefined</span>
<span class="k">if</span> <span class="p">(</span><span class="mi">1</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="dl">'</span><span class="s1">1b</span><span class="dl">'</span> <span class="nx">myName1</span><span class="p">);</span> <span class="c1">// undefined</span>
<span class="kd">var</span> <span class="nx">myName1</span> <span class="o">=</span> <span class="err">‘</span><span class="nx">Sunil</span><span class="err">’</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="dl">'</span><span class="s1">2a</span><span class="dl">'</span><span class="p">,</span> <span class="nx">myName2</span><span class="p">);</span> <span class="c1">// error: myName2 is not defined</span>
<span class="k">if</span> <span class="p">(</span><span class="mi">1</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="dl">'</span><span class="s1">2b</span><span class="dl">'</span><span class="p">,</span> <span class="nx">myName2</span><span class="p">);</span> <span class="c1">// undefined</span>
<span class="kd">let</span> <span class="nx">myName2</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Sunil</span><span class="dl">'</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="dl">'</span><span class="s1">3a</span><span class="dl">'</span><span class="p">,</span> <span class="nx">myName3</span><span class="p">);</span> <span class="c1">// error: myName3 is not defined</span>
<span class="k">if</span> <span class="p">(</span><span class="mi">1</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="dl">'</span><span class="s1">3b</span><span class="dl">'</span><span class="p">,</span> <span class="nx">myName3</span><span class="p">);</span> <span class="c1">// undefined</span>
<span class="kd">const</span> <span class="nx">myName3</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Sunil</span><span class="dl">'</span><span class="p">;</span>
<span class="p">}</span>
</code></pre>
</div>
</div>
<p>
Hoisting is a really tricky part of JavaScript development,
and shouldn’t come up too often. That said, if you ever find
yourself getting weird reference errors and can’t figure out
why, consider hoisting as the culprit.
</p>
<h5>Follow Along</h5>
<p>
Last time we saw our
<code class="highlighter-rouge">flip</code> function it logged
a result of any number of coin flips. Our challenge now is to
include a summative number that calculates the proportion of
<code class="highlighter-rouge">heads</code>/<code
class="highlighter-rouge"
>total flips</code
>. So, if <code class="highlighter-rouge">flip</code> returns
heads 4 times and tails 6 times, our function should not only
return a list of 4 heads and 6 tails, it should also return
<code class="highlighter-rouge">0.4</code>.
</p>
<iframe
height="400px"
width="100%"
src="https://repl.it/@sunjieming/coinFlip-V?lite=true"
scrolling="no"
frameborder="no"
allowtransparency="true"
allowfullscreen="true"
sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"
></iframe>
<p>
In order to perform this calculation we’ll utilize a common
programing trick and create a counter called
<code class="highlighter-rouge">countHeads</code>, this
counter will be declared with
<code class="highlighter-rouge">var</code> and we will add
<code class="highlighter-rouge">1</code> to it every time the
flip lands on heads (or
<code class="highlighter-rouge">1</code>).
</p>
<p>
To make this counter functional we have to rely on scope. It
will be important that we declare the variable
<em>outside</em> the loop but <em>inside</em> the function.
</p>
<p>
This way the variable can be referenced and changed inside of
the for loop, but will reset to 0 every time the function is
run.
</p>
<iframe
height="400px"
width="100%"
src="https://repl.it/@sunjieming/coinFlip-VI?lite=true"
scrolling="no"
frameborder="no"
allowtransparency="true"
allowfullscreen="true"
sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"
></iframe>
<p>
To further demonstrate the importance of scope here let’s lets
consider a few non-examples, where
<code class="highlighter-rouge">countHeads</code> is out of
scope. To be completely clear, the examples below are
<strong>wrong</strong>,
<strong>incorrect programming</strong>, but are used to
illustrate the importance of scope.
</p>
<h3 id="non-example-1">Non-Example 1</h3>
<p>
When countHeads is outside of the function, our program would
work perfectly fine the first time we called
<code class="highlighter-rouge">flip(x)</code>, but would not
reset if we called it again. Make this change in the repl.it
above and see if you get the same errors as outlined below.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">var</span> <span class="nx">countHeads</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1">// won't reset when function is called</span>
<span class="kd">function</span> <span class="nx">flip</span><span class="p">(</span><span class="nx">repeat</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">repeat</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span><span class="p">){</span>
<span class="kd">var</span> <span class="nx">coinFlip</span> <span class="o">=</span> <span class="nb">Math</span><span class="p">.</span><span class="nx">round</span><span class="p">(</span><span class="nb">Math</span><span class="p">.</span><span class="nx">random</span><span class="p">());</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">coinFlip</span> <span class="o">===</span> <span class="mi">1</span><span class="p">){</span>
<span class="nx">countHeads</span> <span class="o">=</span> <span class="nx">countHeads</span> <span class="o">+</span> <span class="mi">1</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">Heads</span><span class="dl">"</span><span class="p">);</span>
<span class="p">}</span> <span class="k">else</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">Tails</span><span class="dl">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span><span class="p">(</span><span class="nx">countHeads</span><span class="o">/</span><span class="nx">repeat</span><span class="p">)</span>
<span class="p">}</span>
<span class="nx">flip</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> <span class="c1">// <- will return a % under 100 (the correct %)</span>
<span class="nx">flip</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> <span class="c1">// <- will return a % close to or over 100 (accumulates heads)</span>
<span class="nx">flip</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> <span class="c1">// <- will return an even higher % (continues to accumulate heads)</span>
</code></pre>
</div>
</div>
<h3 id="non-example-2">Non-Example 2</h3>
<p>
Similarly, if
<code class="highlighter-rouge">countHeads</code> was within
the for loop it would be declared at 0 every time the loop
ran. So
<code class="highlighter-rouge">countHeads</code> would only
ever equal <code class="highlighter-rouge">1</code> or
<code class="highlighter-rouge">0</code> depending on the
result of the flip.
</p>
<p>
Even more problematic, our return statement would throw an
error because
<code class="highlighter-rouge">countHeads</code> only exists
within the for loop. In other words, it is out of scope at the
return statement because we are referencing a variable that
does not exist at that location.
</p>
<p>
Again, make these changes changes in the repl.it above to see
the errors happen in real time.
</p>
<div class="language-js highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">function</span> <span class="nx">flip</span><span class="p">(</span><span class="nx">repeat</span><span class="p">)</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">repeat</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">var</span> <span class="nx">countHeads</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1">// will reset every time the loop starts over</span>
<span class="kd">var</span> <span class="nx">coinFlip</span> <span class="o">=</span> <span class="nb">Math</span><span class="p">.</span><span class="nx">round</span><span class="p">(</span><span class="nb">Math</span><span class="p">.</span><span class="nx">random</span><span class="p">());</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">coinFlip</span> <span class="o">===</span> <span class="mi">1</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">countHeads</span> <span class="o">=</span> <span class="nx">countHeads</span> <span class="o">+</span> <span class="mi">1</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">Heads</span><span class="dl">"</span><span class="p">);</span>
<span class="p">}</span> <span class="k">else</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">Tails</span><span class="dl">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="p">(</span><span class="nx">countHeads</span> <span class="o">/</span> <span class="nx">repeat</span><span class="p">);</span> <span class="c1">// will cause an error because countHeads is out of scope</span>
<span class="nx">flip</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> <span class="c1">// <- will cause an error</span>
</code></pre>
</div>
</div>
<h5>Challenge</h5>
<p>
Check your knowledge with the
<a
href="https://codepen.io/lambdaschool/pen/gOpOjbp?editors=0012"
>function scope challenge</a
>.
</p>
<h5>Dig Deeper</h5>
<ul>
<li>
<a
href="https://scotch.io/tutorials/understanding-hoisting-in-javascript"
target="blank_"
>Understand Hoisting in JavaScript</a
><br />
</li>
</ul>
<hr />
<h4 class="mark mb-3 p-2">
Learn to describe what closure is, how closure is created in a
program and why it is important to understand closures in
JavaScript
</h4>
<h5>Overview</h5>
<p>
Let’s look at the Window/Console in a browser/node
environment. Type
<code class="highlighter-rouge">window</code> to your chrome
console. JavaScript is executed in this context, also known as
the <code class="highlighter-rouge">global scope</code>.
</p>
<p>
When a function is declared and created, a new
<code class="highlighter-rouge">scope</code> is also created.
Any variables declared within that function’s scope will be
<code class="highlighter-rouge">enclosed</code> in a
lexical/private scope that belongs to that function. Also, it
is important to remember that functions look
<code class="highlighter-rouge">outward</code> for context. If
some variable isn’t defined in a function’s scope, the
function will look outside the scope chain and search for a
variable being referenced in the outer scope. This is what
closure is all about.
</p>
<p>In it’s most simplest of forms this is a closure:</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">foo</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">bar</span><span class="dl">'</span><span class="p">;</span>
<span class="kd">function</span> <span class="nx">returnFoo</span> <span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">foo</span><span class="p">;</span>
<span class="p">}</span>
<span class="nx">returnFoo</span><span class="p">();</span>
<span class="c1">// -> reaches outside its scope to find foo because it doesn't exist inside of return Foo's scope when foo is referenced. </span>
</code></pre>
</div>
</div>
<p>
The function body (code inside returnFoo) has access to the
outside scope (code outside of returnFoo) and can access that
‘foo’ variable.
</p>
<p>Lets look at another example:</p>
<div class="language-javascript highlighter-rouge">
<div class="highlight">
<pre
class="highlight"
><code><span class="kd">const</span> <span class="nx">lastName</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Bob</span><span class="dl">'</span><span class="p">;</span>
<span class="kd">function</span> <span class="nx">greet</span><span class="p">()</span> <span class="p">{</span>
<span class="kd">const</span> <span class="nx">firstName</span> <span class="o">=</span> <span class="dl">'</span><span class="s1">Jim</span><span class="dl">'</span><span class="p">;</span>
<span class="nx">alert</span><span class="p">(</span><span class="s2">`The name's </span><span class="p">${</span><span class="nx">lastName</span><span class="p">}</span><span class="s2">, </span><span class="p">${</span><span class="nx">firstName</span><span class="p">}</span><span class="s2"> </span><span class="p">${</span><span class="nx">lastName</span><span class="p">}</span><span class="s2">`</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="nx">lastName</span><span class="p">);</span>
</code></pre>
</div>
</div>
<p>
Not much different than the idea presented above, but the
thought remains the same. When ‘greet’ is called, it has no
context in its local scope of finding
<code class="highlighter-rouge">lastName</code> so, it looks
outside its scope to find it and use the
<code class="highlighter-rouge">lastName</code> that is found
there.
</p>
<p>
Let’s take this a step further. In the given examples, we’ve
seen that we have created two environments for our code. The
first would be the global environment where
<code class="highlighter-rouge">lastName</code> and
<code class="highlighter-rouge">greet</code> exist. The second
would be the local environment where the alert function is