title: Seeking help on explanation for a big combination algorithm
body: <p>A recent <a href="https://app.codesignal.com/challenge/a8GNsYr8FQxZmMhJj?solutionId=MZQa7pP2nzgzzoSev" rel="nofollow noreferrer">CodeSignal Challenge</a> was to calculate 1000C500 (mod 1e9+7) and I got defeated.</p>
  
  <p>All my trials exceeded the time limit. Here is the <a href="https://app.codesignal.com/challenge/a8GNsYr8FQxZmMhJj?solutionId=MZQa7pP2nzgzzoSev" rel="nofollow noreferrer">best JS solution by psr</a>. Could anyone explain what happens in this line? I learnt ES6 but got no idea about its syntax <code>f[o = n + 1/k] = o in f</code> ...</p>
  
  <pre><code>f = nCk = (n, k) =&gt; f[o = n + 1/k] = o in f
      ? ...
      : ... 
          ? n &amp;&amp; ...
          : ...
  </code></pre>
  
  <p>Some lines are masked to avoid violation of rule</p>