title: Arrays inside arrays arent copied/.slice(0)-d correctly
body: <p>Let's say we have a variable a. We want to save it for later, so we make a copy of it with .slice(0):</p>
  
  <pre><code>var a = [[1,2]]
  var b = a.slice(0)
  </code></pre>
  
  <p>This should work, right? Well, if we do this:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var a = [[1,2]]
  var b = a.slice(0)
  console.log(b + " - before change")
  a[0][1] -= 2
  console.log(b + " - after change")</code></pre>
  </div>
  </div>
  </p>
  
  <p>It returns:</p>
  
  <pre><code>1,2 - before change
  1,0 - after change
  </code></pre>
  
  <p>However, if we change variable a to just [1,2] it works normally. Why does this occur?</p>