title: What is "sym.definition" in Uglify3?
body: <p>I'm trying to uglify a javascript function by UglifyJS3, specifically @ <a href="https://skalman.github.io/UglifyJS-online/" rel="nofollow noreferrer">https://skalman.github.io/UglifyJS-online/</a>.  But I'm getting an error</p>
  
  <blockquote>
    <p>TypeError: sym.definition is not a function</p>
  </blockquote>
  
  <p>that I unable to resolve.  </p>
  
  <p>To decipher the error message, I've stripped and manipulated the code down to a bare minimum, trivial code that still produces the error.</p>
  
  <pre><code>function f() {
  
      const a      = Math.abs(2);
      const [b,c]  = [2, 3];
  
  }
  </code></pre>
  
  <p>Despite the obvious trivialness, the uglifier ( with default options) gives me the aforementioned error.   The following, on the other hand, resolves the issue:  simply removing <strong>either</strong> of the lines in <code>f()</code>, lead with the <code>const [b,c]...</code> expression; OR do not use the destructing assignment.   </p>
  
  <p>The easiest and only available fix (for my code) is simply to stop using the destructing, nevertheless, I need to know why this is happening.  What exactly is the meaning of the error <code>TypeError: sym.definition is not a function</code>?</p>
  
  <p>I can't see how this simple code is producing an error, so is it the settings?   Is this a bug in <code>uglify-es 3.3.9</code>?  or specific to <code>https://skalman.github.io/UglifyJS-online/</code>?</p>
  
  <p>Any help would be much appreciated.</p>
  
  <p><strong>UPDATE:</strong>  @SparkFountain  has pointed me to right direction.  While the discussion in the cited link refers to problem in v3.3.10 and the NOT v3.3.9 that I cite, this is because the online uglifier incorrectly cites v3.3.9 as the source.  In fact, it is running v3.3.10.  While I still do not know the cause of the error, I'm satisfied that this simply a BUG, limited only to <a href="https://skalman.github.io/UglifyJS-online/" rel="nofollow noreferrer">https://skalman.github.io/UglifyJS-online/</a> (as of Aug2019) and requires effort, on my part, to move away from that site.  Thanks @SparkFoutain.</p>