="ESLint" markdown="1"><a href="https://eslint.org/">ESLint</a></span>,
which we use to check the code in this book.
Our tool will parse source code to create a data structure,
then go through that data structure and apply rules for each part of the program.
It will also introduce us to one of the key ideas of this book,
which is that source code is just another kind of data.</p>
<div class="callout">
<h3>Don&rsquo;t define your own style</h3>
<p>Just as the world doesn&rsquo;t need more file format (<a cla