title: A javascript 'let' global variable is not a property of 'window' unlike a global 'var'
body: <p>I used to check if a global <code>var</code> has been defined with:</p>
  
  <pre><code>if (window['myvar']==null) ...
  </code></pre>
  
  <p>or</p>
  
  <pre><code>if (window.myvar==null) ...
  </code></pre>
  
  <p>It works with <code>var myvar</code></p>
  
  <p>Now that I am trying to switch to let, this does not work anymore.</p>
  
  <pre><code>var myvar='a';
  console.log(window.myvar); // gives me a
  let mylet='b';
  console.log(window.mylet); // gives me undefined
  </code></pre>
  
  <p>Question:
  With a global <code>let</code>, is there any place I can look if something has been defined like I could with <code>var</code> from the <code>window</code> object?</p>
  
  <p><b>More generally</b>:<br>
  Is <code>var myvar='a'</code> equivalent to <code>window.myvar='a'</code>?<br>
  I hear people say that at the global level, <code>let</code> and <code>var</code> are/behave the same, but this is not what I am seeing.</p>