title: What does immutable mean?
body: <p>If a string is immutable, does that mean that....
  (let's assume JavaScript)</p>
  
  <pre><code>var str = 'foo';
  
  alert(str.substr(1)); // oo
  
  alert(str); // foo
  </code></pre>
  
  <p>Does it mean, when calling methods on a string, it will return the modified string, but it won't change the initial string?</p>
  
  <p>If the string was mutable, does that mean the 2nd <code>alert()</code> would return <code>oo</code> as well?</p>