title: How should I use mangle option in uglifyjs?
body: <p>When running the following command:</p>
  
  <pre><code>echo "let demo = 3; console.log(demo);" | uglifyjs --compress --mangle
  </code></pre>
  
  <p>I would expect the following output:</p>
  
  <pre><code>let a=3;console.log(a);
  </code></pre>
  
  <p>Instead, I get:</p>
  
  <pre><code>let demo=3;console.log(demo);
  </code></pre>
  
  <p>Therefore, I don't understand how I should use <code>--mangle</code> option. <a href="https://github.com/mishoo/UglifyJS2" rel="noreferrer">The official documentation</a> explains how to exclude the names which shouldn't be mangled, how to mangle properties, etc., but how do I just transform the names of ordinary variables?</p>
  
  <p>Or is this option doing something completely different, and I misunderstood its purpose?</p>
  
  <p>Note: I'm using <a href="https://github.com/mishoo/UglifyJS2/tree/harmony" rel="noreferrer">uglify-es</a> 3.2.2.</p>