togakangaroo/is-it-hoisting.html

## is-it-hoisting.html
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<div id="outline-container-orgccecef7" class="outline-2">
<h2 id="orgccecef7"><span class="section-number-2">1</span> Is it hoisting?</h2>
<div class="outline-text-2" id="text-1">
<p>
So first of all, if I just refer to a variable that doesn't exist at all I get a <code>ReferenceError</code>
</p>

<div class="org-src-container">
<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
</pre>
</div>

<pre class="example">
ReferenceError: sayHi is not defined
</pre>


<p>
But if I <code>var</code> that variable&#x2026;
</p>

<div class="org-src-container">
<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
<span style="color: #F92672;">var</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> =&gt; <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
</pre>
</div>

<pre class="example">
TypeError: sayHi is not a function
</pre>


<p>
Looks like <code>sayHi</code> <b>is</b> defined just not a function
</p>

<p>
And if I use a function declaration
</p>

<div class="org-src-container">
<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
<span style="color: #F92672;">function</span> <span style="color: #A6E22E;">sayHi</span><span style="color: #AE81FF;">()</span> <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
</pre>
</div>

<pre class="example">
hi
</pre>


<p>
So what's happening is that when entering a scope, the js interpreter does two passes, one to pick out all var and function declarations and then another to execute. So in the second case it is equivalent to doing
</p>

<div class="org-src-container">
<pre class="src src-js"><span style="color: #F92672;">var</span> <span style="color: #FD971F;">sayHi</span>;
sayHi<span style="color: #AE81FF;">()</span>
sayHi = <span style="color: #AE81FF;">()</span> =&gt; <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
</pre>
</div>

<pre class="example">
TypeError: sayHi is not a function
</pre>


<p>
In the function declaration case, the declaration cannot be separated from the body so its as if the whole thing appeared on the first line.
</p>

<p>
This is hoisting.
</p>

<p>
So what about <code>const</code> and <code>let</code>? They both follow the same rules so we'll use <code>const</code>
</p>

<div class="org-src-container">
<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> =&gt; <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
</pre>
</div>

<pre class="example">
ReferenceError: sayHi is not defined
</pre>


<p>
Ok, so no hoisting. Great.
</p>

<p>
But usually we're not writing things at the top level, and just running right away. So lets emulate something a bit more realistic
</p>

<div class="org-src-container">
<pre class="src src-js"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">greet</span> = <span style="color: #AE81FF;">()</span> =&gt; <span style="color: #AE81FF;">{</span> sayHi<span style="color: #66D9EF;">()</span>; console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`nice to meet you`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
<span style="color: #F92672;">const</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> =&gt; <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
greet<span style="color: #AE81FF;">()</span>
</pre>
</div>

<pre class="example">
hi
nice to meet you
</pre>


<p>
Wait&#x2026;why did that work? <code>sayHi</code> is declared after <code>greet</code> so <code>greet</code> shouldn't be able to see it right? No hoisting?
</p>

<p>
Well no, when you run <code>sayHi</code> its scope is <b>not</b> "from this position in the script downward" but "the current scope" (usually the containing function or module). So when the interpreter <b>does</b> see <code>sayHi</code>, it is created and assigned in the current scope - this happens whether things are hoisted or not.
</p>

<p>
When <code>greet</code> calls it then, it simply checks the current scope (the <code>greet</code> function), then when <code>sayHi</code> is not found there it looks to the parent scope, and <b>does</b> find it.
</p>

<p>
No hoisting involved.
</p>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: George Mauer</p>
<p class="date">Created: 2020-02-06 Thu 17:15</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
</body>
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	<div id="content">
	<div id="outline-container-orgccecef7" class="outline-2">
	<h2 id="orgccecef7"><span class="section-number-2">1</span> Is it hoisting?</h2>
	<div class="outline-text-2" id="text-1">
	<p>
	So first of all, if I just refer to a variable that doesn't exist at all I get a <code>ReferenceError</code>
	</p>

	<div class="org-src-container">
	<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
	</pre>
	</div>

	<pre class="example">
	ReferenceError: sayHi is not defined
	</pre>


	<p>
	But if I <code>var</code> that variable…
	</p>

	<div class="org-src-container">
	<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
	<span style="color: #F92672;">var</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> => <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	</pre>
	</div>

	<pre class="example">
	TypeError: sayHi is not a function
	</pre>


	<p>
	Looks like <code>sayHi</code> <b>is</b> defined just not a function
	</p>

	<p>
	And if I use a function declaration
	</p>

	<div class="org-src-container">
	<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
	<span style="color: #F92672;">function</span> <span style="color: #A6E22E;">sayHi</span><span style="color: #AE81FF;">()</span> <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	</pre>
	</div>

	<pre class="example">
	hi
	</pre>


	<p>
	So what's happening is that when entering a scope, the js interpreter does two passes, one to pick out all var and function declarations and then another to execute. So in the second case it is equivalent to doing
	</p>

	<div class="org-src-container">
	<pre class="src src-js"><span style="color: #F92672;">var</span> <span style="color: #FD971F;">sayHi</span>;
	sayHi<span style="color: #AE81FF;">()</span>
	sayHi = <span style="color: #AE81FF;">()</span> => <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	</pre>
	</div>

	<pre class="example">
	TypeError: sayHi is not a function
	</pre>


	<p>
	In the function declaration case, the declaration cannot be separated from the body so its as if the whole thing appeared on the first line.
	</p>

	<p>
	This is hoisting.
	</p>

	<p>
	So what about <code>const</code> and <code>let</code>? They both follow the same rules so we'll use <code>const</code>
	</p>

	<div class="org-src-container">
	<pre class="src src-js">sayHi<span style="color: #AE81FF;">()</span>
	<span style="color: #F92672;">const</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> => <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	</pre>
	</div>

	<pre class="example">
	ReferenceError: sayHi is not defined
	</pre>


	<p>
	Ok, so no hoisting. Great.
	</p>

	<p>
	But usually we're not writing things at the top level, and just running right away. So lets emulate something a bit more realistic
	</p>

	<div class="org-src-container">
	<pre class="src src-js"><span style="color: #F92672;">const</span> <span style="color: #FD971F;">greet</span> = <span style="color: #AE81FF;">()</span> => <span style="color: #AE81FF;">{</span> sayHi<span style="color: #66D9EF;">()</span>; console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`nice to meet you`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	<span style="color: #F92672;">const</span> <span style="color: #FD971F;">sayHi</span> = <span style="color: #AE81FF;">()</span> => <span style="color: #AE81FF;">{</span> console.log<span style="color: #66D9EF;">(</span><span style="color: #E6DB74;">`hi`</span><span style="color: #66D9EF;">)</span> <span style="color: #AE81FF;">}</span>
	greet<span style="color: #AE81FF;">()</span>
	</pre>
	</div>

	<pre class="example">
	hi
	nice to meet you
	</pre>


	<p>
	Wait…why did that work? <code>sayHi</code> is declared after <code>greet</code> so <code>greet</code> shouldn't be able to see it right? No hoisting?
	</p>

	<p>
	Well no, when you run <code>sayHi</code> its scope is <b>not</b> "from this position in the script downward" but "the current scope" (usually the containing function or module). So when the interpreter <b>does</b> see <code>sayHi</code>, it is created and assigned in the current scope - this happens whether things are hoisted or not.
	</p>

	<p>
	When <code>greet</code> calls it then, it simply checks the current scope (the <code>greet</code> function), then when <code>sayHi</code> is not found there it looks to the parent scope, and <b>does</b> find it.
	</p>

	<p>
	No hoisting involved.
	</p>
	</div>
	</div>
	</div>
	<div id="postamble" class="status">
	<p class="author">Author: George Mauer</p>
	<p class="date">Created: 2020-02-06 Thu 17:15</p>
	<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
	</div>
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